Security Dictionary

Can't tell 'smtp' from 'snmp'? Find the precise meaning of these and hundreds of other security-related terms in our convenient and up-to-date Security Dictionary.

  • Authentication Header (AH)
    (I) An Internet IPsec protocol [R2402] designed to provide connectionless data integrity service and data origin authentication service for IP datagrams, and (optionally) to provide protection against replay attacks.

    (C) Replay protection may be selected by the receiver when a security association is established. AH authenticates upper-layer protocol data units and as much of the IP header as possible. However, some IP header fields may change in transit, and the value of these fields, when the packet arrives at the receiver, may not be predictable by the sender. Thus, the values of such fields cannot be protected end-to-end by AH; protection of the IP header by AH is only partial when such fields are present.

    (C) AH may be used alone, or in combination with the IPsec ESP protocol, or in a nested fashion with tunneling. Security services can be provided between a pair of communicating hosts, between a pair of communicating security gateways, or between a host and a gateway. ESP can provide the same security services as AH, and ESP can also provide data confidentiality service. The main difference between authentication services provided by ESP and AH is the extent of the coverage; ESP does not protect IP header fields unless they are encapsulated by AH.

  • access
    (I) The ability and means to communicate with or otherwise interact with a system in order to use system resources to either handle information or gain knowledge of the information the system contains.

    (O) "A specific type of interaction between a subject and an object that results in the flow of information from one to the other." [NCS04]

    (C) In this Glossary, "access" is intended to cover any ability to communicate with a system, including one-way communication in either direction. In actual practice, however, entities outside a security perimeter that can receive output from the system but cannot provide input or otherwise directly interact with the system, might be treated as not having "access" and, therefore, be exempt from security policy requirements, such as the need for a security clearance.

  • access control
    (I) Protection of system resources against unauthorized access; a process by which use of system resources is regulated according to a security policy and is permitted by only authorized entities (users, programs, processes, or other systems) according to that policy. (See: access, access control service.)

    (O) "The prevention of unauthorized use of a resource, including the prevention of use of a resource in an unauthorized manner." [I7498 Part 2]

  • access control center (ACC)
    (I) A computer containing a database with entries that define a security policy for an access control service.

    (C) An ACC is sometimes used in conjunction with a key center to implement access control in a key distribution system for symmetric cryptography.

  • access control list (ACL)
    (I) A mechanism that implements access control for a system resource by enumerating the identities of the system entities that are permitted to access the resource. (See: capability.)
  • access control service
    (I) A security service that protects against a system entity using a system resource in a way not authorized by the system's security policy; in short, protection of system resources against unauthorized access. (See: access control, discretionary access control, identity-based security policy, mandatory access control, rule-based security policy.)

    (C) This service includes protecting against use of a resource in an unauthorized manner by an entity that is authorized to use the resource in some other manner. The two basic mechanisms for implementing this service are ACLs and tickets.

  • access mode
    (I) A distinct type of data processing operation--e.g., read, write, append, or execute--that a subject can potentially perform on an object in a computer system.
  • accountability
    (I) The property of a system (including all of its system resources) that ensures that the actions of a system entity may be traced uniquely to that entity, which can be held responsible for its actions. (See: audit service.)

    (C) Accountability permits detection and subsequent investigation of security breaches.

  • accreditation
    (I) An administrative declaration by a designated authority that an information system is approved to operate in a particular security configuration with a prescribed set of safeguards. [FP102] (See: certification.)

    (C) An accreditation is usually based on a technical certification of the system's security mechanisms. The terms "certification" and "accreditation" are used more in the U.S. Department of Defense and other government agencies than in commercial organizations. However, the concepts apply any place where managers are required to deal with and accept responsibility for security risks. The American Bar Association is developing accreditation criteria for CAs.

  • add-on security
    (I) "The retrofitting of protection mechanisms, implemented by hardware or software, after the [automatic data processing] system has become operational." [FP039]
  • administrative security
    (I) Management procedures and constraints to prevent unauthorized access to a system. (See: security architecture.)

    (O) "The management constraints, operational procedures, accountability procedures, and supplemental controls established to provide an acceptable level of protection for sensitive data." [FP039]

    (C) Examples include clear delineation and separation of duties, and configuration control.

  • adversary
    (I) An entity that attacks, or is a threat to, a system.
  • aggregation
    (I) A circumstance in which a collection of information items is required to be classified at a higher security level than any of the individual items that comprise it.
  • algorithm
    (I) A finite set of step-by-step instructions for a problem- solving or computation procedure, especially one that can be implemented by a computer. (See: cryptographic algorithm.)
  • alias
    (I) A name that an entity uses in place of its real name, usually for the purpose of either anonymity or deception.
  • anonymous
    (I) The condition of having a name that is unknown or concealed. (See: anonymous login.)

    (C) An application may require security services that maintain anonymity of users or other system entities, perhaps to preserve their privacy or hide them from attack. To hide an entity's real name, an alias may be used. For example, a financial institution may assign an account number. Parties to a transaction can thus remain relatively anonymous, but can also accept the transaction as legitimate. Real names of the parties cannot be easily determined by observers of the transaction, but an authorized third party may be able to map an alias to a real name, such as by presenting the institution with a court order. In other applications, anonymous entities may be completely untraceable.

  • anonymous login
    (I) An access control feature (or, rather, an access control weakness) in many Internet hosts that enables users to gain access to general-purpose or public services and resources on a host (such as allowing any user to transfer data using File Transfer Protocol) without having a pre-established, user-specific account (i.e., user name and secret password).

    (C) This feature exposes a system to more threats than when all the users are known, pre-registered entities that are individually accountable for their actions. A user logs in using a special, publicly known user name (e.g., "anonymous", "guest", or "ftp"). To use the public login name, the user is not required to know a secret password and may not be required to input anything at all except the name. In other cases, to complete the normal sequence of steps in a login protocol, the system may require the user to input a matching, publicly known password (such as "anonymous") or may ask the user for an e-mail address or some other arbitrary character string.

  • archive
    (I) (1.) Noun: A collection of data that is stored for a relatively long period of time for historical and other purposes, such as to support audit service, availability service, or system integrity service. (See: backup.) (2.) Verb: To store data in such a way. (See: back up.)

    (C) A digital signature may need to be verified many years after the signing occurs. The CA--the one that issued the certificate containing the public key needed to verify that signature--may not stay in operation that long. So every CA needs to provide for long-term storage of the information needed to verify the signatures of those to whom it issues certificates.

  • association
    (I) A cooperative relationship between system entities, usually for the purpose of transferring information between them. (See: security association.)
  • assurance
    (I) (1.) An attribute of an information system that provides grounds for having confidence that the system operates such that the system security policy is enforced. (2.) A procedure that ensures a system is developed and operated as intended by the system's security policy.
  • assurance level
    (I) Evaluation usage: A specific level on a hierarchical scale representing successively increased confidence that a target of evaluation adequately fulfills the requirements. (E.g., see: TCSEC.)
  • asymmetric cryptography
    (I) A modern branch of cryptography (popularly known as "public- key cryptography") in which the algorithms employ a pair of keys (a public key and a private key) and use a different component of the pair for different steps of the algorithm. (See: key pair.)

    (C) Asymmetric algorithms have key management advantages over equivalently strong symmetric ones. First, one key of the pair does not need to be known by anyone but its owner; so it can more easily be kept secret. Second, although the other key of the pair is shared by all entities that use the algorithm, that key does not need to be kept secret from other, non-using entities; so the key distribution part of key management can be done more easily.

    (C) For encryption: In an asymmetric encryption algorithm (e.g., see: RSA), when Alice wants to ensure confidentiality for data she sends to Bob, she encrypts the data with a public key provided by Bob. Only Bob has the matching private key that is needed to decrypt the data.

    (C) For signature: In an asymmetric digital signature algorithm (e.g., see: DSA), when Alice wants to ensure data integrity or provide authentication for data she sends to Bob, she uses her private key to sign the data (i.e., create a digital signature based on the data). To verify the signature, Bob uses the matching public key that Alice has provided.

    (C) For key agreement: In an asymmetric key agreement algorithm (e.g., see: Diffie-Hellman), Alice and Bob each send their own public key to the other person. Then each uses their own private key and the other's public key to compute the new key value.

  • attack
    (I) An assault on system security that derives from an intelligent threat, i.e., an intelligent act that is a deliberate attempt (especially in the sense of a method or technique) to evade security services and violate the security policy of a system. (See: penetration, violation, vulnerability.) - Active vs. passive: An "active attack" attempts to alter system resources or affect their operation. A "passive attack" attempts to learn or make use of information from the system but does not affect system resources. (E.g., see: wiretapping.) - Insider vs. outsider: An "inside attack" is an attack initiated by an entity inside the security perimeter (an "insider"), i.e., an entity that is authorized to access system resources but uses them in a way not approved by those who granted the authorization. An "outside attack" is initiated from outside the perimeter, by an unauthorized or illegitimate user of the system (an "outsider"). In the Internet, potential outside attackers range from amateur pranksters to organized criminals, international terrorists, and hostile governments.

    (C) The term "attack" relates to some other basic security terms as shown in the following diagram: + - - - - - - - - - - - - + + - - - - + + - - - - - - - - - - -+ | An Attack: | |Counter- | | A System Resource: | | i.e., A Threat Action | | measure | | Target of the Attack | | +----------+ | | | | +-----------------+ | | | Attacker |<==================||<========= | | | | i.e., | Passive | | | | | Vulnerability | | | | A Threat |<=================>||<========> | | | | Agent | or Active | | | | +-------|||-------+ | | +----------+ Attack | | | | VVV | | | | | | Threat Consequences | + - - - - - - - - - - - - + + - - - - + + - - - - - - - - - - -+

  • attribute authority
    (I) A CA that issues attribute certificates.

    (O) "An authority, trusted by the verifier to delegate privilege, which issues attribute certificates." [FPDAM]

  • attribute certificate
    (I) A digital certificate that binds a set of descriptive data items, other than a public key, either directly to a subject name or to the identifier of another certificate that is a public-key certificate. [X509]

    (O) "A set of attributes of a user together with some other information, rendered unforgeable by the digital signature created using the private key of the CA which issued it." [X509]

    (O) "A data structure that includes some attribute values and identification information about the owner of the attribute certificate, all digitally signed by an Attribute Authority. This authority's signature serves as the guarantee of the binding between the attributes and their owner." [FPDAM]

    (C) A public-key certificate binds a subject name to a public key value, along with information needed to perform certain cryptographic functions. Other attributes of a subject, such as a security clearance, may be certified in a separate kind of digital certificate, called an attribute certificate. A subject may have multiple attribute certificates associated with its name or with each of its public-key certificates.

    (C) An attribute certificate might be issued to a subject in the following situations: - Different lifetimes: When the lifetime of an attribute binding is shorter than that of the related public-key certificate, or when it is desirable not to need to revoke a subject's public key just to revoke an attribute. - Different authorities: When the authority responsible for the attributes is different than the one that issues the public-key certificate for the subject. (There is no requirement that an attribute certificate be issued by the same CA that issued the associated public-key certificate.)

  • audit service
    (I) A security service that records information needed to establish accountability for system events and for the actions of system entities that cause them. (See: security audit.)
  • authentic signature
    (I) A signature (particularly a digital signature) that can be trusted because it can be verified. (See: validate vs. verify.)
  • authenticate
    (I) Verify (i.e., establish the truth of) an identity claimed by or for a system entity. (See: authentication.)

    (D) In general English usage, this term usually means "to prove genuine" (e.g., an art expert authenticates a Michelangelo painting). But the recommended definition carries a much narrower meaning. For example, to be precise, an ISD SHOULD NOT say "the host authenticates each received datagram". Instead, the ISD SHOULD say "the host authenticates the origin of each received datagram". In most cases, we also can say "and verifies the datagram's integrity", because that is usually implied. (See: ("relationship between data integrity service and authentication services" under) data integrity service.)

    (D) ISDs SHOULD NOT talk about authenticating a digital signature or digital certificate. Instead, we "sign" and then "verify" digital signatures, and we "issue" and then "validate" digital certificates. (See: validate vs. verify.)

  • authentication exchange
    (I) A mechanism to verify the identity of an entity by means of information exchange.

    (O) "A mechanism intended to ensure the identity of an entity by means of information exchange." [I7498 Part 2]

  • authentication information
    (I) Information used to verify an identity claimed by or for an entity. (See: authentication, credential.)

    (C) Authentication information may exist as, or be derived from, one of the following: - Something the entity knows. (See: password). - Something the entity possesses. (See: token.) - Something the entity is. (See: biometric authentication.)

  • authentication service
    (I) A security service that verifies an identity claimed by or for an entity. (See: authentication.)

    (C) In a network, there are two general forms of authentication service: data origin authentication service and peer entity authentication service.

  • authenticity
    (I) The property of being genuine and able to be verified and be trusted. (See: authenticate, authentication, validate vs. verify)
  • authority revocation list (ARL)
    (I) A data structure that enumerates digital certificates that were issued to CAs but have been invalidated by their issuer prior to when they were scheduled to expire. (See: certificate expiration, X.509 authority revocation list.)

    (O) "A revocation list containing a list of public-key certificates issued to authorities, which are no longer considered valid by the certificate issuer." [FPDAM]

  • authorize
    (I) (1.) An "authorization" is a right or a permission that is granted to a system entity to access a system resource. (2.) An "authorization process" is a procedure for granting such rights. (3.) To "authorize" means to grant such a right or permission. (See: privilege.)

    (O) SET usage: "The process by which a properly appointed person or persons grants permission to perform some action on behalf of an organization. This process assesses transaction risk, confirms that a given transaction does not raise the account holder's debt above the account's credit limit, and reserves the specified amount of credit. (When a merchant obtains authorization, payment for the authorized amount is guaranteed--provided, of course, that the merchant followed the rules associated with the authorization process.)" [SET2]

  • automated information system
    (I) An organized assembly of resources and procedures--i.e., computing and communications equipment and services, with their supporting facilities and personnel--that collect, record, process, store, transport, retrieve, or display information to accomplish a specified set of functions.
  • availability
    (I) The property of a system or a system resource being accessible and usable upon demand by an authorized system entity, according to performance specifications for the system; i.e., a system is available if it provides services according to the system design whenever users request them. (See: critical, denial of service, reliability, survivability.)

    (O) "The property of being accessible and usable upon demand by an authorized entity." [I7498 Part 2]

  • availability service
    (I) A security service that protects a system to ensure its availability.

    (C) This service addresses the security concerns raised by denial- of-service attacks. It depends on proper management and control of system resources, and thus depends on access control service and other security services.

    (I) Designation for information system equipment or facilities that handle (and for data that contains) only ciphertext (or, depending on the context, only unclassified information), and for such data itself. This term derives from U.S. Government COMSEC terminology. (See: RED, RED/BLACK separation.)
  • Basic Encoding Rules (BER)
    (I) A standard for representing ASN.1 data types as strings of octets. [X690] (See: Distinguished Encoding Rules.)
  • back door

    (I) A hardware or software mechanism that (a) provides access to a system and its resources by other than the usual procedure, (b) was deliberately left in place by the system's designers or maintainers, and (c) usually is not publicly known. (See: trap door.)

    (C) For example, a way to access a computer other than through a normal login. Such access paths do not necessarily have malicious intent; e.g., operating systems sometimes are shipped by the manufacturer with privileged accounts intended for use by field service technicians or the vendor's maintenance programmers. (See: trap door.)

  • back up vs. backup
    (I) Verb "back up": To store data for the purpose of creating a backup copy. (See: archive.)

    (I) Noun/adjective "backup": (1.) A reserve copy of data that is stored separately from the original, for use if the original becomes lost or damaged. (See: archive.) (2.) Alternate means to permit performance of system functions despite a disaster to system resources. (See: contingency plan.)

  • bandwidth
    (I) Commonly used to mean the capacity of a communication channel to pass data through the channel in a given amount of time. Usually expressed in bits per second.
  • bastion host
    (I) A strongly protected computer that is in a network protected by a firewall (or is part of a firewall) and is the only host (or one of only a few hosts) in the network that can be directly accessed from networks on the other side of the firewall.

    (C) Filtering routers in a firewall typically restrict traffic from the outside network to reaching just one host, the bastion host, which usually is part of the firewall. Since only this one host can be directly attacked, only this one host needs to be very strongly protected, so security can be maintained more easily and less expensively. However, to allow legitimate internal and external users to access application resources through the firewall, higher layer protocols and services need to be relayed and forwarded by the bastion host. Some services (e.g., DNS and SMTP) have forwarding built in; other services (e.g., TELNET and FTP) require a proxy server on the bastion host.

  • bind
    (I) To inseparably associate by applying some mechanism, such as when a CA uses a digital signature to bind together a subject and a public key in a public-key certificate.
  • biometric authentication
    (I) A method of generating authentication information for a person by digitizing measurements of a physical characteristic, such as a fingerprint, a hand shape, a retina pattern, a speech pattern (voiceprint), or handwriting.
  • bit
    (I) The smallest unit of information storage; a contraction of the term "binary digit"; one of two symbols--"0" (zero) and "1" (one) --that are used to represent binary numbers.
  • block cipher
    (I) An encryption algorithm that breaks plaintext into fixed-size segments and uses the same key to transform each plaintext segment into a fixed-size segment of ciphertext. (See: mode, stream cipher.)

    (C) For example, Blowfish, DEA, IDEA, RC2, and SKIPJACK. However, a block cipher can be adapted to have a different external interface, such as that of a stream cipher, by using a mode of operation to "package" the basic algorithm.

  • brand
    (I) A distinctive mark or name that identifies a product or business entity.

    (O) SET usage: The name of a payment card. Financial institutions and other companies have founded payment card brands, protect and advertise the brands, establish and enforce rules for use and acceptance of their payment cards, and provide networks to interconnect the financial institutions. These brands combine the roles of issuer and acquirer in interactions with cardholders and merchants. [SET1]

  • break
    (I) Cryptographic usage: To successfully perform cryptanalysis and thus succeed in decrypting data or performing some other cryptographic function, without initially having knowledge of the key that the function requires. (This term applies to encrypted data or, more generally, to a cryptographic algorithm or cryptographic system.)
  • bridge
    (I) A computer that is a gateway between two networks (usually two LANs) at OSI layer 2. (See: router.)
  • browser
    (I) An client computer program that can retrieve and display information from servers on the World Wide Web.

    (C) For example, Netscape's Navigator and Communicator, and Microsoft's Explorer.

  • brute force
    (I) A cryptanalysis technique or other kind of attack method involving an exhaustive procedure that tries all possibilities, one-by-one.

    (C) For example, for ciphertext where the analyst already knows the decryption algorithm, a brute force technique to finding the original plaintext is to decrypt the message with every possible key.

  • byte
    (I) A fundamental unit of computer storage; the smallest addressable unit in a computer's architecture. Usually holds one character of information and, today, usually means eight bits. (See: octet.)

    (C) Larger than a "bit", but smaller than a "word". Although "byte" almost always means "octet" today, bytes had other sizes (e.g., six bits, nine bits) in earlier computer architectures.

  • CA certificate
    (I) "A [digital] certificate for one CA issued by another CA." [X509]

    (C) That is, a digital certificate whose holder is able to issue digital certificates. A v3 X.509 public-key certificate may have a "basicConstraints" extension containing a "cA" value that specifically "indicates whether or not the public key may be used to verify certificate signatures."

  • Challenge Handshake Authentication Protocol (CHAP)
    (I) A peer entity authentication method for PPP, using a randomly- generated challenge and requiring a matching response that depends on a cryptographic hash of the challenge and a secret key. [R1994] (See: challenge-response, PAP.)
  • Challenge-Response Authentication Mechanism (CRAM)
    (I) IMAP4 usage: A mechanism [R2195], intended for use with IMAP4 AUTHENTICATE, by which an IMAP4 client uses a keyed hash [R2104] to authenticate itself to an IMAP4 server. (See: POP3 APOP.)

    (C) The server includes a unique timestamp in its ready response to the client. The client replies with the client's name and the hash result of applying MD5 to a string formed from concatenating the timestamp with a shared secret that is known only to the client and the server.

  • Cryptographic Message Syntax (CMS)
    (I) A encapsulation syntax for digital signatures, hashes, and encryption of arbitrary messages. [R2630]

    (C) CMS was derived from PKCS #7. CMS values are specified with ASN.1 and use BER encoding. The syntax permits multiple encapsulation with nesting, permits arbitrary attributes to be signed along with message content, and supports a variety of architectures for digital certificate-based key management.

  • call back
    (I) An authentication technique for terminals that remotely access a computer via telephone lines. The host system disconnects the caller and then calls back on a telephone number that was previously authorized for that terminal.
  • capability
    (I) A token, usually an unforgeable data value (sometimes called a "ticket") that gives the bearer or holder the right to access a system resource. Possession of the token is accepted by a system as proof that the holder has been authorized to access the resource named or indicated by the token. (See: access control list, credential, digital certificate.)

    (C) This concept can be implemented as a digital certificate. (See: attribute certificate.)

  • cardholder
    (I) An entity that has been issued a card.

    (O) SET usage: "The holder of a valid payment card account and user of software supporting electronic commerce." [SET2] A cardholder is issued a payment card by an issuer. SET ensures that in the cardholder's interactions with merchants, the payment card account information remains confidential. [SET1]

  • category
    (I) A grouping of sensitive information items to which a non- hierarchical restrictive security label is applied to increase protection of the data. (See: compartment.)
  • certificate
    (I) General English usage: A document that attests to the truth of something or the ownership of something.

    (C) Security usage: See: capability, digital certificate.

    (C) PKI usage: See: attribute certificate, public-key certificate.

  • certificate creation
    (I) The act or process by which a CA sets the values of a digital certificate's data fields and signs it. (See: issue.)
  • certificate expiration
    (I) The event that occurs when a certificate ceases to be valid because its assigned lifetime has been exceeded. (See: certificate revocation, validity period.)
  • certificate management
    (I) The functions that a CA may perform during the life cycle of a digital certificate, including the following: - Acquire and verify data items to bind into the certificate. - Encode and sign the certificate. - Store the certificate in a directory or repository. - Renew, rekey, and update the certificate. - Revoke the certificate and issue a CRL. (See: archive management, certificate management, key management, security architecture, token management.)
  • certificate policy
    (I) "A named set of rules that indicates the applicability of a certificate to a particular community and/or class of application with common security requirements." [X509] (See: certification practice statement.)

    (C) A certificate policy can help a certificate user decide whether a certificate should be trusted in a particular application. "For example, a particular certificate policy might indicate applicability of a type of certificate for the authentication of electronic data interchange transactions for the trading goods within a given price range." [R2527]

    (C) A v3 X.509 public-key certificate may have a "certificatePolicies" extension that lists certificate policies, recognized by the issuing CA, that apply to the certificate and govern its use. Each policy is denoted by an object identifier and may optionally have certificate policy qualifiers.

    (C) SET usage: Every SET certificate specifies at least one certificate policy, that of the SET root CA. SET uses certificate policy qualifiers to point to the actual policy statement and to add qualifying policies to the root policy. (See: SET qualifier.)

  • certificate policy qualifier
    (I) Information that pertains to a certificate policy and is included in a "certificatePolicies" extension in a v3 X.509 public-key certificate.
  • certificate reactivation
    (I) The act or process by which a digital certificate, which a CA has designated for revocation but not yet listed on a CRL, is returned to the valid state.
  • certificate rekey
    (I) The act or process by which an existing public-key certificate has its public key value changed by issuing a new certificate with a different (usually new) public key. (See: certificate renewal, certificate update, rekey.)

    (C) For an X.509 public-key certificate, the essence of rekey is that the subject stays the same and a new public key is bound to that subject. Other changes are made, and the old certificate is revoked, only as required by the PKI and CPS in support of the rekey. If changes go beyond that, the process is a "certificate update".

    (O) MISSI usage: To rekey a MISSI X.509 public-key certificate means that the issuing authority creates a new certificate that is identical to the old one, except the new one has a new, different KEA key; or a new, different DSS key; or new, different KEA and DSS keys. The new certificate also has a different serial number and may have a different validity period. A new key creation date and maximum key lifetime period are assigned to each newly generated key. If a new KEA key is generated, that key is assigned a new KMID. The old certificate remains valid until it expires, but may not be further renewed, rekeyed, or updated.

  • certificate renewal
    (I) The act or process by which the validity of the data binding asserted by an existing public-key certificate is extended in time by issuing a new certificate. (See: certificate rekey, certificate update.)

    (C) For an X.509 public-key certificate, this term means that the validity period is extended (and, of course, a new serial number is assigned) but the binding of the public key to the subject and to other data items stays the same. The other data items are changed, and the old certificate is revoked, only as required by the PKI and CPS to support the renewal. If changes go beyond that, the process is a "certificate rekey" or "certificate update".

  • certificate revocation
    (I) The event that occurs when a CA declares that a previously valid digital certificate issued by that CA has become invalid; usually stated with a revocation date.

    (C) In X.509, a revocation is announced to potential certificate users by issuing a CRL that mentions the certificate. Revocation and listing on a CRL is only necessary before certificate expiration.

  • certificate revocation list (CRL)
    (I) A data structure that enumerates digital certificates that have been invalidated by their issuer prior to when they were scheduled to expire. (See: certificate expiration, X.509 certificate revocation list.)

    (O) "A signed list indicating a set of certificates that are no longer considered valid by the certificate issuer. After a certificate appears on a CRL, it is deleted from a subsequent CRL after the certificate's expiry. CRLs may be used to identify revoked public-key certificates or attribute certificates and may represent revocation of certificates issued to authorities or to users. The term CRL is also commonly used as a generic term applying to all the different types of revocation lists, including CRLs, ARLs, ACRLs, etc." [FPDAM]

  • certificate revocation tree
    (I) A mechanism for distributing notice of certificate revocations; uses a tree of hash results that is signed by the tree's issuer. Offers an alternative to issuing a CRL, but is not supported in X.509. (See: certificate status responder.)
  • certificate serial number
    (I) An integer value that (a) is associated with, and may be carried in, a digital certificate; (b) is assigned to the certificate by the certificate's issuer; and (c) is unique among all the certificates produced by that issuer.

    (O) "An integer value, unique within the issuing CA, which is unambiguously associated with a certificate issued by that CA." [X509]

  • certificate update
    (I) The act or process by which non-key data items bound in an existing public-key certificate, especially authorizations granted to the subject, are changed by issuing a new certificate. (See: certificate rekey, certificate renewal.)

    (C) For an X.509 public-key certificate, the essence of this process is that fundamental changes are made in the data that is bound to the public key, such that it is necessary to revoke the old certificate. (Otherwise, the process is only a "certificate rekey" or "certificate renewal".)

  • certificate user
    (I) A system entity that depends on the validity of information (such as another entity's public key value) provided by a digital certificate. (See: relying party.)

    (O) "An entity that needs to know, with certainty, the public key of another entity." [X509]

    (C) The system entity may be a human being or an organization, or a device or process under the control of a human or an organization.

    (D) ISDs SHOULD NOT use this term as a synonym for the "subject" of a certificate.

  • certificate validation
    (I) An act or process by which a certificate user establishes that the assertions made by a digital certificate can be trusted. (See: valid certificate, validate vs. verify.)

    (O) "The process of ensuring that a certificate is valid including possibly the construction and processing of a certification path, and ensuring that all certificates in that path have not expired or been revoked." [FPDAM]

    (C) To validate a certificate, a certificate user checks that the certificate is properly formed and signed and currently in force: - Checks the signature: Employs the issuer's public key to verify the digital signature of the CA who issued the certificate in question. If the verifier obtains the issuer's public key from the issuer's own public-key certificate, that certificate should be validated, too. That validation may lead to yet another certificate to be validated, and so on. Thus, in general, certificate validation involves discovering and validating a certification path. - Checks the syntax and semantics: Parses the certificate's syntax and interprets its semantics, applying rules specified for and by its data fields, such as for critical extensions in an X.509 certificate. - Checks currency and revocation: Verifies that the certificate is currently in force by checking that the current date and time are within the validity period (if that is specified in the certificate) and that the certificate is not listed on a CRL or otherwise announced as invalid. (CRLs themselves require a similar validation process.)

  • certification
    (I) Information system usage: Technical evaluation (usually made in support of an accreditation action) of an information system's security features and other safeguards to establish the extent to which the system's design and implementation meet specified security requirements. [FP102] (See: accreditation.)

    (I) Digital certificate usage: The act or process of vouching for the truth and accuracy of the binding between data items in a certificate. (See: certify.)

    (I) Public key usage: The act or process of vouching for the ownership of a public key by issuing a public-key certificate that binds the key to the name of the entity that possesses the matching private key. In addition to binding a key to a name, a public-key certificate may bind those items to other restrictive or explanatory data items. (See: X.509 public-key certificate.)

    (O) SET usage: "The process of ascertaining that a set of requirements or criteria has been fulfilled and attesting to that fact to others, usually with some written instrument. A system that has been inspected and evaluated as fully compliant with the SET protocol by duly authorized parties and process would be said to have been certified compliant." [SET2]

  • certification authority (CA)
    (I) An entity that issues digital certificates (especially X.509 certificates) and vouches for the binding between the data items in a certificate.

    (O) "An authority trusted by one or more users to create and assign certificates. Optionally, the certification authority may create the user's keys." [X509]

    (C) Certificate users depend on the validity of information provided by a certificate. Thus, a CA should be someone that certificate users trust, and usually holds an official position created and granted power by a government, a corporation, or some other organization. A CA is responsible for managing the life cycle of certificates (see: certificate management) and, depending on the type of certificate and the CPS that applies, may be responsible for the life cycle of key pairs associated with the certificates (see: key management).

  • certification authority workstation (CAW)
    (I) A computer system that enables a CA to issue digital certificates and supports other certificate management functions as required.
  • certification hierarchy
    (I) A tree-structured (loop-free) topology of relationships among CAs and the entities to whom the CAs issue public-key certificates. (See: hierarchical PKI.)

    (C) In this structure, one CA is the top CA, the highest level of the hierarchy. (See: root, top CA.) The top CA may issue public- key certificates to one or more additional CAs that form the second highest level. Each of these CAs may issue certificates to more CAs at the third highest level, and so on. The CAs at the second-lowest of the hierarchy issue certificates only to non-CA entities, called "end entities" that form the lowest level. (See: end entity.) Thus, all certification paths begin at the top CA and descend through zero or more levels of other CAs. All certificate users base path validations on the top CA's public key.

    (O) MISSI usage: A MISSI certification hierarchy has three or four levels of CAs: - A CA at the highest level, the top CA, is a "policy approving authority". - A CA at the second-highest level is a "policy creation authority". - A CA at the third-highest level is a local authority called a "certification authority". - A CA at the fourth-highest (optional) level is a "subordinate certification authority".

    (O) PEM usage: A PEM certification hierarchy has three levels of CAs [R1422]: - The highest level is the "Internet Policy Registration Authority". - A CA at the second-highest level is a "policy certification authority". - A CA at the third-highest level is a "certification authority".

    (O) SET usage: A SET certification hierarchy has three or four levels of CAs: - The highest level is a "SET root CA". - A CA at the second-highest level is a "brand certification authority". - A CA at the third-highest (optional) level is a "geopolitical certification authority". - A CA at the fourth-highest level is a "cardholder CA", a "merchant CA", or a "payment gateway CA".

  • certification path
    (I) An ordered sequence of public-key certificates (or a sequence of public-key certificates followed by one attribute certificate) that enables a certificate user to verify the signature on the last certificate in the path, and thus enables the user to obtain a certified public key (or certified attributes) of the entity that is the subject of that last certificate. (See: certificate validation, valid certificate.)

    (O) "An ordered sequence of certificates of objects in the [X.500 Directory Information Tree] which, together with the public key of the initial object in the path, can be processed to obtain that of the final object in the path." [X509, R2527]

    (C) The path is the "list of certificates needed to allow a particular user to obtain the public key of another." [X509] The list is "linked" in the sense that the digital signature of each certificate (except the first) is verified by the public key contained in the preceding certificate; i.e., the private key used to sign a certificate and the public key contained in the preceding certificate form a key pair owned by the entity that signed.

    (C) In the X.509 quotation in the previous "C" paragraph, the word "particular" points out that a certification path that can be validated by one certificate user might not be able to be validated by another. That is because either the first certificate should be a trusted certificate (it might be a root certificate) or the signature on the first certificate should be verified by a trusted key (it might be a root key), but such trust is defined relative to each user, not absolutely for all users.

  • certification practice statement (CPS)
    (I) "A statement of the practices which a certification authority employs in issuing certificates." [ABA96, R2527] (See: certificate policy.)

    (C) A CPS is a published security policy that can help a certificate user to decide whether a certificate issued by a particular CA can be trusted enough to use in a particular application. A CPS may be (a) a declaration by a CA of the details of the system and practices it employs in its certificate management operations, (b) part of a contract between the CA and an entity to whom a certificate is issued, (c) a statute or regulation applicable to the CA, or (d) a combination of these types involving multiple documents. [ABA]

    (C) A CPS is usually more detailed and procedurally oriented than a certificate policy. A CPS applies to a particular CA or CA community, while a certificate policy applies across CAs or communities. A CA with a single CPS may support multiple certificate policies, which may be used for different application purposes or by different user communities. Multiple CAs, each with a different CPS, may support the same certificate policy. [R2527]

  • certification request
    (I) A algorithm-independent transaction format, defined by PCKS #10 and used in PKIX, that contains a DN, a public key, and optionally a set of attributes, collectively signed by the entity requesting certification, and sent to a CA, which transforms the request to an X.509 public-key certificate or another type of certificate.
  • challenge-response
    (I) An authentication process that verifies an identity by requiring correct authentication information to be provided in response to a challenge. In a computer system, the authentication information is usually a value that is required to be computed in response to an unpredictable challenge value.
  • channel
    (I) An information transfer path within a system. (See: covert channel.)
  • checksum
    (I) A value that (a) is computed by a function that is dependent on the contents of a data object and (b) is stored or transmitted together with the object, for the purpose of detecting changes in the data. (See: cyclic redundancy check, data integrity service, error detection code, hash, keyed hash, protected checksum.)

    (C) To gain confidence that a data object has not been changed, an entity that later uses the data can compute a checksum and compare it with the checksum that was stored or transmitted with the object.

    (C) Computer systems and networks employ checksums (and other mechanisms) to detect accidental changes in data. However, active wiretapping that changes data could also change an accompanying checksum to match the changed data. Thus, some checksum functions by themselves are not good countermeasures for active attacks. To protect against active attacks, the checksum function needs to be well-chosen (see: cryptographic hash), and the checksum result needs to be cryptographically protected (see: digital signature, keyed hash).

  • chosen-ciphertext attack
    (I) A cryptanalysis technique in which the analyst tries to determine the key from knowledge of plaintext that corresponds to ciphertext selected (i.e., dictated) by the analyst.
  • chosen-plaintext attack
    (I) A cryptanalysis technique in which the analyst tries to determine the key from knowledge of ciphertext that corresponds to plaintext selected (i.e., dictated) by the analyst.
  • cipher
    (I) A cryptographic algorithm for encryption and decryption.
  • cipher block chaining (CBC)
    (I) An block cipher mode that enhances electronic codebook mode by chaining together blocks of ciphertext it produces. [FP081] (See: [R1829], [R2451].)

    (C) This mode operates by combining (exclusive OR-ing) the algorithm's ciphertext output block with the next plaintext block to form the next input block for the algorithm.

  • cipher feedback (CFB)
    (I) An block cipher mode that enhances electronic code book mode by chaining together the blocks of ciphertext it produces and operating on plaintext segments of variable length less than or equal to the block length. [FP081]

    (C) This mode operates by using the previously generated ciphertext segment as the algorithm's input (i.e., by "feeding back" the ciphertext) to generate an output block, and then combining (exclusive OR-ing) that output block with the next plaintext segment (block length or less) to form the next ciphertext segment.

  • ciphertext
    (I) Data that has been transformed by encryption so that its semantic information content (i.e., its meaning) is no longer intelligible or directly available. (See: cleartext, plaintext.)

    (O) "Data produced through the use of encipherment. The semantic content of the resulting data is not available." [I7498 Part 2]

  • ciphertext-only attack
    (I) A cryptanalysis technique in which the analyst tries to determine the key solely from knowledge of intercepted ciphertext (although the analyst may also know other clues, such as the cryptographic algorithm, the language in which the plaintext was written, the subject matter of the plaintext, and some probable plaintext words.)
  • classification level
    (I) (1.) A grouping of classified information to which a hierarchical, restrictive security label is applied to increase protection of the data. (2.) The level of protection that is required to be applied to that information. (See: security level.)
  • classified
    (I) Refers to information (stored or conveyed, in any form) that is formally required by a security policy to be given data confidentiality service and to be marked with a security label (which in some cases might be implicit) to indicate its protected status. (See: unclassified.)

    (C) The term is mainly used in government, especially in the military, although the concept underlying the term also applies outside government. In the U.S. Department of Defense, for example, it means information that has been determined pursuant to Executive Order 12958 ("Classified National Security Information", 20 April 1995) or any predecessor order to require protection against unauthorized disclosure and is marked to indicate its classified status when in documentary form.

  • clean system
    (I) A computer system in which the operating system and application system software and files have just been freshly installed from trusted software distribution media.

    (C) A clean system is not necessarily in a secure state.

  • clearance level
    (I) The security level of information to which a security clearance authorizes a person to have access.
  • cleartext
    (I) Data in which the semantic information content (i.e., the meaning) is intelligible or is directly available. (See: plaintext.)

    (O) "Intelligible data, the semantic content of which is available." [I7498 Part 2]

    (D) ISDs SHOULD NOT use this term as a synonym for "plaintext", the input to an encryption operation, because the plaintext input to encryption may itself be ciphertext that was output from another operation. (See: superencryption.)

  • client
    (I) A system entity that requests and uses a service provided by another system entity, called a "server". (See: server.)

    (C) Usually, the requesting entity is a computer process, and it makes the request on behalf of a human user. In some cases, the server may itself be a client of some other server.

  • code
    (I) noun: A system of symbols used to represent information, which might originally have some other representation. (See: encode.)

    (D) ISDs SHOULD NOT use this term as synonym for the following: (a) "cipher", "hash", or other words that mean "a cryptographic algorithm"; (b) "ciphertext"; or (c) "encrypt", "hash", or other words that refer to applying a cryptographic algorithm.

    (D) ISDs SHOULD NOT this word as an abbreviation for the following terms: country code, cyclic redundancy code, Data Authentication Code, error detection code, Message Authentication Code, object code, or source code. To avoid misunderstanding, use the fully qualified term, at least at the point of first usage.

  • color change
    (I) In a system that is being operated in periods processing mode, the act of purging all information from one processing period and then changing over to the next processing period.
  • common name
    (I) A character string that (a) may be a part of the X.500 DN of a Directory object ("commonName" attribute), (b) is a (possibly ambiguous) name by which the object is commonly known in some limited scope (such as an organization), and (c) conforms to the naming conventions of the country or culture with which it is associated. [X520] (See: ("subject" and "issuer" under) X.509 public-key certificate.)

    (C) For example, "Dr. E. F. Moore", "The United Nations", or "12-th Floor Laser Printer".

  • communication security (COMSEC)
    (I) Measures that implement and assure security services in a communication system, particularly those that provide data confidentiality and data integrity and that authenticate communicating entities.

    (C) Usually understood to include cryptographic algorithms and key management methods and processes, devices that implement them, and the life cycle management of keying material and devices.

  • community string
    (I) A community name in the form of an octet string that serves as a cleartext password in SNMP version 1. [R1157]
  • compartment
    (I) A grouping of sensitive information items that require special access controls beyond those normally provided for the basic classification level of the information. (See: category.)

    (C) The term is usually understood to include the special handling procedures to be used for the information.

  • computer emergency response team (CERT)
    (I) An organization that studies computer and network INFOSEC in order to provide incident response services to victims of attacks, publish alerts concerning vulnerabilities and threats, and offer other information to help improve computer and network security. (See: CSIRT, security incident.)

    (C) For example, the CERT Coordination Center at Carnegie-Mellon University (sometimes called "the" CERT) and the Computer Incident Advisory Capability.

  • computer network
    (I) A collection of host computers together with the subnetwork or internetwork through which they can exchange data.

    (C) This definition is intended to cover systems of all sizes and types, ranging from the complex Internet to a simple system composed of a personal computer dialing in as a remote terminal of another computer.

  • computer security (COMPUSEC)
    (I) Measures that implement and assure security services in a computer system, particularly those that assure access control service.

    (C) Usually understood to include functions, features, and technical characteristics of computer hardware and software, especially operating systems.

  • computer security incident response team (CSIRT)
    (I) An organization "that coordinates and supports the response to security incidents that involve sites within a defined constituency." [R2350] (See: CERT, FIRST, security incident.)

    (C) To be considered a CSIRT, an organization must do as follows: - Provide a (secure) channel for receiving reports about suspected security incidents. - Provide assistance to members of its constituency in handling the incidents. - Disseminate incident-related information to its constituency and other involved parties.

  • computer security object
    (I) The definition or representation of a resource, tool, or mechanism used to maintain a condition of security in computerized environments. Includes many elements referred to in standards that are either selected or defined by separate user communities. [CSOR] (See: object identifier, Computer Security Objects Register.)
  • configuration control
    (I) The process of regulating changes to hardware, firmware, software, and documentation throughout the development and operational life of a system. (See: administrative security.)

    (C) Configuration control helps protect against unauthorized or malicious alteration of a system and thus provides assurance of system integrity. (See: malicious logic.)

  • connectionless data integrity service
    (I) A security service that provides data integrity service for an individual IP datagram, by detecting modification of the datagram, without regard to the ordering of the datagram in a stream of datagrams.

    (C) A connection-oriented data integrity service would be able to detect lost or reordered datagrams within a stream of datagrams.

  • contingency plan
    (I) A plan for emergency response, backup operations, and post- disaster recovery in a system as part of a security program to ensure availability of critical system resources and facilitate continuity of operations in a crisis. [NCS04] (See: availability.)
  • cookie
    (I) access control usage: A synonym for "capability" or "ticket" in an access control system.

    (I) IPsec usage: Data exchanged by ISAKMP to prevent certain denial-of-service attacks during the establishment of a security association.

    (I) HTTP usage: Data exchanged between an HTTP server and a browser (a client of the server) to store state information on the client side and retrieve it later for server use.

    (C) An HTTP server, when sending data to a client, may send along a cookie, which the client retains after the HTTP connection closes. A server can use this mechanism to maintain persistent client-side state information for HTTP-based applications, retrieving the state information in later connections. A cookie may include a description of the range of URLs for which the state is valid. Future requests made by the client in that range will also send the current value of the cookie to the server. Cookies can be used to generate profiles of web usage habits, and thus may infringe on personal privacy.

  • correctness integrity
    (I) Accuracy and consistency of the information that data values represent, rather than of the data itself. Closely related to issues of accountability and error handling. (See: data integrity, source integrity.)
  • correctness proof
    (I) A mathematical proof of consistency between a specification for system security and the implementation of that specification. (See: formal specification.)
  • countermeasure
    (I) An action, device, procedure, or technique that reduces a threat, a vulnerability, or an attack by eliminating or preventing it, by minimizing the harm it can cause, or by discovering and reporting it so that corrective action can be taken.

    (C) In an Internet protocol, a countermeasure may take the form of a protocol feature, an element function, or a usage constraint.

  • country code
    (I) An identifier that is defined for a nation by ISO. [I3166]

    (C) For each nation, ISO Standard 3166 defines a unique two- character alphabetic code, a unique three-character alphabetic code, and a three-digit code. Among many uses of these codes, the two-character codes are used as top-level domain names.

  • covert channel
    (I) A intra-system channel that permits two cooperating entities, without exceeding their access authorizations, to transfer information in a way that violates the system's security policy. (See: channel, out of band.)

    (O) "A communications channel that allows two cooperating processes to transfer information in a manner that violates the system's security policy." [NCS04]

    (C) The cooperating entities can be either two insiders or an insider and an outsider. Of course, an outsider has no access authorization at all. A covert channel is a system feature that the system architects neither designed nor intended for information transfer: - "Timing channel": A system feature that enable one system entity to signal information to another by modulating its own use of a system resource in such a way as to affect system response time observed by the second entity. - "Storage channel": A system feature that enables one system entity to signal information to another entity by directly or indirectly writing a storage location that is later directly or indirectly read by the second entity.

  • cracker
    (I) Someone who tries to break the security of, and gain access to, someone else's system without being invited to do so. (See: hacker and intruder.)
  • credential(s)
    (I) Data that is transferred or presented to establish either a claimed identity or the authorizations of a system entity. (See: authentication information, capability, ticket.)

    (O) "Data that is transferred to establish the claimed identity of an entity." [I7498 Part 2]

  • cross-certification
    (I) The act or process by which two CAs each certify a public key of the other, issuing a public-key certificate to that other CA.

    (C) Cross-certification enables users to validate each other's certificate when the users are certified under different certification hierarchies.

  • cryptanalysis
    (I) The mathematical science that deals with analysis of a cryptographic system in order to gain knowledge needed to break or circumvent the protection that the system is designed to provide. (See: cryptology.)

    (O) "The analysis of a cryptographic system and/or its inputs and outputs to derive confidential variables and/or sensitive data including cleartext." [I7498 Part 2]

    (C) The "O" definition states the traditional goal of cryptanalysis--convert the ciphertext to plaintext (which usually is cleartext) without knowing the key--but that definition applies only to encryption systems. Today, the term is used with reference to all kinds of cryptographic algorithms and key management, and the "I" definition reflects that. In all cases, however, a cryptanalyst tries to uncover or reproduce someone else's sensitive data, such as cleartext, a key, or an algorithm. The basic cryptanalytic attacks on encryption systems are ciphertext- only, known-plaintext, chosen-plaintext, and chosen-ciphertext; and these generalize to the other kinds of cryptography.

  • cryptographic algorithm
    (I) An algorithm that employs the science of cryptography, including encryption algorithms, cryptographic hash algorithms, digital signature algorithms, and key agreement algorithms.
  • cryptographic application programming interface (CAPI)
    (I) The source code formats and procedures through which an application program accesses cryptographic services, which are defined abstractly compared to their actual implementation. For example, see: PKCS #11, [R2628].
  • cryptographic card
    (I) A cryptographic token in the form of a smart card or a PC card.
  • cryptographic component
    (I) A generic term for any system component that involves cryptography. (See: cryptographic module.)
  • cryptographic ignition key (CIK)
    (I) A physical (usually electronic) token used to store, transport, and protect cryptographic keys. (Sometimes abbreviated as "crypto ignition key".)

    (C) A typical use is to divide a split key between a CIK and a cryptographic module, so that it is necessary to combine the two to regenerate a key-encrypting key and thus activate the module and other keys it contains.

  • cryptographic key
    (I) Usually shortened to just "key". An input parameter that varies the transformation performed by a cryptographic algorithm.

    (O) "A sequence of symbols that controls the operations of encipherment and decipherment." [I7498 Part 2]

    (C) If a key value needs to be kept secret, the sequence of symbols (usually bits) that comprise it should be random, or at least pseudo-random, because that makes the key hard for an adversary to guess. (See: cryptanalysis, brute force attack.)

  • cryptographic module
    (I) A set of hardware, software, firmware, or some combination thereof that implements cryptographic logic or processes, including cryptographic algorithms, and is contained within the module's cryptographic boundary, which is an explicitly defined contiguous perimeter that establishes the physical bounds of the module. [FP140]
  • cryptographic system
    (I) A set of cryptographic algorithms together with the key management processes that support use of the algorithms in some application context.

    (C) This "I" definition covers a wider range of algorithms than the following "O" definition:

    (O) "A collection of transformations from plaintext into ciphertext and vice versa [which would exclude digital signature, cryptographic hash, and key agreement algorithms], the particular transformation(s) to be used being selected by keys. The transformations are normally defined by a mathematical algorithm." [X509]

  • cryptographic token
    (I) A portable, user-controlled, physical device used to store cryptographic information and possibly perform cryptographic functions. (See: cryptographic card, token.)

    (C) A smart token may implement some set of cryptographic algorithms and may implement related algorithms and key management functions, such as a random number generator. A smart cryptographic token may contain a cryptographic module or may not be explicitly designed that way.

  • cryptography
    (I) The mathematical science that deals with transforming data to render its meaning unintelligible (i.e., to hide its semantic content), prevent its undetected alteration, or prevent its unauthorized use. If the transformation is reversible, cryptography also deals with restoring encrypted data to intelligible form. (See: cryptology, steganography.)

    (O) "The discipline which embodies principles, means, and methods for the transformation of data in order to hide its information content, prevent its undetected modification and/or prevent its unauthorized use. . . . Cryptography determines the methods used in encipherment and decipherment." [I7498 Part 2]

  • cryptology
    (I) The science that includes both cryptography and cryptanalysis, and sometimes is said to include steganography.
  • cryptonet
    (I) A group of system entities that share a secret cryptographic key for a symmetric algorithm.
  • cryptoperiod
    (I) The time span during which a particular key is authorized to be used in a cryptographic system. (See: key management.)

    (C) A cryptoperiod is usually stated in terms of calendar or clock time, but sometimes is stated in terms of the maximum amount of data permitted to be processed by a cryptographic algorithm using the key. Specifying a cryptoperiod involves a tradeoff between the cost of rekeying and the risk of successful cryptanalysis.

    (C) Although we deprecate its prefix, this term is long- established in COMPUSEC usage. (See: crypto) In the context of certificates and public keys, "key lifetime" and "validity period" are often used instead.

  • cut-and-paste attack
    (I) An active attack on the data integrity of ciphertext, effected by replacing sections of ciphertext with other ciphertext, such that the result appears to decrypt correctly but actually decrypts to plaintext that is forged to the satisfaction of the attacker.
  • cyclic redundancy check (CRC)
    (I) Sometimes called "cyclic redundancy code". A type of checksum algorithm that is not a cryptographic hash but is used to implement data integrity service where accidental changes to data are expected.
  • data
    (I) Information in a specific physical representation, usually a sequence of symbols that have meaning; especially a representation of information that can be processed or produced by a computer.
  • data compromise
    (I) A security incident in which information is exposed to potential unauthorized access, such that unauthorized disclosure, alteration, or use of the information may have occurred. (See: compromise.)
  • data confidentiality
    (I) "The property that information is not made available or disclosed to unauthorized individuals, entities, or processes [i.e., to any unauthorized system entity]." [I7498 Part 2]. (See: data confidentiality service.)

    (D) ISDs SHOULD NOT use this term as a synonym for "privacy", which is a different concept.

  • data confidentiality service
    (I) A security service that protects data against unauthorized disclosure. (See: data confidentiality.)

    (D) ISDs SHOULD NOT use this term as a synonym for "privacy", which is a different concept.

  • data encryption key (DEK)
    (I) A cryptographic key that is used to encipher application data. (See: key-encrypting key.)
  • data integrity
    (I) The property that data has not been changed, destroyed, or lost in an unauthorized or accidental manner. (See: data integrity service.)

    (O) "The property that information has not been modified or destroyed in an unauthorized manner." [I7498 Part 2]

    (C) Deals with constancy of and confidence in data values, not with the information that the values represent (see: correctness integrity) or the trustworthiness of the source of the values (see: source integrity).

  • data integrity service
    (I) A security service that protects against unauthorized changes to data, including both intentional change or destruction and accidental change or loss, by ensuring that changes to data are detectable. (See: data integrity.)

    (C) A data integrity service can only detect a change and report it to an appropriate system entity; changes cannot be prevented unless the system is perfect (error-free) and no malicious user has access. However, a system that offers data integrity service might also attempt to correct and recover from changes.

    (C) Relationship between data integrity service and authentication services: Although data integrity service is defined separately from data origin authentication service and peer entity authentication service, it is closely related to them. Authentication services depend, by definition, on companion data integrity services. Data origin authentication service provides verification that the identity of the original source of a received data unit is as claimed; there can be no such verification if the data unit has been altered. Peer entity authentication service provides verification that the identity of a peer entity in a current association is as claimed; there can be no such verification if the claimed identity has been altered.

  • data origin authentication
    (I) "The corroboration that the source of data received is as claimed." [I7498 Part 2] (See: authentication.)
  • data origin authentication service
    (I) A security service that verifies the identity of a system entity that is claimed to be the original source of received data. (See: authentication, authentication service.)

    (C) This service is provided to any system entity that receives or holds the data. Unlike peer entity authentication service, this service is independent of any association between the originator and the recipient, and the data in question may have originated at any time in the past.

    (C) A digital signature mechanism can be used to provide this service, because someone who does not know the private key cannot forge the correct signature. However, by using the signer's public key, anyone can verify the origin of correctly signed data.

    (C) This service is usually bundled with connectionless data integrity service. (See: (relationship between data integrity service and authentication services under) data integrity service.

  • data security
    (I) The protection of data from disclosure, alteration, destruction, or loss that either is accidental or is intentional but unauthorized.

    (C) Both data confidentiality service and data integrity service are needed to achieve data security.

  • datagram
    (I) "A self-contained, independent entity of data carrying sufficient information to be routed from the source to the destination." [R1983]
  • decode
    (I) Convert encoded data back to its original form of representation. (See: decrypt.)

    (D) ISDs SHOULD NOT use this term as a synonym for "decrypt", because that would mix concepts in a potentially misleading way.

  • decrypt
    (I) Cryptographically restore ciphertext to the plaintext form it had before encryption.
  • dedicated security mode
    (I) A mode of operation of an information system, wherein all users have the clearance or authorization, and the need-to-know, for all data handled by the system. In this mode, the system may handle either a single classification level or category of information or a range of levels and categories. [DOD2]

    (C) This mode is defined formally in U.S. Department of Defense policy regarding system accreditation, but the term is also used outside the Defense Department and outside the Government.

  • default account
    (I) A system login account (usually accessed with a user name and password) that has been predefined in a manufactured system to permit initial access when the system is first put into service.

    (C) Sometimes, the default user name and password are the same in each copy of the system. In any case, when the system is put into service, the default password should immediately be changed or the default account should be disabled.

  • delta CRL
    (I) A partial CRL that only contains entries for X.509 certificates that have been revoked since the issuance of a prior, base CRL. This method can be used to partition CRLs that become too large and unwieldy.
  • denial of service
    (I) The prevention of authorized access to a system resource or the delaying of system operations and functions. (See: availability, critical (resource of a system), flooding.)
  • dictionary attack
    (I) An attack that uses a brute-force technique of successively trying all the words in some large, exhaustive list.

    (C) For example, an attack on an authentication service by trying all possible passwords; or an attack on encryption by encrypting some known plaintext phrase with all possible keys so that the key for any given encrypted message containing that phrase may be obtained by lookup.

  • digital certificate
    (I) A certificate document in the form of a digital data object (a data object used by a computer) to which is appended a computed digital signature value that depends on the data object. (See: attribute certificate, capability, public-key certificate.)

    (D) ISDs SHOULD NOT use this term to refer to a signed CRL or CKL. Although the recommended definition can be interpreted to include those items, the security community does not use the term with those meanings.

  • digital document
    (I) An electronic data object that represents information originally written in a non-electronic, non-magnetic medium (usually ink on paper) or is an analogue of a document of that type.
  • digital envelope
    (I) A digital envelope for a recipient is a combination of (a) encrypted content data (of any kind) and (b) the content encryption key in an encrypted form that has been prepared for the use of the recipient.

    (C) In ISDs, this term should be defined at the point of first use because, although the term is defined in PKCS #7 and used in S/MIME, it is not yet widely established.

    (C) Digital enveloping is not simply a synonym for implementing data confidentiality with encryption; digital enveloping is a hybrid encryption scheme to "seal" a message or other data, by encrypting the data and sending both it and a protected form of the key to the intended recipient, so that no one other than the intended recipient can "open" the message. In PCKS #7, it means first encrypting the data using a symmetric encryption algorithm and a secret key, and then encrypting the secret key using an asymmetric encryption algorithm and the public key of the intended recipient. In S/MIME, additional methods are defined for conveying the content encryption key.

  • digital notary
    (I) Analogous to a notary public. Provides a trusted date-and-time stamp for a document, so that someone can later prove that the document existed at a point in time. May also verify the signature(s) on a signed document before applying the stamp. (See: notarization.)
  • digital signature
    (I) A value computed with a cryptographic algorithm and appended to a data object in such a way that any recipient of the data can use the signature to verify the data's origin and integrity. (See: data origin authentication service, data integrity service, digitized signature, electronic signature, signer.)

    (I) "Data appended to, or a cryptographic transformation of, a data unit that allows a recipient of the data unit to prove the source and integrity of the data unit and protect against forgery, e.g. by the recipient." [I7498 Part 2]

    (C) Typically, the data object is first input to a hash function, and then the hash result is cryptographically transformed using a private key of the signer. The final resulting value is called the digital signature of the data object. The signature value is a protected checksum, because the properties of a cryptographic hash ensure that if the data object is changed, the digital signature will no longer match it. The digital signature is unforgeable because one cannot be certain of correctly creating or changing the signature without knowing the private key of the supposed signer.

    (C) Some digital signature schemes use a asymmetric encryption algorithm (e.g., see: RSA) to transform the hash result. Thus, when Alice needs to sign a message to send to Bob, she can use her private key to encrypt the hash result. Bob receives both the message and the digital signature. Bob can use Alice's public key to decrypt the signature, and then compare the plaintext result to the hash result that he computes by hashing the message himself. If the values are equal, Bob accepts the message because he is certain that it is from Alice and has arrived unchanged. If the values are not equal, Bob rejects the message because either the message or the signature was altered in transit.

    (C) Other digital signature schemes (e.g., see: DSS) transform the hash result with an algorithm (e.g., see: DSA, El Gamal) that cannot be directly used to encrypt data. Such a scheme creates a signature value from the hash and provides a way to verify the signature value, but does not provide a way to recover the hash result from the signature value. In some countries, such a scheme may improve exportability and avoid other legal constraints on usage.

  • digital watermarking
    (I) Computing techniques for inseparably embedding unobtrusive marks or labels as bits in digital data--text, graphics, images, video, or audio--and for detecting or extracting the marks later.

    (C) The set of embedded bits (the digital watermark) is sometimes hidden, usually imperceptible, and always intended to be unobtrusive. Depending on the particular technique that is used, digital watermarking can assist in proving ownership, controlling duplication, tracing distribution, ensuring data integrity, and performing other functions to protect intellectual property rights. [ACM]

  • discretionary access control (DAC)
    (I) An access control service that enforces a security policy based on the identity of system entities and their authorizations to access system resources. (See: access control list, identity- based security policy, mandatory access control.)

    (C) This service is termed "discretionary" because an entity might have access rights that permit the entity, by its own volition, to enable another entity to access some resource.

    (O) "A means of restricting access to objects based on the identity of subjects and/or groups to which they belong. The controls are discretionary in the sense that a subject with a certain access permission is capable of passing that permission (perhaps indirectly) on to any other subject." [DOD1]

  • distinguished name (DN)
    (I) An identifier that uniquely represents an object in the X.500 Directory Information Tree (DIT) [X501]. (See: domain name.)

    (C) A DN is a set of attribute values that identify the path leading from the base of the DIT to the object that is named. An X.509 public-key certificate or CRL contains a DN that identifies its issuer, and an X.509 attribute certificate contains a DN or other form of name that identifies its subject.

  • distribution point
    (I) An X.500 Directory entry or other information source that is named in a v3 X.509 public-key certificate extension as a location from which to obtain a CRL that might list the certificate.

    (C) A v3 X.509 public-key certificate may have a "cRLDistributionPoints" extension that names places to get CRLs on which the certificate might be listed. A CRL obtained from a distribution point may (a) cover either all reasons for which a certificate might be revoked or only some of the reasons, (b) be issued by either the authority that signed the certificate or some other authority, and (c) contain revocation entries for only a subset of the full set of certificates issued by one CA or (c') contain revocation entries for multiple CAs.

  • domain
    (I) Security usage: An environment or context that is defined by a security policy, security model, or security architecture to include a set of system resources and the set of system entities that have the right to access the resources. (See: domain of interpretation, security perimeter.)

    (I) Internet usage: That part of the Internet domain name space tree [R1034] that is at or below the name the specifies the domain. A domain is a subdomain of another domain if it is contained within that domain. For example, D.C.B.A is a subdomain of C.B.A. (See: Domain Name System.)

    (O) MISSI usage: The domain of a MISSI CA is the set of MISSI users whose certificates are signed by the CA.

    (O) OSI usage: An administrative partition of a complex distributed OSI system.

  • domain name
    (I) The style of identifier--a sequence of case-insensitive ASCII labels separated by dots ("")--defined for subtrees in the Internet Domain Name System [R1034] and used in other Internet identifiers, such as host names (e.g., ""), mailbox names (e.g., ""), and URLs (e.g., ""). (See: distinguished name, domain.)

    (C) The domain name space of the DNS is a tree structure in which each node and leaf holds records describing a resource. Each node has a label. The domain name of a node is the list of labels on the path from the node to the root of the tree. The labels in a domain name are printed or read left to right, from the most specific (lowest, farthest from the root) to the least specific (highest, closest to the root). The root's label is the null string, so a complete domain name properly ends in a dot. The top- level domains, those immediately below the root, include COM, EDU, GOV, INT, MIL, NET, ORG, and two-letter country codes (such as US) from ISO-3166. [R1591] (See: country code.)

  • domain of interpretation (DOI)
    (I) IPsec usage: An ISAKMP/IKE DOI defines payload formats, exchange types, and conventions for naming security-relevant information such as security policies or cryptographic algorithms and modes.

    (C) For example, see [R2407]. The DOI concept is based on work by the TSIG's CIPSO Working Group.

  • dominate
    (I) Security level A is said to "dominate" security level B if the hierarchical classification level of A is greater (higher) than or equal to that of B and the nonhierarchical categories of A include all of those of B.
  • dongle
    (I) A portable, physical, electronic device that is required to be attached to a computer to enable a particular software program to run. (See: token.)

    (C) A dongle is essentially a physical key used for copy protection of software, because the program will not run unless the matching dongle is attached. When the software runs, it periodically queries the dongle and quits if the dongle does not reply with the proper authentication information. Dongles were originally constructed as an EPROM (erasable programmable read- only memory) to be connected to a serial input-output port of a personal computer.

  • downgrade
    (I) Reduce the classification level of information in an authorized manner.
  • dual control
    (I) A procedure that uses two or more entities (usually persons) operating in concert to protect a system resource, such that no single entity acting alone can access that resource. (See: no-lone zone, separation of duties, split knowledge.)
  • EMV
    (I) An abbreviation of "Europay, MasterCard, Visa". Refers to a specification for smart cards that are used as payment cards, and for related terminals and applications. [EMV1, EMV2, EMV3]
  • Encapsulating Security Payload (ESP)
    (I) An Internet IPsec protocol [R2406] designed to provide a mix of security services--especially data confidentiality service--in the Internet Protocol. (See: Authentication Header.)

    (C) ESP may be used alone, or in combination with the IPsec AH protocol, or in a nested fashion with tunneling. Security services can be provided between a pair of communicating hosts, between a pair of communicating security gateways, or between a host and a gateway. The ESP header is encapsulated by the IP header, and the ESP header encapsulates either the upper layer protocol header (transport mode) or an IP header (tunnel mode). ESP can provide data confidentiality service, data origin authentication service, connectionless data integrity service, an anti-replay service, and limited traffic flow confidentiality. The set of services depends on the placement of the implementation and on options selected when the security association is established.

  • Extensible Authentication Protocol
    (I) A framework that supports multiple, optional authentication mechanisms for PPP, including cleartext passwords, challenge- response, and arbitrary dialog sequences. [R2284]

    (C) This protocol is intended for use primarily by a host or router that connects to a PPP network server via switched circuits or dial-up lines.

  • File Transfer Protocol (FTP)
    (I) A TCP-based, application-layer, Internet Standard protocol [R0959] for moving data files from one computer to another.
  • GRIP
    (I) A contraction of "Guidelines and Recommendations for Security Incident Processing", the name of the IETF working group that seeks to facilitate consistent handling of security incidents in the Internet community. (See: security incident.)

    (C) Guidelines to be produced by the WG will address technology vendors, network service providers, and response teams in their roles assisting organizations in resolving security incidents. These relationships are functional and can exist within and across organizational boundaries.

  • GULS
    (I) Generic Upper Layer Security service element (ISO 11586), a five-part standard for the exchange of security information and security-transformation functions that protect confidentiality and integrity of application data.
  • Generic Security Service Application Program Interface (GSS-API)
    (I) An Internet Standard protocol [R2078] that specifies calling conventions by which an application (typically another communication protocol) can obtain authentication, integrity, and confidentiality security services independently of the underlying security mechanisms and technologies, thus allowing the application source code to be ported to different environments.

    (C) "A GSS-API caller accepts tokens provided to it by its local GSS-API implementation and transfers the tokens to a peer on a remote system; that peer passes the received tokens to its local GSS-API implementation for processing. The security services available through GSS-API in this fashion are implementable (and have been implemented) over a range of underlying mechanisms based on [symmetric] and [asymmetric cryptography]." [R2078]

  • gateway
    (I) A relay mechanism that attaches to two (or more) computer networks that have similar functions but dissimilar implementations and that enables host computers on one network to communicate with hosts on the other; an intermediate system that is the interface between two computer networks. (See: bridge, firewall, guard, internetwork, proxy server, router, and subnetwork.)

    (C) In theory, gateways are conceivable at any OSI layer. In practice, they operate at OSI layer 3 (see: bridge, router) or layer 7 (see: proxy server). When the two networks differ in the protocol by which they offer service to hosts, the gateway may translate one protocol into another or otherwise facilitate interoperation of hosts (see: Internet Protocol).

  • guard
    (I) A gateway that is interposed between two networks (or computers, or other information systems) operating at different security levels (one level is usually higher than the other) and is trusted to mediate all information transfers between the two levels, either to ensure that no sensitive information from the first (higher) level is disclosed to the second (lower) level, or to protect the integrity of data on the first (higher) level. (See: firewall.)
  • HMAC
    (I) A keyed hash [R2104] that can be based on any iterated cryptographic hash (e.g., MD5 or SHA-1), so that the cryptographic strength of HMAC depends on the properties of the selected cryptographic hash. (See: [R2202, R2403, R2404].)

    (C) Assume that H is a generic cryptographic hash in which a function is iterated on data blocks of length B bytes. L is the length of the of hash result of H. K is a secret key of length L <= K <= B. The values IPAD and OPAD are fixed strings used as inner and outer padding and defined as follows: IPAD = the byte 0x36 repeated B times, OPAD = the byte 0x5C repeated B times. HMAC is computed by H(K XOR OPAD, H(K XOR IPAD, inputdata)).

    (C) The goals of HMAC are as follows: - To use available cryptographic hash functions without modification, particularly functions that perform well in software and for which software is freely and widely available. - To preserve the original performance of the selected hash without significant degradation. - To use and handle keys in a simple way. - To have a well-understood cryptographic analysis of the strength of the mechanism based on reasonable assumptions about the underlying hash function. - To enable easy replacement of the hash function in case a faster or stronger hash is found or required.

  • Hypertext Markup Language (HTML)
    (I) A platform-independent system of syntax and semantics for adding characters to data files (particularly text files) to represent the data's structure and to point to related data, thus creating hypertext for use in the World Wide Web and other applications. [R1866]
  • Hypertext Transfer Protocol (HTTP)
    (I) A TCP-based, application-layer, client-server, Internet protocol [R2616] used to carry data requests and responses in the World Wide Web. (See: hypertext.)
  • hacker
    (I) Someone with a strong interest in computers, who enjoys learning about them and experimenting with them. (See: cracker.)

    (C) The recommended definition is the original meaning of the term (circa 1960), which then had a neutral or positive connotation of "someone who figures things out and makes something cool happen". Today, the term is frequently misused, especially by journalists, to have the pejorative meaning of cracker.

  • handle
    (I) (1.) Verb: Perform processing operations on data, such as receive and transmit, collect and disseminate, create and delete, store and retrieve, read and write, and compare. (2.) Noun: An on- line pseudonym, particularly one used by a cracker; derived from citizens band radio culture.
  • hardware
    (I) The material physical components of a computer system. (See: firmware, software.)
  • hash function
    (I) An algorithm that computes a value based on a data object (such as a message or file; usually variable-length; possibly very large), thereby mapping the data object to a smaller data object (the "hash result") which is usually a fixed-size value. (See: checksum, keyed hash.)

    (O) "A (mathematical) function which maps values from a large (possibly very large) domain into a smaller range. A 'good' hash function is such that the results of applying the function to a (large) set of values in the domain will be evenly distributed (and apparently at random) over the range." [X509]

    (C) The kind of hash function needed for security applications is called a "cryptographic hash function", an algorithm for which it is computationally infeasible (because no attack is significantly more efficient than brute force) to find either (a) a data object that maps to a pre-specified hash result (the "one-way" property) or (b) two data objects that map to the same hash result (the "collision-free" property). (See: MD2, MD4, MD5, SHA-1.)

    (C) A cryptographic hash is "good" in the sense stated in the "O" definition for hash function. Any change to an input data object will, with high probability, result in a different hash result, so that the result of a cryptographic hash makes a good checksum for a data object.

  • hash result
    (I) The output of a hash function. (See: hash code, hash value.)

    (O) "The output produced by a hash function upon processing a message" (where "message" is broadly defined as "a digital representation of data"). [ABA] (The recommended definition is compatible with this ABA definition, but we avoid the unusual definition of "message".)

  • hierarchical PKI
    (I) A PKI architecture based on a certification hierarchy. (See: mesh PKI, trust-file PKI.)
  • hierarchy management
    (I) The process of generating configuration data and issuing public-key certificates to build and operate a certification hierarchy.
  • hijack attack
    (I) A form of active wiretapping in which the attacker seizes control of a previously established communication association. (See: man-in-the-middle attack, pagejacking, piggyback attack.)
  • honey pot
    (I) A system (e.g., a web server) or a system resource (e.g., a file on a server), that is designed to be attractive to potential crackers and intruders, like honey is attractive to bears. (See: entrapment.)

    (D) It is likely that other cultures have different metaphors for this concept. To ensure international understanding, ISDs should not use this term unless they also provide an explanation like this one. (See: (usage note under) Green Book.)

  • host
    (I) General computer network usage: A computer that is attached to a communication subnetwork or internetwork and can use services provided by the network to exchange data with other attached systems. (See: end system.)

    (I) Specific Internet Protocol Suite usage: A networked computer that does not forward Internet Protocol packets that are not addressed to the computer itself. (See: router.)

    (C) Derivation: As viewed by its users, a host "entertains" guests, providing application layer services or access to other computers attached to the network. However, even though some traditional peripheral service devices, such as printers, can now be independently connected to networks, they are not usually called hosts.

  • https
    (I) When used in the first part of a URL (the part that precedes the colon and specifies an access scheme or protocol), this term specifies the use of HTTP enhanced by a security mechanism, which is usually SSL. (See: S-HTTP.)
  • hybrid encryption
    (I) An application of cryptography that combines two or more encryption algorithms, particularly a combination of symmetric and asymmetric encryption. (E.g., see: digital envelope.)

    (C) Asymmetric algorithms require more computation than equivalently strong symmetric ones. Thus, asymmetric encryption is not normally used for data confidentiality except in distributing symmetric keys in applications where the key data is usually short (in terms of bits) compared to the data it protects. (E.g., see: MSP, PEM, PGP.)

  • hyperlink
    (I) In hypertext or hypermedia, an information object (such as a word, a phrase, or an image; usually highlighted by color or underscoring) that points (indicates how to connect) to related information that is located elsewhere and can be retrieved by activating the link (e.g., by selecting the object with a mouse pointer and then clicking).
  • hypermedia
    (I) A generalization of hypertext; any media that contain hyperlinks that point to material in the same or another data object.
  • hypertext
    (I) A computer document, or part of a document, that contains hyperlinks to other documents; i.e., text that contains active pointers to other text. Usually written in Hypertext Markup Language and accessed using a web browser. (See: hypermedia.)
  • ICMP flood
    (I) A denial of service attack that sends a host more ICMP echo request ("ping") packets than the protocol implementation can handle. (See: flooding, smurf.)
    (I) A IMAP4 "command" (better described as a transaction type, or a protocol-within-a-protocol) by which an IMAP4 client optionally proposes a mechanism to an IMAP4 server to authenticate the client to the server and provide other security services. (See: POP3.)

    (C) If the server accepts the proposal, the command is followed by performing a challenge-response authentication protocol and, optionally, negotiating a protection mechanism for subsequent POP3 interactions. The security mechanisms that are used by IMAP4 AUTHENTICATE--including Kerberos, GSSAPI, and S/Key--are described in [R1731].

    (I) Abbreviation for "information security", referring to security measures that implement and assure security services in computer systems (i.e., COMPUSEC) and communication systems (i.e., COMSEC).
  • IP address
    (I) A computer's internetwork address that is assigned for use by the Internet Protocol and other protocols.

    (C) An IP version 4 [R0791] address is written as a series of four 8-bit numbers separated by periods. For example, the address of the host named "" is

    (C) An IP version 6 [R2373] address is written as x:x:x:x:x:x:x:x, where each "x" is the hexadecimal value of one of the eight 16-bit parts of the address. For example, 1080:0:0:0:8:800:200C:417A and FEDC:BA98:7654:3210:FEDC:BA98:7654:3210.

  • IPsec Key Exchange (IKE)
    (I) An Internet, IPsec, key-establishment protocol [R2409] (partly based on OAKLEY) that is intended for putting in place authenticated keying material for use with ISAKMP and for other security associations, such as in AH and ESP.
  • ISO
    (I) International Organization for Standardization, a voluntary, non-treaty, non-government organization, established in 1947, with voting members that are designated standards bodies of participating nations and non-voting observer organizations. (See: ANSI, ITU-T.)

    (C) Legally, ISO is a Swiss, non-profit, private organization. ISO and the IEC (the International Electrotechnical Commission) form the specialized system for worldwide standardization. National bodies that are members of ISO or IEC participate in developing international standards through ISO and IEC technical committees that deal with particular fields of activity. Other international governmental and non-governmental organizations, in liaison with ISO and IEC, also take part. (ANSI is the U.S. voting member of ISO. ISO is a class D member of ITU-T.)

    (C) The ISO standards development process has four levels of increasing maturity: Working Draft (WD), Committee Draft (CD), Draft International Standard (DIS), and International Standard (IS). (See: (standards track levels under) Internet Standard.) In information technology, ISO and IEC have a joint technical committee, ISO/IEC JTC 1. DISs adopted by JTC 1 are circulated to national bodies for voting, and publication as an IS requires approval by at least 75% of the national bodies casting a vote.

  • Identification Protocol
    (I) An client-server Internet protocol [R1413] for learning the identity of a user of a particular TCP connection.

    (C) Given a TCP port number pair, the server returns a character string that identifies the owner of that connection on the server's system. The protocol is not intended for authorization or access control. At best, it provides additional auditing information with respect to TCP.

  • Internet Architecture Board (IAB)
    (I) A technical advisory group of the ISOC, chartered by the ISOC Trustees to provide oversight of Internet architecture and protocols and, in the context of Internet Standards, a body to which decisions of the IESG may be appealed. Responsible for approving appointments to the IESG from among nominees submitted by the IETF nominating committee. [R2026]
  • Internet Assigned Numbers Authority (IANA)
    (I) From the early days of the Internet, the IANA was chartered by the ISOC and the U.S. Government's Federal Network Council to be the central coordination, allocation, and registration body for parameters for Internet protocols. Superseded by ICANN.
  • Internet Control Message Protocol (ICMP)
    (I) An Internet Standard protocol [R0792] that is used to report error conditions during IP datagram processing and to exchange other information concerning the state of the IP network.
  • Internet Corporation for Assigned Names and Numbers (ICANN)
    (I) The non-profit, private corporation that has assumed responsibility for the IP address space allocation, protocol parameter assignment, domain name system management, and root server system management functions formerly performed under U.S. Government contract by IANA and other entities.

    (C) The Internet Protocol Suite, as defined by the IETF and the IESG, contains numerous parameters, such as internet addresses, domain names, autonomous system numbers, protocol numbers, port numbers, management information base object identifiers, including private enterprise numbers, and many others. The Internet community requires that the values used in these parameter fields be assigned uniquely. ICANN makes those assignments as requested and maintains a registry of the current values.

    (C) ICANN was formed in October 1998, by a coalition of the Internet's business, technical, and academic communities. The U.S. Government designated ICANN to serve as the global consensus entity with responsibility for coordinating four key functions for the Internet: the allocation of IP address space, the assignment of protocol parameters, the management of the DNS, and the management of the DNS root server system.

  • Internet Draft
    (I) A working document of the IETF, its areas, and its working groups. (Other groups may also distribute working documents as Internet Drafts.) An Internet Draft is not an archival document like an RFC is. Instead, an Internet Draft is a preliminary or working document that is valid for a maximum of six months and may be updated, replaced, or made obsolete by other documents at any time. It is inappropriate to use an Internet Draft as reference material or to cite it other than as "work in progress."
  • Internet Engineering Steering Group (IESG)
    (I) The part of the ISOC responsible for technical management of IETF activities and administration of the Internet Standards Process according to procedures approved by the ISOC Trustees. Directly responsible for actions along the "standards track", including final approval of specifications as Internet Standards. Composed of IETF Area Directors and the IETF chairperson, who also chairs the IESG. [R2026]
  • Internet Engineering Task Force (IETF)
    (I) A self-organized group of people who make contributions to the development of Internet technology. The principal body engaged in developing Internet Standards, although not itself a part of the ISOC. Composed of Working Groups, which are arranged into Areas (such as the Security Area), each coordinated by one or more Area Directors. Nominations to the IAB and the IESG are made by a committee selected at random from regular IETF meeting attendees who have volunteered. [R2026, R2323]
  • Internet Message Access Protocol, version 4 (IMAP4)
    (I) An Internet protocol [R2060] by which a client workstation can dynamically access a mailbox on a server host to manipulate and retrieve mail messages that the server has received and is holding for the client. (See: POP3.)

    (C) IMAP4 has mechanisms for optionally authenticating a client to a server and providing other security services. (See: IMAP4 AUTHENTICATE.)

  • Internet Policy Registration Authority (IPRA)
    (I) An X.509-compliant CA that is the top CA of the Internet certification hierarchy operated under the auspices of the ISOC [R1422]. (See: (PEM usage under) certification hierarchy.)
  • Internet Protocol (IP)
    (I) A Internet Standard protocol (version 4 [R0791] and version 6 [R2460]) that moves datagrams (discrete sets of bits) from one computer to another across an internetwork but does not provide reliable delivery, flow control, sequencing, or other end-to-end services that TCP provides. (See: IP address, TCP/IP.)

    (C) In the OSIRM, IP would be located at the top of layer 3.

  • Internet Protocol security (IPsec)
    (I) (1.) The name of the IETF working group that is specifying a security architecture [R2401] and protocols to provide security services for Internet Protocol traffic. (2.) A collective name for that architecture and set of protocols. (Implementation of IPsec protocols is optional for IP version 4, but mandatory for IP version 6.) (See: Internet Protocol Security Option.)

    (C) Note that the letters "sec" are lower-case.

    (C) The IPsec architecture specifies (a) security protocols (AH and ESP), (b) security associations (what they are, how they work, how they are managed, and associated processing), (c) key management (IKE), and (d) algorithms for authentication and encryption. The set of security services include access control service, connectionless data integrity service, data origin authentication service, protection against replays (detection of the arrival of duplicate datagrams, within a constrained window), data confidentiality service, and limited traffic flow confidentiality.

  • Internet Security Association and Key Management Protocol (ISAKMP)
    (I) An Internet IPsec protocol [R2408] to negotiate, establish, modify, and delete security associations, and to exchange key generation and authentication data, independent of the details of any specific key generation technique, key establishment protocol, encryption algorithm, or authentication mechanism.

    (C) ISAKMP supports negotiation of security associations for protocols at all TCP/IP layers. By centralizing management of security associations, ISAKMP reduces duplicated functionality within each protocol. ISAKMP can also reduce connection setup time, by negotiating a whole stack of services at once. Strong authentication is required on ISAKMP exchanges, and a digital signature algorithm based on asymmetric cryptography is used within ISAKMP's authentication component.

  • Internet Society (ISOC)
    (I) A professional society concerned with Internet development (including technical Internet Standards); with how the Internet is and can be used; and with social, political, and technical issues that result. The ISOC Board of Trustees approves appointments to the IAB from among nominees submitted by the IETF nominating committee. [R2026]
  • Internet Standard
    (I) A specification, approved by the IESG and published as an RFC, that is stable and well-understood, is technically competent, has multiple, independent, and interoperable implementations with substantial operational experience, enjoys significant public support, and is recognizably useful in some or all parts of the Internet. [R2026] (See: RFC.)

    (C) The Internet Standards Process is an activity of the ISOC and is organized and managed by the IAB and the IESG. The process is concerned with all protocols, procedures, and conventions used in or by the Internet, whether or not they are part of the Internet Protocol Suite. The "Internet Standards Track" has three levels of increasing maturity: Proposed Standard, Draft Standard, and Standard. (See: (standards levels under) ISO.)

  • key agreement (algorithm or protocol)
    (I) A key establishment method (especially one involving asymmetric cryptography) by which two or more entities, without prior arrangement except a public exchange of data (such as public keys), each computes the same key value. I.e., each can independently generate the same key value, but that key cannot be computed by other entities. (See: Diffie-Hellman, key establishment, Key Exchange Algorithm, key transport.)

    (O) "A method for negotiating a key value on line without transferring the key, even in an encrypted form, e.g., the Diffie- Hellman technique." [X509]

    (O) "The procedure whereby two different parties generate shared symmetric keys such that any of the shared symmetric keys is a function of the information contributed by all legitimate participants, so that no party [alone] can predetermine the value of the key." [A9042]

    (C) For example, a message originator and the intended recipient can each use their own private key and the other's public key with the Diffie-Hellman algorithm to first compute a shared secret value and, from that value, derive a session key to encrypt the message.

  • key center
    (I) A centralized key distribution process (used in symmetric cryptography), usually a separate computer system, that uses key- encrypting keys (master keys) to encrypt and distribute session keys needed in a community of users.

    (C) An ANSI standard [A9017] defines two types of key center: key distribution center and key translation center.

  • key distribution
    (I) A process that delivers a cryptographic key from the location where it is generated to the locations where it is used in a cryptographic algorithm. (See: key management.)
  • key distribution center (KDC)
    (I) A type of key center (used in symmetric cryptography) that implements a key distribution protocol to provide keys (usually, session keys) to two (or more) entities that wish to communicate securely. (See: key translation center.)

    (C) A KDC distributes keys to Alice and Bob, who (a) wish to communicate with each other but do not currently share keys, (b) each share a KEK with the KDC, and (c) may not be able to generate or acquire keys by themselves. Alice requests the keys from the KDC. The KDC generates or acquires the keys and makes two identical sets. The KDC encrypts one set in the KEK it shares with Alice, and sends that encrypted set to Alice. The KDC encrypts the second set in the KEK it shares with Bob, and either sends that encrypted set to Alice for her to forward to Bob, or sends it directly to Bob (although the latter option is not supported in the ANSI standard [A9017]).

  • key establishment (algorithm or protocol)
    (I) A process that combines the key generation and key distribution steps needed to set up or install a secure communication association. (See: key agreement, key transport.)

    (O) "The procedure to share a symmetric key among different parties by either key agreement or key transport." [A9042]

    (C) Key establishment involves either key agreement or key transport: - Key transport: One entity generates a secret key and securely sends it to the other entity. (Or each entity generates a secret value and securely sends it to the other entity, where the two values are combined to form a secret key.) - Key agreement: No secret is sent from one entity to another. Instead, both entities, without prior arrangement except a public exchange of data, compute the same secret value. I.e., each can independently generate the same value, but that value cannot be computed by other entities.

  • key generation
    (I) A process that creates the sequence of symbols that comprise a cryptographic key. (See: key management.)
  • key length
    (I) The number of symbols (usually bits) needed to be able to represent any of the possible values of a cryptographic key. (See: key space.)
  • key management
    (I) The process of handling and controlling cryptographic keys and related material (such as initialization values) during their life cycle in a cryptographic system, including ordering, generating, distributing, storing, loading, escrowing, archiving, auditing, and destroying the material. (See: key distribution, key escrow, keying material, public-key infrastructure.)

    (O) "The generation, storage, distribution, deletion, archiving and application of keys in accordance with a security policy." [I7498 Part 2]

    (O) "The activities involving the handling of cryptographic keys and other related security parameters (e.g., IVs, counters) during the entire life cycle of the keys, including their generation, storage, distribution, entry and use, deletion or destruction, and archiving." [FP140]

  • key pair
    (I) A set of mathematically related keys--a public key and a private key--that are used for asymmetric cryptography and are generated in a way that makes it computationally infeasible to derive the private key from knowledge of the public key (e.g., see: Diffie-Hellman, Rivest-Shamir-Adleman).

    (C) A key pair's owner discloses the public key to other system entities so they can use the key to encrypt data, verify a digital signature, compute a protected checksum, or generate a key in a key agreement algorithm. The matching private key is kept secret by the owner, who uses it to decrypt data, generate a digital signature, verify a protected checksum, or generate a key in a key agreement algorithm.

  • key space
    (I) The range of possible values of a cryptographic key; or the number of distinct transformations supported by a particular cryptographic algorithm. (See: key length.)
  • key translation center
    (I) A type of key center (used in a symmetric cryptography) that implements a key distribution protocol to convey keys between two (or more) parties who wish to communicate securely. (See: key distribution center.)

    (C) A key translation center translates keys for future communication between Bob and Alice, who (a) wish to communicate with each other but do not currently share keys, (b) each share a KEK with the center, and (c) have the ability to generate or acquire keys by themselves. Alice generates or acquires a set of keys for communication with Bob. Alice encrypts the set in the KEK she shares with the center and sends the encrypted set to the center. The center decrypts the set, reencrypts the set in the KEK it shares with Bob, and either sends that encrypted set to Alice for her to forward to Bob, or sends it directly to Bob (although direct distribution is not supported in the ANSI standard [A9017]).

  • key transport (algorithm or protocol)
    (I) A key establishment method by which a secret key is generated by one entity in a communication association and securely sent to another entity in the association. (See: key agreement.)

    (O) "The procedure to send a symmetric key from one party to other parties. As a result, all legitimate participants share a common symmetric key in such a way that the symmetric key is determined entirely by one party." [A9042]

    (C) For example, a message originator can generate a random session key and then use the Rivest-Shamir-Adleman algorithm to encrypt that key with the public key of the intended recipient.

  • key update
    (I) Derive a new key from an existing key. (See: certificate rekey.)
  • key-encrypting key (KEK)
    (I) A cryptographic key that is used to encrypt other keys, either DEKs or other KEKs, but usually is not used to encrypt application data.
  • keyed hash
    (I) A cryptographic hash (e.g., [R1828]) in which the mapping to a hash result is varied by a second input parameter that is a cryptographic key. (See: checksum.)

    (C) If the input data object is changed, a new hash result cannot be correctly computed without knowledge of the secret key. Thus, the secret key protects the hash result so it can be used as a checksum even when there is a threat of an active attack on the data. There are least two forms of keyed hash: - A function based on a keyed encryption algorithm. (E.g., see: Data Authentication Code.) - A function based on a keyless hash that is enhanced by combining (e.g., by concatenating) the input data object parameter with a key parameter before mapping to the hash result. (E.g., see: HMAC.)

  • keying material
    (I) Data (such as keys, key pairs, and initialization values) needed to establish and maintain a cryptographic security association.
  • known-plaintext attack
    (I) A cryptanalysis technique in which the analyst tries to determine the key from knowledge of some plaintext-ciphertext pairs (although the analyst may also have other clues, such as the knowing the cryptographic algorithm).
  • lattice model
    (I) A security model for flow control in a system, based on the lattice that is formed by the finite security levels in a system and their partial ordering. [Denn] (See: flow control, security level, security model.)

    (C) The model describes the semantic structure formed by a finite set of security levels, such as those used in military organizations.

    (C) A lattice is a finite set together with a partial ordering on its elements such that for every pair of elements there is a least upper bound and a greatest lower bound. For example, a lattice is formed by a finite set S of security levels -- i.e., a set S of all ordered pairs (x, c), where x is one of a finite set X of hierarchically ordered classification levels (X1, ..., Xm), and c is a (possibly empty) subset of a finite set C of non-hierarchical categories (C1, ..., Cn) -- together with the "dominate" relation. (See: dominate.)

  • least privilege
    (I) The principle that a security architecture should be designed so that each system entity is granted the minimum system resources and authorizations that the entity needs to do its work. (See: economy of mechanism.)

    (C) This principle tends to limit damage that can be caused by an accident, error, or unauthorized act.

  • link
    (I) World Wide Web usage: See: hyperlink.

    (I) Subnetwork usage: A point-to-point communication channel connecting two subnetwork relays (especially one between two packet switches) that is implemented at OSI layer 2. (See: link encryption.)

    (C) The relay computers assume that links are logically passive. If a computer at one end of a link sends a sequence of bits, the sequence simply arrives at the other end after a finite time, although some bits may have been changed either accidentally (errors) or by active wiretapping.

  • link encryption
    (I) Stepwise protection of data that flows between two points in a network, provided by encrypting data separately on each network link, i.e., by encrypting data when it leaves a host or subnetwork relay and decrypting when it arrives at the next host or relay. Each link may use a different key or even a different algorithm. [R1455] (See: end-to-end encryption.)
  • logic bomb
    (I) Malicious logic that activates when specified conditions are met. Usually intended to cause denial of service or otherwise damage system resources. (See: Trojan horse, virus, worm.)
  • login
    (I) The act of a system entity gaining access to a session in which the entity can use system resources; usually accomplished by providing a user name and password to an access control system that authenticates the user.

    (C) Derives from "log" file", a security audit trail that records security events, such as the beginning of sessions, and who initiates them.

  • MIME Object Security Services (MOSS)
    (I) An Internet protocol [R1848] that applies end-to-end encryption and digital signature to MIME message content, using symmetric cryptography for encryption and asymmetric cryptography for key distribution and signature. MOSS is based on features and specifications of PEM. (See: S/MIME.)
  • Message Handling Systems
    (I) A ITU-T/ISO system concept, which encompasses the notion of electronic mail but defines more comprehensive OSI systems and services that enable users to exchange messages on a store-and- forward basis. (The ISO equivalent is "Message Oriented Text Interchange System".) (See: X.400.)
  • Morris Worm
    (I) A worm program written by Robert T. Morris, Jr. that flooded the ARPANET in November, 1988, causing problems for thousands of hosts. (See: worm.)
  • Multipurpose Internet Mail Extensions (MIME)
    (I) An Internet protocol [R2045] that enhances the basic format of Internet electronic mail messages [R0822] to be able to use character sets other than US-ASCII for textual headers and text content, and to carry non-textual and multi-part content. (See: S/MIME.)
  • malicious logic
    (I) Hardware, software, or firmware that is intentionally included or inserted in a system for a harmful purpose. (See: logic bomb, Trojan horse, virus, worm.)
  • malware
    (I) A contraction of "malicious software". (See: malicious logic.)

    (D) ISDs SHOULD NOT use this term because it is not listed in most dictionaries and could confuse international readers.

  • man-in-the-middle
    (I) A form of active wiretapping attack in which the attacker intercepts and selectively modifies communicated data in order to masquerade as one or more of the entities involved in a communication association. (See: hijack attack, piggyback attack.)

    (C) For example, suppose Alice and Bob try to establish a session key by using the Diffie-Hellman algorithm without data origin authentication service. A "man in the middle" could (a) block direct communication between Alice and Bob and then (b) masquerade as Alice sending data to Bob, (c) masquerade as Bob sending data to Alice, (d) establish separate session keys with each of them, and (e) function as a clandestine proxy server between them in order to capture or modify sensitive information that Alice and Bob think they are sending only to each other.

  • mandatory access control (MAC)
    (I) An access control service that enforces a security policy based on comparing (a) security labels (which indicate how sensitive or critical system resources are) with (b) security clearances (which indicate system entities are eligible to access certain resources). (See: discretionary access control, rule-based security policy.)

    (C) This kind of access control is called "mandatory" because an entity that has clearance to access a resource may not, just by its own volition, enable another entity to access that resource.

    (O) "A means of restricting access to objects based on the sensitivity (as represented by a label) of the information contained in the objects and the formal authorization (i.e., clearance) of subjects to access information of such sensitivity." [DOD1]

  • masquerade attack
    (I) A type of attack in which one system entity illegitimately poses as (assumes the identity of) another entity. (See: spoofing attack.)
  • mesh PKI
    (I) A non-hierarchical PKI architecture in which there are several trusted CAs rather than a single root. Each certificate user bases path validations on the public key of one of the trusted CAs, usually the one that issued that user's own public-key certificate. Rather than having superior-to-subordinate relationships between CAs, the relationships are peer-to-peer, and CAs issue cross-certificates to each other. (See: hierarchical PKI, trust-file PKI.)
  • mode of operation
    (I) Encryption usage: A technique for enhancing the effect of a cryptographic algorithm or adapting the algorithm for an application, such as applying a block cipher to a sequence of data blocks or a data stream. (See: electronic codebook, cipher block chaining, cipher feedback, output feedback.)

    (I) System operation usage: A type of security policy that states the range of classification levels of information that a system is permitted to handle and the range of clearances and authorizations of users who are permitted to access the system. (See: dedicated security mode, multilevel security mode, partitioned security mode, system high security mode.)

  • modulus
    (I) The defining constant in modular arithmetic, and usually a part of the public key in asymmetric cryptography that is based on modular arithmetic. (See: Diffie-Hellman, Rivest-Shamir-Adleman.)
  • multilevel secure (MLS)
    (I) A class of system that has system resources (particularly stored information) at more than one security level (i.e., has different types of sensitive resources) and that permits concurrent access by users who differ in security clearance and need-to-know, but is able to prevent each user from accessing resources for which the user lacks authorization.
  • multilevel security mode
    (I) A mode of operation of an information system, that allows two or more classification levels of information to be processed concurrently within the same system when not all users have a clearance or formal access authorization for all data handled by the system.

    (C) This mode is defined formally in U.S. Department of Defense policy regarding system accreditation [DOD2], but the term is also used outside the Defense Department and outside the Government.

  • mutual suspicion
    (I) The state that exists between two interacting system entities in which neither entity can trust the other to function correctly with regard to some security requirement.
  • NULL encryption algorithm
    (I) An algorithm [R2410] that does nothing to transform plaintext data; i.e., a no-op. It originated because of IPsec ESP, which always specifies the use of an encryption algorithm to provide confidentiality. The NULL encryption algorithm is a convenient way to represent the option of not applying encryption in ESP (or in any other context where this is needed).
    (I) A key establishment protocol (proposed for IPsec but superseded by IKE) based on the Diffie-Hellman algorithm and designed to be a compatible component of ISAKMP. [R2412]

    (C) OAKLEY establishes a shared key with an assigned identifier and associated authenticated identities for parties. I.e., OAKLEY provides authentication service to ensure the entities of each other's identity, even if the Diffie-Hellman exchange is threatened by active wiretapping. Also, provides public-key forward secrecy for the shared key and supports key updates, incorporation of keys distributed by out-of-band mechanisms, and user-defined abstract group structures for use with Diffie- Hellman.

  • On-line Certificate Status Protocol (OCSP)
    (I) An Internet protocol used by a client to obtain from a server the validity status and other information concerning a digital certificate.

    (C) In some applications, such as those involving high-value commercial transactions, it may be necessary to obtain certificate revocation status that is more timely than is possible with CRLs or to obtain other kinds of status information. OCSP may be used to determine the current revocation status of a digital certificate, in lieu of or as a supplement to checking against a periodic CRL. An OCSP client issues a status request to an OCSP server and suspends acceptance of the certificate in question until the server provides a response.

  • PKIX
    (I) (1.) A contraction of "Public-Key Infrastructure (X.509)", the name of the IETF working group that is specifying an architecture and set of protocols needed to support an X.509-based PKI for the Internet. (2.) A collective name for that architecture and set of protocols.

    (C) The goal of PKIX is to facilitate the use of X.509 public-key certificates in multiple Internet applications and to promote interoperability between different implementations that use those certificates. The resulting PKI is intended to provide a framework that supports a range of trust and hierarchy environments and a range of usage environments. PKIX specifies (a) profiles of the v3 X.509 public-key certificate standards and the v2 X.509 CRL standards for the Internet; (b) operational protocols used by relying parties to obtain information such as certificates or certificate status; (c) management protocols used by system entities to exchange information needed for proper management of the PKI; and (d) information about certificate policies and CPSs, covering the areas of PKI security not directly addressed in the rest of PKIX.

  • PKIX private extension
    (I) PKIX defines a private extension to identify an on-line verification service supporting the issuing CA.
    (I) A POP3 "command" (better described as a transaction type, or a protocol-within-a-protocol) by which a POP3 client optionally uses a keyed hash (based on MD5) to authenticate itself to a POP3 server and, depending on the server implementation, to protect against replay attacks. (See: CRAM, POP3 AUTH, IMAP4 AUTHENTICATE.)

    (C) The server includes a unique timestamp in its greeting to the client. The subsequent APOP command sent by the client to the server contains the client's name and the hash result of applying MD5 to a string formed from both the timestamp and a shared secret that is known only to the client and the server. APOP was designed to provide as an alternative to using POP3's USER and PASS (i.e., password) command pair, in which the client sends a cleartext password to the server.

    (I) A "command" [R1734] (better described as a transaction type, or a protocol-within-a-protocol) in POP3, by which a POP3 client optionally proposes a mechanism to a POP3 server to authenticate the client to the server and provide other security services. (See: POP3 APOP, IMAP4 AUTHENTICATE.)

    (C) If the server accepts the proposal, the command is followed by performing a challenge-response authentication protocol and, optionally, negotiating a protection mechanism for subsequent POP3 interactions. The security mechanisms used by POP3 AUTH are those used by IMAP4.

  • Password Authentication Protocol (PAP)
    (I) A simple authentication mechanism in PPP. In PAP, a user identifier and password are transmitted in cleartext. [R1334] (See: CHAP.)
  • Photuris
    (I) A UDP-based, key establishment protocol for session keys, designed for use with the IPsec protocols AH and ESP. Superseded by IKE.
  • Point-to-Point Protocol (PPP)
    (I) An Internet Standard protocol [R1661] for encapsulation and full-duplex transportation of network layer (mainly OSI layer 3) protocol data packets over a link between two peers, and for multiplexing different network layer protocols over the same link. Includes optional negotiation to select and use a peer entity authentication protocol to authenticate the peers to each other before they exchange network layer data. (See: CHAP, EAP, PAP.)
  • Point-to-Point Tunneling Protocol (PPTP)
    (I) An Internet client-server protocol (originally developed by Ascend and Microsoft) that enables a dial-up user to create a virtual extension of the dial-up link across a network by tunneling PPP over IP. (See: L2TP.)

    (C) PPP can encapsulate any Internet Protocol Suite network layer protocol (or OSI layer 3 protocol). Therefore, PPTP does not specify security services; it depends on protocols above and below it to provide any needed security. PPTP makes it possible to divorce the location of the initial dial-up server (i.e., the PPTP Access Concentrator, the client, which runs on a special-purpose host) from the location at which the dial-up protocol (PPP) connection is terminated and access to the network is provided (i.e., the PPTP Network Server, which runs on a general-purpose host).

  • Post Office Protocol, version 3 (POP3)
    (I) An Internet Standard protocol [R1939] by which a client workstation can dynamically access a mailbox on a server host to retrieve mail messages that the server has received and is holding for the client. (See: IMAP4.)

    (C) POP3 has mechanisms for optionally authenticating a client to a server and providing other security services. (See: POP3 APOP, POP3 AUTH.)

  • Privacy Enhanced Mail (PEM)
    (I) An Internet protocol to provide data confidentiality, data integrity, and data origin authentication for electronic mail. [R1421, R1422]. (See: MOSS, MSP, PGP, S/MIME.)

    (C) PEM encrypts messages with DES in CBC mode, provides key distribution of DES keys by encrypting them with RSA, and signs messages with RSA over either MD2 or MD5. To establish ownership of public keys, PEM uses a certification hierarchy, with X.509 public-key certificates and X.509 CRLs that are signed with RSA and MD2. (See: Pretty Good Privacy.)

    (C) PEM is designed to be compatible with a wide range of key management methods, but is limited to specifying security services only for text messages and, like MOSS, has not been widely implemented in the Internet.

  • Public-Key Cryptography Standards (PKCS)
    (I) A series of specifications published by RSA Laboratories for data structures and algorithm usage for basic applications of asymmetric cryptography. (See: PKCS #7, PKCS #10, PKCS #11.)

    (C) The PKCS were begun in 1991 in cooperation with industry and academia, originally including Apple, Digital, Lotus, Microsoft, Northern Telecom, Sun, and MIT. Today, the specifications are widely used, but they are not sanctioned by an official standards organization, such as ANSI, ITU-T, or IETF. RSA Laboratories retains sole decision-making authority over the PKCS.

  • pagejacking
    (I) A contraction of "Web page hijacking". A masquerade attack in which the attacker copies (steals) a home page or other material from the target server, rehosts the page on a server the attacker controls, and causes the rehosted page to be indexed by the major Web search services, thereby diverting browsers from the target server to the attacker's server.

    (D) ISDs SHOULD NOT use this term without including a definition, because the term is not listed in most dictionaries and could confuse international readers. (See: (usage note under) Green Book.)

  • password
    (I) A secret data value, usually a character string, that is used as authentication information. (See: challenge-response.)

    (C) A password is usually matched with a user identifier that is explicitly presented in the authentication process, but in some cases the identity may be implicit.

    (C) Using a password as authentication information assumes that the password is known only by the system entity whose identity is being authenticated. Therefore, in a network environment where wiretapping is possible, simple authentication that relies on transmission of static (i.e., repetitively used) passwords as cleartext is inadequate. (See: one-time password, strong authentication.)

  • password sniffing
    (I) Passive wiretapping, usually on a local area network, to gain knowledge of passwords. (See: (usage note under) sniffing.)
  • path discovery
    (I) For a digital certificate, the process of finding a set of public-key certificates that comprise a certification path from a trusted key to that specific certificate.
  • path validation
    (I) The process of validating (a) all of the digital certificates in a certification path and (b) the required relationships between those certificates, thus validating the contents of the last certificate on the path. (See: certificate validation.)
  • peer entity authentication
    (I) "The corroboration that a peer entity in an association is the one claimed." [I7498 Part 2] (See: authentication.)
  • peer entity authentication service
    (I) A security service that verifies an identity claimed by or for a system entity in an association. (See: authentication, authentication service.)

    (C) This service is used at the establishment of, or at times during, an association to confirm the identity of one entity to another, thus protecting against a masquerade by the first entity. However, unlike data origin authentication service, this service requires an association to exist between the two entities, and the corroboration provided by the service is valid only at the current time that the service is provided.

    (C) See: "relationship between data integrity service and authentication services" under data integrity service.

  • penetration
    (I) Successful, repeatable, unauthorized access to a protected system resource. (See: attack, violation.)
  • penetration test
    (I) A system test, often part of system certification, in which evaluators attempt to circumvent the security features of the system. [NCS04]

    (C) Penetration testing may be performed under various constraints and conditions. However, for a TCSEC evaluation, testers are assumed to have all system design and implementation documentation, including source code, manuals, and circuit diagrams, and to work under no greater constraints than those applied to ordinary users.

  • periods processing
    (I) A mode of system operation in which information of different sensitivities is processed at distinctly different times by the same system, with the system being properly purged or sanitized between periods. (See: color change.)
  • permission
    (I) A synonym for "authorization", but "authorization" is preferred in the PKI context. (See: privilege.)
  • personal identification number (PIN)
    (I) A character string used as a password to gain access to a system resource. (See: authentication information.)

    (C) Despite the words "identification" and "number", a PIN seldom serves as a user identifier, and a PIN's characters are not necessarily all numeric. A better name for this concept would have been "personal authentication system string (PASS)".

    (C) Retail banking applications commonly use 4-digit PINs. FORTEZZA PC card's use up to 12 characters for user or SSO PINs.

  • personnel security
    (I) Procedures to ensure that persons who access a system have proper clearance, authorization, and need-to-know as required by the system's security policy.
  • phreaking
    (I) A contraction of "telephone breaking". An attack on or penetration of a telephone system or, by extension, any other communication or information system. [Raym]

    (D) ISDs SHOULD NOT use this term because it is not listed in most dictionaries and could confuse international readers.

  • physical security
    (I) Tangible means of preventing unauthorized physical access to a system. E.g., fences, walls, and other barriers; locks, safes, and vaults; dogs and armed guards; sensors and alarm bells. [FP031, R1455]
  • piggyback attack
    (I) A form of active wiretapping in which the attacker gains access to a system via intervals of inactivity in another user's legitimate communication connection. Sometimes called a "between- the-lines" attack. (See: hijack attack, man-in-the-middle attack.)
  • ping of death
    (I) An attack that sends an improperly large ICMP [R0792] echo request packet (a "ping") with the intent of overflowing the input buffers of the destination machine and causing it to crash.
  • ping sweep
    (I) An attack that sends ICMP [R0792] echo requests ("pings") to a range of IP addresses, with the goal of finding hosts that can be probed for vulnerabilities.
  • plaintext
    (I) Data that is input to and transformed by an encryption process, or that is output by a decryption process.

    (C) Usually, the plaintext input to an encryption operation is cleartext. But in some cases, the input is ciphertext that was output from another encryption operation. (See: superencryption.)

  • policy certification authority (Internet PCA)
    (I) An X.509-compliant CA at the second level of the Internet certification hierarchy, under the Internet Policy Registration Authority (IPRA). Each PCA operates in accordance with its published security policy (see: certification practice statement) and within constraints established by the IPRA for all PCAs. [R1422]. (See: policy creation authority.)
  • policy mapping
    (I) "Recognizing that, when a CA in one domain certifies a CA in another domain, a particular certificate policy in the second domain may be considered by the authority of the first domain to be equivalent (but not necessarily identical in all respects) to a particular certificate policy in the first domain." [X509]
  • port scan
    (I) An attack that sends client requests to a range of server port addresses on a host, with the goal of finding an active port and exploiting a known vulnerability of that service.
  • pre-authorization
    (I) A capability of a CAW that enables certification requests to be automatically validated against data provided in advance to the CA by an authorizing entity.
  • privacy
    (I) The right of an entity (normally a person), acting in its own behalf, to determine the degree to which it will interact with its environment, including the degree to which the entity is willing to share information about itself with others. (See: anonymity.)

    (O) "The right of individuals to control or influence what information related to them may be collected and stored and by whom and to whom that information may be disclosed." [I7498 Part 2]

    (D) ISDs SHOULD NOT use this term as a synonym for "data confidentiality" or "data confidentiality service", which are different concepts. Privacy is a reason for security rather than a kind of security. For example, a system that stores personal data needs to protect the data to prevent harm, embarrassment, inconvenience, or unfairness to any person about whom data is maintained, and to protect the person's privacy. For that reason, the system may need to provide data confidentiality service.

  • private component
    (I) A synonym for "private key".

    (D) In most cases, ISDs SHOULD NOT use this term; to avoid confusing readers, use "private key" instead. However, the term MAY be used when specifically discussing a key pair; e.g., "A key pair has a public component and a private component."

  • private key
    (I) The secret component of a pair of cryptographic keys used for asymmetric cryptography. (See: key pair, public key.)

    (O) "(In a public key cryptosystem) that key of a user's key pair which is known only by that user." [X509]

  • privilege
    (I) An authorization or set of authorizations to perform security- relevant functions, especially in the context of a computer operating system.
  • privileged process
    (I) An computer process that is authorized (and, therefore, trusted) to perform some security-relevant functions that ordinary processes are not. (See: privilege, trusted process.)
  • proprietary
    (I) Refers to information (or other property) that is owned by an individual or organization and for which the use is restricted by that entity.
  • protected checksum
    (I) A checksum that is computed for a data object by means that protect against active attacks that would attempt to change the checksum to make it match changes made to the data object. (See: digital signature, keyed hash, (discussion under) checksum.
  • protected distribution system
    (I) A wireline or fiber-optic system that includes sufficient safeguards (acoustic, electric, electromagnetic, and physical) to permit its use for unencrypted transmission of (cleartext) data.
  • protection ring
    (I) One of a hierarchy of privileged operation modes of a system that gives certain access rights to processes authorized to operate in that mode.
  • protocol
    (I) A set of rules (i.e., formats and procedures) to implement and control some type of association (e.g., communication) between systems. (E.g., see: Internet Protocol.)

    (C) In particular, a series of ordered steps involving computing and communication that are performed by two or more system entities to achieve a joint objective. [A9042]

  • protocol suite
    (I) A complementary collection of communication protocols used in a computer network. (See: Internet, OSI.)
  • proxy server
    (I) A computer process--often used as, or as part of, a firewall-- that relays a protocol between client and server computer systems, by appearing to the client to be the server and appearing to the server to be the client. (See: SOCKS.)

    (C) In a firewall, a proxy server usually runs on a bastion host, which may support proxies for several protocols (e.g., FTP, HTTP, and TELNET). Instead of a client in the protected enclave connecting directly to an external server, the internal client connects to the proxy server which in turn connects to the external server. The proxy server waits for a request from inside the firewall, forwards the request to the remote server outside the firewall, gets the response, then sends the response back to the client. The proxy may be transparent to the clients, or they may need to connect first to the proxy server, and then use that association to also initiate a connection to the real server.

    (C) Proxies are generally preferred over SOCKS for their ability to perform caching, high-level logging, and access control. A proxy can provide security service beyond that which is normally part of the relayed protocol, such as access control based on peer entity authentication of clients, or peer entity authentication of servers when clients do not have that capability. A proxy at OSI layer 7 can also provide finer-grained security service than can a filtering router at OSI layer 3. For example, an FTP proxy could permit transfers out of, but not into, a protected network.

  • pseudo-random
    (I) A sequence of values that appears to be random (i.e., unpredictable) but is actually generated by a deterministic algorithm. (See: random.)
  • pseudo-random number generator
    (I) A process used to deterministically generate a series of numbers (usually integers) that appear to be random according to certain statistical tests, but actually are pseudo-random.

    (C) Pseudo-random number generators are usually implemented in software.

  • public component
    (I) A synonym for "public key".

    (D) In most cases, ISDs SHOULD NOT use this term; to avoid confusing readers, use "private key" instead. However, the term MAY be used when specifically discussing a key pair; e.g., "A key pair has a public component and a private component."

  • public key
    (I) The publicly-disclosable component of a pair of cryptographic keys used for asymmetric cryptography. (See: key pair, private key.)

    (O) "(In a public key cryptosystem) that key of a user's key pair which is publicly known." [X509]

  • public-key certificate
    (I) A digital certificate that binds a system entity's identity to a public key value, and possibly to additional data items; a digitally-signed data structure that attests to the ownership of a public key. (See: X.509 public-key certificate.)

    (C) The digital signature on a public-key certificate is unforgeable. Thus, the certificate can be published, such as by posting it in a directory, without the directory having to protect the certificate's data integrity.

    (O) "The public key of a user, together with some other information, rendered unforgeable by encipherment with the private key of the certification authority which issued it." [X509]

  • public-key cryptography
    (I) The popular synonym for "asymmetric cryptography".
  • public-key forward secrecy (PFS)
    (I) For a key agreement protocol based on asymmetric cryptography, the property that ensures that a session key derived from a set of long-term public and private keys will not be compromised if one of the private keys is compromised in the future.

    (C) Some existing RFCs use the term "perfect forward secrecy" but either do not define it or do not define it precisely. While preparing this Glossary, we tried to find a good definition for that term, but found this to be a muddled area. Experts did not agree. For all practical purposes, the literature defines "perfect forward secrecy" by stating the Diffie-Hellman algorithm. The term "public-key forward secrecy" (suggested by Hilarie Orman) and the "I" definition stated for it here were crafted to be compatible with current Internet documents, yet be narrow and leave room for improved terminology.

    (C) Challenge to the Internet security community: We need a taxonomy--a family of mutually exclusive and collectively exhaustive terms and definitions to cover the basic properties discussed here--for the full range of cryptographic algorithms and protocols used in Internet Standards:

    (C) Involvement of session keys vs. long-term keys: Experts disagree about the basic ideas involved. - One concept of "forward secrecy" is that, given observations of the operation of a key establishment protocol up to time t, and given some of the session keys derived from those protocol runs, you cannot derive unknown past session keys or future session keys. - A related property is that, given observations of the protocol and knowledge of the derived session keys, you cannot derive one or more of the long-term private keys. - The "I" definition presented above involves a third concept of "forward secrecy" that refers to the effect of the compromise of long-term keys. - All three concepts involve the idea that a compromise of "this" encryption key is not supposed to compromise the "next" one. There also is the idea that compromise of a single key will compromise only the data protected by the single key. In Internet literature, the focus has been on protection against decryption of back traffic in the event of a compromise of secret key material held by one or both parties to a communication.

    (C) Forward vs. backward: Experts are unhappy with the word "forward", because compromise of "this" encryption key also is not supposed to compromise the "previous" one, which is "backward" rather than forward. In S/KEY, if the key used at time t is compromised, then all keys used prior to that are compromised. If the "long-term" key (i.e., the base of the hashing scheme) is compromised, then all keys past and future are compromised; thus, you could say that S/KEY has neither forward nor backward secrecy.

    (C) Asymmetric cryptography vs. symmetric: Experts disagree about forward secrecy in the context of symmetric cryptographic systems. In the absence of asymmetric cryptography, compromise of any long- term key seems to compromise any session key derived from the long-term key. For example, Kerberos isn't forward secret, because compromising a client's password (thus compromising the key shared by the client and the authentication server) compromises future session keys shared by the client and the ticket-granting server.

    (C) Ordinary forward secrecy vs. "perfect" forward secret: Experts disagree about the difference between these two. Some say there is no difference, and some say that the initial naming was unfortunate and suggest dropping the word "perfect". Some suggest using "forward secrecy" for the case where one long-term private key is compromised, and adding "perfect" for when both private keys (or, when the protocol is multi-party, all private keys) are compromised.

    (C) Acknowledgements: Bill Burr, Burt Kaliski, Steve Kent, Paul Van Oorschot, Michael Wiener, and, especially, Hilarie Orman contributed ideas to this discussion.

  • public-key infrastructure (PKI)
    (I) A system of CAs (and, optionally, RAs and other supporting servers and agents) that perform some set of certificate management, archive management, key management, and token management functions for a community of users in an application of asymmetric cryptography. (See: hierarchical PKI, mesh PKI, security management infrastructure, trust-file PKI.)

    (O) PKIX usage: The set of hardware, software, people, policies, and procedures needed to create, manage, store, distribute, and revoke digital certificates based on asymmetric cryptography.

    (C) The core PKI functions are (a) to register users and issue their public-key certificates, (b) to revoke certificates when required, and (c) to archive data needed to validate certificates at a much later time. Key pairs for data confidentiality may be generated (and perhaps escrowed) by CAs or RAs, but requiring a PKI client to generate its own digital signature key pair helps maintain system integrity of the cryptographic system, because then only the client ever possesses the private key it uses. Also, an authority may be established to approve or coordinate CPSs, which are security policies under which components of a PKI operate.

    (C) A number of other servers and agents may support the core PKI, and PKI clients may obtain services from them. The full range of such services is not yet fully understood and is evolving, but supporting roles may include archive agent, certified delivery agent, confirmation agent, digital notary, directory, key escrow agent, key generation agent, naming agent who ensures that issuers and subjects have unique identifiers within the PKI, repository, ticket-granting agent, and time stamp agent.

  • RA domains
    (I) A capability of a CAW that allows a CA to divide the responsibility for certification requests among multiple RAs.

    (C) This capability might be used to restrict access to private authorization data that is provided with a certification request, and to distribute the responsibility to review and approve certification requests in high volume environments. RA domains might segregate certification requests according to an attribute of the certificate subject, such as an organizational unit.

  • RED
    (I) Designation for information system equipment or facilities that handle (and for data that contains) only plaintext (or, depending on the context, classified information), and for such data itself. This term derives from U.S. Government COMSEC terminology. (See: BLACK, RED/BLACK separation.)
  • RED/BLACK separation
    (I) An architectural concept for cryptographic systems that strictly separates the parts of a system that handle plaintext (i.e., RED information) from the parts that handle ciphertext (i.e., BLACK information). This term derives from U.S. Government COMSEC terminology. (See: BLACK, RED.)
  • Remote Authentication Dial-In User Service (RADIUS)
    (I) An Internet protocol [R2138] for carrying dial-in users' authentication information and configuration information between a shared, centralized authentication server (the RADIUS server) and a network access server (the RADIUS client) that needs to authenticate the users of its network access ports. (See: TACACS.)

    (C) A user of the RADIUS client presents authentication information to the client, and the client passes that information to the RADIUS server. The server authenticates the client using a shared secret value, then checks the user's authentication information, and finally returns to the client all authorization and configuration information needed by the client to deliver service to the user.

  • Request for Comment (RFC)
    (I) One of the documents in the archival series that is the official channel for ISDs and other publications of the Internet Engineering Steering Group, the Internet Architecture Board, and the Internet community in general. [R2026, R2223] (See: Internet Standard.)

    (C) This term is *not* a synonym for "Internet Standard".

  • random
    (I) General usage: In mathematics, random means "unpredictable". A sequence of values is called random if each successive value is obtained merely by chance and does not depend on the preceding values of the sequence, and a selected individual value is called random if each of the values in the total population of possibilities has equal probability of being selected. [Knuth] (See: cryptographic key, pseudo-random, random number generator.)

    (I) Security usage: In cryptography and other security applications, random means not only unpredictable, but also "unguessable". When selecting data values to use for cryptographic keys, "the requirement is for data that an adversary has a very low probability of guessing or determining." It is not sufficient to use data that "only meets traditional statistical tests for randomness or which is based on limited range sources, such as clocks. Frequently such random quantities are determinable [i.e., guessable] by an adversary searching through an embarrassingly small space of possibilities." [R1750]

  • random number generator
    (I) A process used to generate an unpredictable, uniformly distributed series of numbers (usually integers). (See: pseudo- random, random.)

    (C) True random number generators are hardware-based devices that depend on the output of a "noisy diode" or other physical phenomena. [R1750]

  • reference monitor
    (I) "An access control concept that refers to an abstract machine that mediates all accesses to objects by subjects." [NCS04] (See: security kernel.)

    (C) A reference monitor should be (a) complete (i.e., it mediates every access), (b) isolated (i.e., it cannot be modified by other system entities), and (c) verifiable (i.e., small enough to be subjected to analysis and tests to ensure that it is correct).

  • reflection attack
    (I) A type of replay attack in which transmitted data is sent back to its originator.
  • registration
    (I) An administrative act or process whereby an entity's name and other attributes are established for the first time at a CA, prior to the CA issuing a digital certificate that has the entity's name as the subject. (See: registration authority.)

    (C) Registration may be accomplished either directly, by the CA, or indirectly, by a separate RA. An entity is presented to the CA or RA, and the authority either records the name(s) claimed for the entity or assigns the entity's name(s). The authority also determines and records other attributes of the entity that are to be bound in a certificate (such as a public key or authorizations) or maintained in the authority's database (such as street address and telephone number). The authority is responsible, possibly assisted by an RA, for authenticating the entity's identity and verifying the correctness of the other attributes, in accordance with the CA's CPS.

    (C) Among the registration issues that a CPS may address are the following [R2527]: - How a claimed identity and other attributes are verified. - How organization affiliation or representation is verified. - What forms of names are permitted, such as X.500 DN, domain name, or IP address. - Whether names are required to be meaningful or unique, and within what domain. - How naming disputes are resolved, including the role of trademarks. - Whether certificates are issued to entities that are not persons. - Whether a person is required to appear before the CA or RA, or can instead be represented by an agent. - Whether and how an entity proves possession of the private key matching a public key.

  • registration authority (RA)
    (I) An optional PKI entity (separate from the CAs) that does not sign either digital certificates or CRLs but has responsibility for recording or verifying some or all of the information (particularly the identities of subjects) needed by a CA to issue certificates and CRLs and to perform other certificate management functions. (See: organizational registration authority, registration.)

    (C) Sometimes, a CA may perform all certificate management functions for all end users for which the CA signs certificates. Other times, such as in a large or geographically dispersed community, it may be necessary or desirable to offload secondary CA functions and delegate them to an assistant, while the CA retains the primary functions (signing certificates and CRLs). The tasks that are delegated to an RA by a CA may include personal authentication, name assignment, token distribution, revocation reporting, key generation, and archiving. An RA is an optional PKI component, separate from the CA, that is assigned secondary functions. The duties assigned to RAs vary from case to case but may include the following: - Verifying a subject's identity, i.e., performing personal authentication functions. - Assigning a name to a subject. (See: distinguished name.) - Verifying that a subject is entitled to have the attributes requested for a certificate. - Verifying that a subject possesses the private key that matches the public key requested for a certificate. - Performing functions beyond mere registration, such as generating key pairs, distributing tokens, and handling revocation reports. (Such functions may be assigned to a PKI element that is separate from both the CA and the RA.)

    (I) PKIX usage: An optional PKI component, separate from the CA(s). The functions that the RA performs will vary from case to case but may include identity authentication and name assignment, key generation and archiving of key pairs, token distribution, and revocation reporting. [R2510]

    (O) SET usage: "An independent third-party organization that processes payment card applications for multiple payment card brands and forwards applications to the appropriate financial institutions." [SET2]

  • regrade
    (I) Deliberately change the classification level of information in an authorized manner.
  • rekey
    (I) Change the value of a cryptographic key that is being used in an application of a cryptographic system. (See: certificate rekey.)

    (C) For example, rekey is required at the end of a cryptoperiod or key lifetime.

  • reliability
    (I) The ability of a system to perform a required function under stated conditions for a specified period of time. (See: availability, survivability.)
  • replay attack
    (I) An attack in which a valid data transmission is maliciously or fraudulently repeated, either by the originator or by an adversary who intercepts the data and retransmits it, possibly as part of a masquerade attack. (See: active wiretapping.)
  • repository
    (I) A system for storing and distributing digital certificates and related information (including CRLs, CPSs, and certificate policies) to certificate users. (See: directory.)

    (O) "A trustworthy system for storing and retrieving certificates or other information relevant to certificates." [ABA]

    (C) A certificate is published to those who might need it by putting it in a repository. The repository usually is a publicly accessible, on-line server. In the Federal Public-key Infrastructure, for example, the expected repository is a directory that uses LDAP, but also may be the X.500 Directory that uses DAP, or an HTTP server, or an FTP server that permits anonymous login.

  • repudiation
    (I) Denial by a system entity that was involved in an association (especially an association that transfers information) of having participated in the relationship. (See: accountability, non- repudiation service.)

    (O) "Denial by one of the entities involved in a communication of having participated in all or part of the communication." [I7498 Part 2]

  • residual risk
    (I) The risk that remains after countermeasures have been applied.
  • risk
    (I) An expectation of loss expressed as the probability that a particular threat will exploit a particular vulnerability with a particular harmful result.

    (O) SET usage: "The possibility of loss because of one or more threats to information (not to be confused with financial or business risk)." [SET2]

  • risk assessment
    (I) A process that systematically identifies valuable system resources and threats to those resources, quantifies loss exposures (i.e., loss potential) based on estimated frequencies and costs of occurrence, and (optionally) recommends how to allocate resources to countermeasures so as to minimize total exposure.

    (C) The analysis lists risks in order of cost and criticality, thereby determining where countermeasures should be applied first. It is usually financially and technically infeasible to counteract all aspects of risk, and so some residual risk will remain, even after all available countermeasures have been deployed. [FP031, R2196]

  • risk management
    (I) The process of identifying, controlling, and eliminating or minimizing uncertain events that may affect system resources. (See: risk analysis.)
  • role-based access control (RBAC)
    (I) A form of identity-based access control where the system entities that are identified and controlled are functional positions in an organization or process.
  • root
    (I) A CA that is directly trusted by an end entity. Acquiring the value of a root CA's public key involves an out-of-band procedure.

    (I) Hierarchical PKI usage: The CA that is the highest level (most trusted) CA in a certification hierarchy; i.e., the authority upon whose public key all certificate users base their trust. (See: top CA.)

    (C) In a hierarchical PKI, a root issues public-key certificates to one or more additional CAs that form the second highest level. Each of these CAs may issue certificates to more CAs at the third highest level, and so on. To initialize operation of a hierarchical PKI, the root's initial public key is securely distributed to all certificate users in a way that does not depend on the PKI's certification relationships. The root's public key may be distributed simply as a numerical value, but typically is distributed in a self-signed certificate in which the root is the subject. The root's certificate is signed by the root itself because there is no higher authority in a certification hierarchy. The root's certificate is then the first certificate in every certification path.

    (O) MISSI usage: A name previously used for a MISSI policy creation authority, which is not a root as defined above for general usage, but is a CA at the second level of the MISSI hierarchy, immediately subordinate to a MISSI policy approving authority.

    (O) UNIX usage: A user account (also called "superuser") that has all privileges (including all security-related privileges) and thus can manage the system and its other user accounts.

  • root certificate
    (I) A certificate for which the subject is a root.

    (I) Hierarchical PKI usage: The self-signed public-key certificate at the top of a certification hierarchy.

  • root key
    (I) A public key for which the matching private key is held by a root.
  • router
    (I) A computer that is a gateway between two networks at OSI layer 3 and that relays and directs data packets through that internetwork. The most common form of router operates on IP packets. (See: bridge.)

    (I) Internet usage: In the context of the Internet protocol suite, a networked computer that forwards Internet Protocol packets that are not addressed to the computer itself. (See: host.)

  • rule-based security policy
    (I) "A security policy based on global rules imposed for all users. These rules usually rely on comparison of the sensitivity of the resource being accessed and the possession of corresponding attributes of users, a group of users, or entities acting on behalf of users." [I7498 Part 2] (See: identity-based security policy.)
  • S/Key
    (I) A security mechanism that uses a cryptographic hash function to generate a sequence of 64-bit, one-time passwords for remote user login. [R1760]

    (C) The client generates a one-time password by applying the MD4 cryptographic hash function multiple times to the user's secret key. For each successive authentication of the user, the number of hash applications is reduced by one. (Thus, an intruder using wiretapping cannot compute a valid password from knowledge of one previously used.) The server verifies a password by hashing the currently presented password (or initialization value) one time and comparing the hash result with the previously presented password.

    (I) An Internet protocol [R1928] that provides a generalized proxy server that enables client-server applications--such as TELNET, FTP, and HTTP; running over either TCP or UDP--to use the services of a firewall.

    (C) SOCKS is layered under the application layer and above the transport layer. When a client inside a firewall wishes to establish a connection to an object that is reachable only through the firewall, it uses TCP to connect to the SOCKS server, negotiates with the server for the authentication method to be used, authenticates with the chosen method, and then sends a relay request. The SOCKS server evaluates the request, typically based on source and destination addresses, and either establishes the appropriate connection or denies it.

  • SSH
    (I) A protocol for secure remote login and other secure network services over an insecure network.

    (C) Consists of three major components: - Transport layer protocol: Provides server authentication, confidentiality, and integrity. It may optionally also provide compression. The transport layer will typically be run over a TCP/IP connection, but might also be used on top of any other reliable data stream. - User authentication protocol: Authenticates the client-side user to the server. It runs over the transport layer protocol. - Connection protocol: Multiplexes the encrypted tunnel into several logical channels. It runs over the user authentication protocol.

  • SYN flood
    (I) A denial of service attack that sends a host more TCP SYN packets (request to synchronize sequence numbers, used when opening a connection) than the protocol implementation can handle. (See: flooding.)
  • Secure Hypertext Transfer Protocol (Secure-HTTP, S-HTTP)
    (I) A Internet protocol for providing client-server security services for HTTP communications. (See: https.)

    (C) S-HTTP was originally specified by CommerceNet, a coalition of businesses interested in developing the Internet for commercial uses. Several message formats may be incorporated into S-HTTP clients and servers, particularly CMS and MOSS. S-HTTP supports choice of security policies, key management mechanisms, and cryptographic algorithms through option negotiation between parties for each transaction. S-HTTP supports both asymmetric and symmetric key operation modes. S-HTTP attempts to avoid presuming a particular trust model, but it attempts to facilitate multiply- rooted hierarchical trust and anticipates that principals may have many public key certificates.

  • Secure/MIME (S/MIME)
    (I) Secure/Multipurpose Internet Mail Extensions, an Internet protocol [R2633] to provide encryption and digital signatures for Internet mail messages.
  • Simple Authentication and Security Layer (SASL)
    (I) An Internet specification [R2222] for adding authentication service to connection-based protocols. To use SASL, a protocol includes a command for authenticating a user to a server and for optionally negotiating protection of subsequent protocol interactions. The command names a registered security mechanism. SASL mechanisms include Kerberos, GSSAPI, S/KEY, and others. Some protocols that use SASL are IMAP4 and POP3.
  • Simple Key-management for Internet Protocols (SKIP)
    (I) A key distribution protocol that uses hybrid encryption to convey session keys that are used to encrypt data in IP packets. [R2356] (See: IKE, IPsec.)

    (C) SKIP uses the Diffie-Hellman algorithm (or could use another key agreement algorithm) to generate a key-encrypting key for use between two entities. A session key is used with a symmetric algorithm to encrypt data in one or more IP packets that are to be sent from one of the entities to the other. The KEK is used with a symmetric algorithm to encrypt the session key, and the encrypted session key is placed in a SKIP header that is added to each IP packet that is encrypted with that session key.

  • Simple Mail Transfer Protocol (SMTP)
    (I) A TCP-based, application-layer, Internet Standard protocol [R0821] for moving electronic mail messages from one computer to another.
  • Simple Network Management Protocol (SNMP)
    (I) A UDP-based, application-layer, Internet Standard protocol [R2570, R2574] for conveying management information between managers and agents.

    (C) SNMP version 1 uses cleartext passwords for authentication and access control. (See: community string.) Version 2 adds cryptographic mechanisms based on DES and MD5. Version 3 provides enhanced, integrated support for security services, including data confidentiality, data integrity, data origin authentication, and message timeliness and limited replay protection.

  • safety
    (I) The property of a system being free from risk of causing harm to system entities and outside entities.
  • salt
    (I) A random value that is concatenated with a password before applying the one-way encryption function used to protect passwords that are stored in the database of an access control system. (See: initialization value.)

    (C) Salt protects a password-based access control system against a dictionary attack.

  • sanitize
    (I) Delete sensitive data from a file, a device, or a system; or modify data so as to be able to downgrade its classification level.
  • screening router
    (I) A synonym for "filtering router".
  • secret
    (I) (1.) Adjective: The condition of information being protected from being known by any system entities except those who are intended to know it. (2.) Noun: An item of information that is protected thusly.

    (C) This term applies to symmetric keys, private keys, and passwords.

  • secret-key cryptography
    (I) A synonym for "symmetric cryptography".
  • secure state
    (I) A system condition in which no subject can access any object in an unauthorized manner. (See: (secondary definition under) Bell-LaPadula Model, clean system.)
  • security
    (I) (1.) Measures taken to protect a system. (2.) The condition of a system that results from the establishment and maintenance of measures to protect the system. (3.) The condition of system resources being free from unauthorized access and from unauthorized or accidental change, destruction, or loss.
  • security architecture
    (I) A plan and set of principles that describe (a) the security services that a system is required to provide to meet the needs of its users, (b) the system elements required to implement the services, and (c) the performance levels required in the elements to deal with the threat environment. (See: (discussion under) security policy.)

    (C) A security architecture is the result of applying the system engineering process. A complete system security architecture includes administrative security, communication security, computer security, emanations security, personnel security, and physical security (e.g., see: [R2179]). A complete security architecture needs to deal with both intentional, intelligent threats and accidental kinds of threats.

  • security association
    (I) A relationship established between two or more entities to enable them to protect data they exchange. The relationship is used to negotiate characteristics of protection mechanisms, but does not include the mechanisms themselves. (See: association.)

    (C) A security association describes how entities will use security services. The relationship is represented by a set of information that is shared between the entities and is agreed upon and considered a contract between them.

    (O) IPsec usage: A simplex (uni-directional) logical connection created for security purposes and implemented with either AH or ESP (but not both). The security services offered by a security association depend on the protocol selected, the IPsec mode (transport or tunnel), the endpoints, and the election of optional services within the protocol. A security association is identified by a triple consisting of (a) a destination IP address, (b) a protocol (AH or ESP) identifier, and (c) a Security Parameter Index.

  • security association identifier (SAID)
    (I) A data field in a security protocol (such as NLSP or SDE), used to identify the security association to which a protocol data unit is bound. The SAID value is usually used to select a key for decryption or authentication at the destination. (See: Security Parameter Index.)
  • security audit
    (I) An independent review and examination of a system's records and activities to determine the adequacy of system controls, ensure compliance with established security policy and procedures, detect breaches in security services, and recommend any changes that are indicated for countermeasures. [I7498 Part 2, NCS01]

    (C) The basic audit objective is to establish accountability for system entities that initiate or participate in security-relevant events and actions. Thus, means are needed to generate and record a security audit trail and to review and analyze the audit trail to discover and investigate attacks and security compromises.

  • security audit trail
    (I) A chronological record of system activities that is sufficient to enable the reconstruction and examination of the sequence of environments and activities surrounding or leading to an operation, procedure, or event in a security-relevant transaction from inception to final results. [NCS04] (See: security audit.)
  • security clearance
    (I) A determination that a person is eligible, under the standards of a specific security policy, for authorization to access sensitive information or other system resources. (See: clearance level.)
  • security compromise
    (I) A security violation in which a system resource is exposed, or is potentially exposed, to unauthorized access. (See: data compromise, violation.)
  • security environment
    (I) The set of external entities, procedures, and conditions that affect secure development, operation, and maintenance of a system.
  • security event
    (I) A occurrence in a system that is relevant to the security of the system. (See: security incident.)

    (C) The term includes both events that are security incidents and those that are not. In a CA workstation, for example, a list of security events might include the following: - Performing a cryptographic operation, e.g., signing a digital certificate or CRL. - Performing a cryptographic card operation: creation, insertion, removal, or backup. - Performing a digital certificate lifecycle operation: rekey, renewal, revocation, or update. - Posting information to an X.500 Directory. - Receiving a key compromise notification. - Receiving an improper certification request. - Detecting an alarm condition reported by a cryptographic module. - Logging the operator in or out. - Failing a built-in hardware self-test or a software system integrity check.

  • security fault analysis
    (I) A security analysis, usually performed on hardware at a logic gate level, gate-by-gate, to determine the security properties of a device when a hardware fault is encountered.
  • security gateway
    (I) A gateway that separates trusted (or relatively more trusted) hosts on the internal network side from untrusted (or less trusted) hosts on the external network side. (See: firewall and guard.)

    (O) IPsec usage: "An intermediate system that implements IPsec protocols." [R2401] Normally, AH or ESP is implemented to serve a set of internal hosts, providing security services for the hosts when they communicate with other, external hosts or gateways that also implement IPsec.

  • security incident
    (I) A security event that involves a security violation. (See: CERT, GRIP, security event, security intrusion, security violation.)

    (C) In other words, a security-relevant system event in which the system's security policy is disobeyed or otherwise breached.

    (O) "Any adverse event which compromises some aspect of computer or network security." [R2350]

    (D) ISDs SHOULD NOT use this "O" definition because (a) a security incident may occur without actually being harmful (i.e., adverse) and (b) this Glossary defines "compromise" more narrowly in relation to unauthorized access.

  • security intrusion
    (I) A security event, or a combination of multiple security events, that constitutes a security incident in which an intruder gains, or attempts to gain, access to a system (or system resource) without having authorization to do so.
  • security kernel
    (I) "The hardware, firmware, and software elements of a trusted computing base that implement the reference monitor concept. It must mediate all accesses, be protected from modification, and be verifiable as correct." [NCS04] (See: reference monitor.)

    (C) That is, a security kernel is an implementation of a reference monitor for a given hardware base.

  • security label
    (I) A marking that is bound to a system resource and that names or designates the security-relevant attributes of that resource. [I7498 Part 2, R1457]

    (C) The recommended definition is usefully broad, but usually the term is understood more narrowly as a marking that represents the security level of an information object, i.e., a marking that indicates how sensitive an information object is. [NCS04]

    (C) System security mechanisms interpret security labels according to applicable security policy to determine how to control access to the associated information, otherwise constrain its handling, and affix appropriate security markings to visible (printed and displayed) images thereof. [FP188]

  • security level
    (I) The combination of a hierarchical classification level and a set of non-hierarchical category designations that represents how sensitive information is. (See: (usage note under) classification level, dominate, lattice model.)
  • security management infrastructure (SMI)
    (I) System elements and activities that support security policy by monitoring and controlling security services and mechanisms, distributing security information, and reporting security events. The associated functions are as follows [I7498-4]: - Controlling (granting or restricting) access to system resources: This includes verifying authorizations and identities, controlling access to sensitive security data, and modifying access priorities and procedures in the event of attacks. - Retrieving (gathering) and archiving (storing) security information: This includes logging security events and analyzing the log, monitoring and profiling usage, and reporting security violations. - Managing and controlling the encryption process: This includes performing the functions of key management and reporting on key management problems. (See: public-key infrastructure.)
  • security mechanism
    (I) A process (or a device incorporating such a process) that can be used in a system to implement a security service that is provided by or within the system. (See: (discussion under) security policy.)

    (C) Some examples of security mechanisms are authentication exchange, checksum, digital signature, encryption, and traffic padding.

  • security model
    (I) A schematic description of a set of entities and relationships by which a specified set of security services are provided by or within a system. (See: (discussion under) security policy.)

    (C) An example is the Bell-LaPadula Model.

  • security parameters index (SPI)
    (I) IPsec usage: The type of security association identifier used in IPsec protocols. A 32-bit value used to distinguish among different security associations terminating at the same destination (IP address) and using the same IPsec security protocol (AH or ESP). Carried in AH and ESP to enable the receiving system to determine under which security association to process a received packet.
  • security perimeter
    (I) The boundary of the domain in which a security policy or security architecture applies; i.e., the boundary of the space in which security services protect system resources.
  • security policy
    (I) A set of rules and practices that specify or regulate how a system or organization provides security services to protect sensitive and critical system resources. (See: identity-based security policy, rule-based security policy, security architecture, security mechanism, security model.)

    (O) "The set of rules laid down by the security authority governing the use and provision of security services and facilities." [X509]

    (C) Ravi Sandhu notes that security policy is one of four layers of the security engineering process (as shown in the following diagram). Each layer provides a different view of security, ranging from what services are needed to how services are implemented. What Security Services Should Be Provided? ^ | + - - - - - - - - - - - + | | Security Policy | | + - - - - - - - - - - - + + - - - - - - - - - - - - - - + | | Security Model | | A "top-level specification" | | + - - - - - - - - - - - + <- | is at a level below "model" | | | Security Architecture | | but above "architecture". | | + - - - - - - - - - - - + + - - - - - - - - - - - - - - + | | Security Mechanism | | + - - - - - - - - - - - + v How Are Security Services Implemented?

  • security service
    (I) A processing or communication service that is provided by a system to give a specific kind of protection to system resources. (See: access control service, audit service, availability service, data confidentiality service, data integrity service, data origin authentication service, non-repudiation service, peer entity authentication service, system integrity service.)

    (O) "A service, provided by a layer of communicating open systems, which ensures adequate security of the systems or the data transfers." [I7498 Part 2]

    (C) Security services implement security policies, and are implemented by security mechanisms.

  • security situation
    (I) ISAKMP usage: The set of all security-relevant information-- e.g., network addresses, security classifications, manner of operation (normal or emergency)--that is needed to decide the security services that are required to protect the association that is being negotiated.
  • security violation
    (I) An act or event that disobeys or otherwise breaches security policy. (See: compromise, penetration, security incident.)
  • self-signed certificate
    (I) A public-key certificate for which the public key bound by the certificate and the private key used to sign the certificate are components of the same key pair, which belongs to the signer. (See: root certificate.)

    (C) In a self-signed X.509 public-key certificate, the issuer's DN is the same as the subject's DN.

  • semantic security
    (I) An attribute of a encryption algorithm that is a formalization of the notion that the algorithm not only hides the plaintext but also reveals no partial information about the plaintext. Whatever is efficiently computable about the plaintext when given the ciphertext, is also efficiently computable without the ciphertext. (See: indistinguishability.)
  • sensitive (information)
    (I) Information is sensitive if disclosure, alteration, destruction, or loss of the information would adversely affect the interests or business of its owner or user. (See: critical.)
  • separation of duties
    (I) The practice of dividing the steps in a system function among different individuals, so as to keep a single individual from subverting the process. (See: dual control, administrative security.)
  • server
    (I) A system entity that provides a service in response to requests from other system entities called clients.
  • session key
    (I) In the context of symmetric encryption, a key that is temporary or is used for a relatively short period of time. (See: ephemeral key, key distribution center, master key.)

    (C) Usually, a session key is used for a defined period of communication between two computers, such as for the duration of a single connection or transaction set, or the key is used in an application that protects relatively large amounts of data and, therefore, needs to be rekeyed frequently.

  • shared secret
    (I) A synonym for "keying material" or "cryptographic key".
  • sign
    (I) Create a digital signature for a data object.
  • signature certificate
    (I) A public-key certificate that contains a public key that is intended to be used for verifying digital signatures, rather than for encrypting data or performing other cryptographic functions.

    (C) A v3 X.509 public-key certificate may have a "keyUsage" extension which indicates the purpose for which the certified public key is intended.

  • simple authentication
    (I) An authentication process that uses a password as the information needed to verify an identity claimed for an entity. (See: strong authentication.)

    (O) "Authentication by means of simple password arrangements." [X509]

  • single sign-on
    (I) A system that enables a user to access multiple computer platforms (usually a set of hosts on the same network) or application systems after being authenticated just one time. (See: Kerberos.)

    (C) Typically, a user logs in just once, and then is transparently granted access to a variety of permitted resources with no further login being required until after the user logs out. Such a system has the advantages of being user friendly and enabling authentication to be managed consistently across an entire enterprise, and has the disadvantage of requiring all hosts and applications to trust the same authentication mechanism.

  • smart card
    (I) A credit-card sized device containing one or more integrated circuit chips, which perform the functions of a computer's central processor, memory, and input/output interface. (See: PC card.)

    (C) Sometimes this term is used rather strictly to mean a card that closely conforms to the dimensions and appearance of the kind of plastic credit card issued by banks and merchants. At other times, the term is used loosely to include cards that are larger than credit cards, especially cards that are thicker, such as PC cards.

    (C) A "smart token" is a device that conforms to the definition of smart card except that rather than having standard credit card dimensions, the token is packaged in some other form, such as a dog tag or door key shape.

  • smurf
    (I) Software that mounts a denial-of-service attack ("smurfing") by exploiting IP broadcast addressing and ICMP ping packets to cause flooding. (See: flood, ICMP flood.)

    (D) ISDs SHOULD NOT use this term because it is not listed in most dictionaries and could confuse international readers.

    (C) A smurf program builds a network packet that appears to originate from another address, that of the "victim", either a host or an IP router. The packet contains an ICMP ping message that is addressed to an IP broadcast address, i.e., to all IP addresses in a given network. The echo responses to the ping message return to the victim's address. The goal of smurfing may be either to deny service at a particular host or to flood all or part of an IP network.

  • social engineering
    (I) A euphemism for non-technical or low-technology means--such as lies, impersonation, tricks, bribes, blackmail, and threats--used to attack information systems. (See: masquerade attack.)

    (D) ISDs SHOULD NOT use this term because it is vague; instead, use a term that is specific with regard to the means of attack.

  • software
    (I) Computer programs (which are stored in and executed by computer hardware) and associated data (which also is stored in the hardware) that may be dynamically written or modified during execution. (See: firmware, hardware.)
  • source integrity
    (I) The degree of confidence that can be placed in information based on the trustworthiness of its sources. (See: integrity.)
  • spam
    (I) (1.) Verb: To indiscriminately send unsolicited, unwanted, irrelevant, or inappropriate messages, especially commercial advertising in mass quantities. (2.) Noun: electronic "junk mail". [R2635]

    (D) This term SHOULD NOT be written in upper-case letters, because SPAM(trademark) is a trademark of Hormel Foods Corporation. Hormel says, "We do not object to use of this slang term [spam] to describe [unsolicited commercial email (UCE)], although we do object to the use of our product image in association with that term. Also, if the term is to be used, it should be used in all lower-case letters to distinguish it from our trademark SPAM, which should be used with all uppercase letters."

    (C) In sufficient volume, spam can cause denial of service. (See: flooding.) According to the SPAM Web site, the term was adopted as a result of the Monty Python skit in which a group of Vikings sang a chorus of 'SPAM, SPAM, SPAM . . .' in an increasing crescendo, drowning out other conversation. Hence, the analogy applied because UCE was drowning out normal discourse on the Internet.

  • split key
    (I) A cryptographic key that is divided into two or more separate data items that individually convey no knowledge of the whole key that results from combining the items. (See: dual control, split knowledge.)
  • split knowledge
    (I) A security technique in which two or more entities separately hold data items that individually convey no knowledge of the information that results from combining the items. (See: dual control, split key.)

    (O) "A condition under which two or more entities separately have key components which individually convey no knowledge of the plaintext key which will be produced when the key components are combined in the cryptographic module." [FP140]

  • spoofing attack
    (I) A synonym for "masquerade attack".
  • star property
    (I) (Written "*-property".) See: "confinement property" under Bell-LaPadula Model.
  • steganography
    (I) Methods of hiding the existence of a message or other data. This is different than cryptography, which hides the meaning of a message but does not hide the message itself. (See: cryptology.)

    (C) An example of a steganographic method is "invisible" ink. (See: digital watermark.)

  • stream cipher
    (I) An encryption algorithm that breaks plaintext into a stream of successive bits (or characters) and encrypts the n-th plaintext bit with the n-th element of a parallel key stream, thus converting the plaintext bit stream into a ciphertext bit stream. [Schn] (See: block cipher.)
  • strong authentication
    (I) An authentication process that uses cryptography--particularly public-key certificates--to verify the identity claimed for an entity. (See: X.509.)

    (O) "Authentication by means of cryptographically derived credentials." [X509]

  • subordinate certification authority (SCA)
    (I) A CA whose public-key certificate is issued by another (superior) CA. (See: certification hierarchy.)

    (O) MISSI usage: The fourth-highest (bottom) level of a MISSI certification hierarchy; a MISSI CA whose public-key certificate is signed by a MISSI CA rather than by a MISSI PCA. A MISSI SCA is the administrative authority for a subunit of an organization, established when it is desirable to organizationally distribute or decentralize the CA service. The term refers both to that authoritative office or role, and to the person who fills that office. A MISSI SCA registers end users and issues their certificates and may also register ORAs, but may not register other CAs. An SCA periodically issues a CRL.

  • subordinate distinguished name
    (I) An X.500 DN is subordinate to another X.500 DN if it begins with a set of attributes that is the same as the entire second DN except for the terminal attribute of the second DN (which is usually the name of a CA). For example, the DN is subordinate to the DN .
  • superencryption
    (I) An encryption operation for which the plaintext input to be transformed is the ciphertext output of a previous encryption operation.
  • survivability
    (I) The ability of a system to remain in operation or existence despite adverse conditions, including both natural occurrences, accidental actions, and attacks on the system. (See: availability, reliability.)
  • symmetric cryptography
    (I) A branch of cryptography involving algorithms that use the same key for two different steps of the algorithm (such as encryption and decryption, or signature creation and signature verification). (See: asymmetric cryptography.)

    (C) Symmetric cryptography has been used for thousands of years [Kahn]. A modern example of a symmetric encryption algorithm is the U.S. Government's Data Encryption Algorithm. (See: DEA, DES.)

    (C) Symmetric cryptography is sometimes called "secret-key cryptography" (versus public-key cryptography) because the entities that share the key, such as the originator and the recipient of a message, need to keep the key secret. For example, when Alice wants to ensure confidentiality for data she sends to Bob, she encrypts the data with a secret key, and Bob uses the same key to decrypt. Keeping the shared key secret entails both cost and risk when the key is distributed to both Alice and Bob. Thus, symmetric cryptography has a key management disadvantage compared to asymmetric cryptography.

  • symmetric key
    (I) A cryptographic key that is used in a symmetric cryptographic algorithm.
  • system entity
    (I) An active element of a system--e.g., an automated process, a subsystem, a person or group of persons--that incorporates a specific set of capabilities.
  • system high
    (I) The highest security level supported by a system at a particular time or in a particular environment. (See: system high security mode.)
  • system high security mode
    (I) A mode of operation of an information system, wherein all users having access to the system possess a security clearance or authorization, but not necessarily a need-to-know, for all data handled by the system. (See: mode of operation.)

    (C) This mode is defined formally in U.S. Department of Defense policy regarding system accreditation [DOD2], but the term is widely used outside the Defense Department and outside the Government.

  • system integrity
    (I) "The quality that a system has when it can perform its intended function in a unimpaired manner, free from deliberate or inadvertent unauthorized manipulation." [NCS04] (See: system integrity service.)
  • system integrity service
    (I) A security service that protects system resources in a verifiable manner against unauthorized or accidental change, loss, or destruction. (See: system integrity.)
  • system low
    (I) The lowest security level supported by a system at a particular time or in a particular environment. (See: system high.)
  • system resource
    (I) Data contained in an information system; or a service provided by a system; or a system capability, such as processing power or communication bandwidth; or an item of system equipment (i.e., a system component--hardware, firmware, software, or documentation); or a facility that houses system operations and equipment.
  • system security officer (SSO)
    (I) A person responsible for enforcement or administration of the security policy that applies to the system.
  • TCP/IP
    (I) A synonym for "Internet Protocol Suite", in which the Transmission Control Protocol (TCP) and the Internet Protocol (IP) are important parts.
    (I) A TCP-based, application-layer, Internet Standard protocol [R0854] for remote login from one host to another.
  • Terminal Access Controller (TAC) Access Control System (TACACS)
    (I) A UDP-based authentication and access control protocol [R1492] in which a network access server receives an identifier and password from a remote terminal and passes them to a separate authentication server for verification.

    (C) TACACS was developed for ARPANET and has evolved for use in commercial equipment. TACs were a type of network access server computer used to connect terminals to the early Internet, usually using dial-up modem connections. TACACS used centralized authentication servers and served not only network access servers like TACs but also routers and other networked computing devices. TACs are no longer in use, but TACACS+ is. [R1983] - "XTACACS": The name of Cisco Corporation's implementation, which enhances and extends the original TACACS. - "TACACS+": A TCP-based protocol that improves on TACACS and XTACACS by separating the functions of authentication, authorization, and accounting and by encrypting all traffic between the network access server and authentication server. It is extensible to allow any authentication mechanism to be used with TACACS+ clients.

  • The Exponential Encryption System (TESS)
    (I) A system of separate but cooperating cryptographic mechanisms and functions for the secure authenticated exchange of cryptographic keys, the generation of digital signatures, and the distribution of public keys. TESS employs asymmetric cryptography, based on discrete exponentiation, and a structure of self- certified public keys. [R1824]
  • Transmission Control Protocol (TCP)
    (I) An Internet Standard protocol [R0793] that reliably delivers a sequence of datagrams (discrete sets of bits) from one computer to another in a computer network. (See: TCP/IP.)

    (C) TCP is designed to fit into a layered hierarchy of protocols that support internetwork applications. TCP assumes it can obtain a simple, potentially unreliable datagram service (such as the Internet Protocol) from the lower-layer protocols.

  • Transport Layer Security (TLS)
    (I) TLS Version 1.0 is an Internet protocol [R2246] based-on and very similar to SSL Version 3.0. (See: TLSP.)

    (C) The TLS protocol is misnamed, because it operates well above the transport layer (OSI layer 4).

  • Transport Layer Security Protocol (TLSP)
    (I) An end-to-end encryption protocol(ISO Standard 10736) that provides security services at the bottom of OSI layer 4, i.e., directly above layer 3. (See: TLS.)

    (C) TLSP evolved directly from the SP4 protocol of SDNS.

  • Trojan horse
    (I) A computer program that appears to have a useful function, but also has a hidden and potentially malicious function that evades security mechanisms, sometimes by exploiting legitimate authorizations of a system entity that invokes the program.
  • User Datagram Protocol (UDP)
    (I) An Internet Standard protocol [R0768] that provides a datagram mode of packet-switched computer communication in an internetwork.

    (C) UDP is a transport layer protocol, and it assumes that IP is the underlying protocol. UDP enables application programs to send transaction-oriented data to other programs with minimal protocol mechanism. UDP does not provide reliable delivery, flow control, sequencing, or other end-to-end services that TCP provides.

  • unclassified
    (I) Not classified.
  • unencrypted
    (I) Not encrypted.
  • unforgeable
    (I) Cryptographic usage: The property of a cryptographic data structure (i.e., a data structure that is defined using one or more cryptographic functions) that makes it computationally infeasible to construct (i.e., compute) an unauthorized but correct value of the structure without having knowledge of one of more keys. (E.g., see: digital certificate.)

    (C) This definition is narrower than general English usage, where "unforgeable" means unable to be fraudulently created or duplicated. In that broader sense, anyone can forge a digital certificate containing any set of data items whatsoever by generating the to-be-signed certificate and signing it with any private key whatsoever. But for PKI purposes, the forged data structure is invalid if it is not signed with the true private key of the claimed issuer; thus, the forgery will be detected when a certificate user uses the true public key of the claimed issuer to verify the signature.

  • uniform resource identifier (URI)
    (I) A type of formatted identifier that encapsulates the name of an Internet object, and labels it with an identification of the name space, thus producing a member of the universal set of names in registered name spaces and of addresses referring to registered protocols or name spaces. [R1630]

    (C) URIs are used in HTML to identify the target of hyperlinks. In common practice, URIs include uniform resource locators [R2368] and relative URLs, and may be URNs. [R1808]

  • uniform resource locator (URL)
    (I) A type of formatted identifier that describes the access method and location of an information resource object on the Internet. [R1738]

    (C) A URL is a URI that provides explicit instructions on how to access the named object. For example, "" is a URL. The part before the colon specifies the access scheme or protocol, and the part after the colon is interpreted according to that access method. Usually, two slashes after the colon indicate the host name of a server (written as a domain name). In an FTP or HTTP URL, the host name is followed by the path name of a file on the server. The last (optional) part of a URL may be either a fragment identifier that indicates a position in the file, or a query string.

  • uniform resource name (URN)
    (I) A URI that has an institutional commitment to persistence and availability.
  • untrusted process
    (I) A system process that is not able to affect the state of system security through incorrect or malicious operation, usually because its operation is confined by a security kernel. (See: trusted process.)
  • user
    (I) A person, organization entity, or automated process that accesses a system, whether authorized to do so or not. (See: [R2504].)

    (C) Any ISD that uses this term SHOULD provide an explicit definition, because this term is used in many ways and can easily be misunderstood.

  • user identifier
    (I) A character string or symbol that is used in a system to uniquely name a specific user or group of users.

    (C) Often verified by a password in an authentication process.

  • v1 CRL
    (I) An abbreviation for "X.509 CRL in version 1 format".

    (C) ISDs should use this abbreviation only after using the full term at its first occurrence and defining the abbreviation.

  • v2 CRL
    (I) An abbreviation for "X.509 CRL in version 2 format".

    (C) ISDs should use this abbreviation only after using the full term at its first occurrence and defining the abbreviation.

  • v2 certificate
    (I) An abbreviation for "X.509 public-key certificate in version 2 format".

    (C) ISDs should use this abbreviation only after using the full term at its first occurrence and defining the abbreviation.

  • v3 certificate
    (I) An abbreviation for "X.509 public-key certificate in version 3 format".

    (C) ISDs should use this abbreviation only after using the full term at its first occurrence and defining the abbreviation.

  • valid certificate
    (I) A digital certificate for which the binding of the data items can be trusted; one that can be validated successfully. (See: validate vs. verify.)
  • validity period
    (I) A data item in a digital certificate that specifies the time period for which the binding between data items (especially between the subject name and the public key value in a public-key certificate) is valid, except if the certificate appears on a CRL or the key appears on a CKL.
  • value-added network (VAN)
    (I) A computer network or subnetwork (which is usually a commercial enterprise) that transmits, receives, and stores EDI transactions on behalf of its customers.

    (C) A VAN may also provide additional services, ranging from EDI format translation, to EDI-to-FAX conversion, to integrated business systems.

  • virtual private network (VPN)
    (I) A restricted-use, logical (i.e., artificial or simulated) computer network that is constructed from the system resources of a relatively public, physical (i.e., real) network (such as the Internet), often by using encryption (located at hosts or gateways), and often by tunneling links of the virtual network across the real network.

    (C) For example, if a corporation has LANs at several different sites, each connected to the Internet by a firewall, the corporation could create a VPN by (a) using encrypted tunnels to connect from firewall to firewall across the Internet and (b) not allowing any other traffic through the firewalls. A VPN is generally less expensive to build and operate than a dedicated real network, because the virtual network shares the cost of system resources with other users of the real network.

  • virus
    (I) A hidden, self-replicating section of computer software, usually malicious logic, that propagates by infecting--i.e., inserting a copy of itself into and becoming part of--another program. A virus cannot run by itself; it requires that its host program be run to make the virus active.
  • vulnerability
    (I) A flaw or weakness in a system's design, implementation, or operation and management that could be exploited to violate the system's security policy.

    (C) Most systems have vulnerabilities of some sort, but this does not mean that the systems are too flawed to use. Not every threat results in an attack, and not every attack succeeds. Success depends on the degree of vulnerability, the strength of attacks, and the effectiveness of any countermeasures in use. If the attacks needed to exploit a vulnerability are very difficult to carry out, then the vulnerability may be tolerable. If the perceived benefit to an attacker is small, then even an easily exploited vulnerability may be tolerable. However, if the attacks are well understood and easily made, and if the vulnerable system is employed by a wide range of users, then it is likely that there will be enough benefit for someone to make an attack.

  • war dialer
    (I) A computer program that automatically dials a series of telephone numbers to find lines connected to computer systems, and catalogs those numbers so that a cracker can try to break into the systems.
  • web server
    (I) A software process that runs on a host computer connected to the Internet to respond to HTTP requests for documents from client web browsers.
  • wiretapping
    (I) An attack that intercepts and accesses data and other information contained in a flow in a communication system.

    (C) Although the term originally referred to making a mechanical connection to an electrical conductor that links two nodes, it is now used to refer to reading information from any sort of medium used for a link or even directly from a node, such as gateway or subnetwork switch.

    (C) "Active wiretapping" attempts to alter the data or otherwise affect the flow; "passive wiretapping" only attempts to observe the flow and gain knowledge of information it contains. (See: active attack, end-to-end encryption, passive attack.)

  • work factor
    (I) General security usage: The estimated amount of effort or time that can be expected to be expended by a potential intruder to penetrate a system, or defeat a particular countermeasure, when using specified amounts of expertise and resources.

    (I) Cryptography usage: The estimated amount of computing time and power needed to break a cryptographic system.

  • worm
    (I) A computer program that can run independently, can propagate a complete working version of itself onto other hosts on a network, and may consume computer resources destructively. (See: Morris Worm, virus.)
  • zeroize
    (I) Use erasure or other means to render stored data unusable and unrecoverable, particularly a key stored in a cryptographic module or other device.

    (O) Erase electronically stored data by altering the contents of the data storage so as to prevent the recovery of the data. [FP140]