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(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.
(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.
(O) "The prevention of unauthorized use of a resource, including the prevention of use of a resource in an unauthorized manner." [I7498 Part 2]
(C) An ACC is sometimes used in conjunction with a key center to implement access control in a key distribution system for symmetric cryptography.
(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.
(C) Accountability permits detection and subsequent investigation of security breaches.
(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.
(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.
(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.
(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.
(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.
(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.
(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 | + - - - - - - - - - - - - + + - - - - + + - - - - - - - - - - -+
(O) "An authority, trusted by the verifier to delegate privilege, which issues attribute certificates." [FPDAM]
(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.)
(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.)
(O) "A mechanism intended to ensure the identity of an entity by means of information exchange." [I7498 Part 2]
(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.)
(C) In a network, there are two general forms of authentication service: data origin authentication service and peer entity authentication service.
(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]
(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]
(O) "The property of being accessible and usable upon demand by an authorized entity." [I7498 Part 2]
(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) 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.)
(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.)
(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.
(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.
(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]
(C) For example, Netscape's Navigator and Communicator, and Microsoft's Explorer.
(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.
(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.
(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."
(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.
(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.
(C) This concept can be implemented as a digital certificate. (See: attribute certificate.)
(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]
(C) Security usage: See: capability, digital certificate.
(C) PKI usage: See: attribute certificate, public-key certificate.
(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.)
(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.
(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".
(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.
(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]
(O) "An integer value, unique within the issuing CA, which is unambiguously associated with a certificate issued by that CA." [X509]
(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".)
(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.
(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.)
(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]
(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).
(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".
(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.
(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]
(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).
(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.
(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.
(O) "Data produced through the use of encipherment. The semantic content of the resulting data is not available." [I7498 Part 2]
(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.
(C) A clean system is not necessarily in a secure state.
(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.)
(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.
(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.
(C) For example, "Dr. E. F. Moore", "The United Nations", or "12-th Floor Laser Printer".
(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.
(C) The term is usually understood to include the special handling procedures to be used for the information.
(C) For example, the CERT Coordination Center at Carnegie-Mellon University (sometimes called "the" CERT) and the Computer Incident Advisory Capability.
(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.
(C) Usually understood to include functions, features, and technical characteristics of computer hardware and software, especially operating systems.
(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.
(C) Configuration control helps protect against unauthorized or malicious alteration of a system and thus provides assurance of system integrity. (See: malicious logic.)
(C) A connection-oriented data integrity service would be able to detect lost or reordered datagrams within a stream of datagrams.
(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.
(C) In an Internet protocol, a countermeasure may take the form of a protocol feature, an element function, or a usage constraint.
(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.
(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.
(O) "Data that is transferred to establish the claimed identity of an entity." [I7498 Part 2]
(C) Cross-certification enables users to validate each other's certificate when the users are certified under different certification hierarchies.
(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.
(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.
(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.)
(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]
(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.
(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]
(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.
(D) ISDs SHOULD NOT use this term as a synonym for "privacy", which is a different concept.
(D) ISDs SHOULD NOT use this term as a synonym for "privacy", which is a different concept.
(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).
(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.
(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.
(C) Both data confidentiality service and data integrity service are needed to achieve data security.
(D) ISDs SHOULD NOT use this term as a synonym for "decrypt", because that would mix concepts in a potentially misleading way.
(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.
(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.
(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.
(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.
(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.
(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.
(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]
(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]
(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.
(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.
(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.
(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.)
(C) For example, see [R2407]. The DOI concept is based on work by the TSIG's CIPSO Working Group.
(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.
(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.
(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.
(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.
(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]
(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).
(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.
(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.
(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.
(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".)
(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.)
(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.
(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.)
(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].
(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 "rosslyn.bbn.com" is 220.127.116.11.
(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.
(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.
(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.
(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.
(C) IMAP4 has mechanisms for optionally authenticating a client to a server and providing other security services. (See: IMAP4 AUTHENTICATE.)
(C) In the OSIRM, IP would be located at the top of layer 3.
(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.
(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.
(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.)
(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.
(C) An ANSI standard [A9017] defines two types of key center: key distribution center and 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]).
(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.
(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]
(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.
(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]).
(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.
(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.)
(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.)
(C) This principle tends to limit damage that can be caused by an accident, error, or unauthorized act.
(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.
(C) Derives from "log" file", a security audit trail that records security events, such as the beginning of sessions, and who initiates them.
(D) ISDs SHOULD NOT use this term because it is not listed in most dictionaries and could confuse international readers.
(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.
(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]
(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.)
(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.
(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.
(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.
(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.
(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.
(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.
(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).
(C) POP3 has mechanisms for optionally authenticating a client to a server and providing other security services. (See: POP3 APOP, POP3 AUTH.)
(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.
(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.
(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.)
(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.)
(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.
(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.
(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.
(D) ISDs SHOULD NOT use this term because it is not listed in most dictionaries and could confuse international readers.
(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.)
(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.
(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."
(O) "(In a public key cryptosystem) that key of a user's key pair which is known only by that user." [X509]
(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]
(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.
(C) Pseudo-random number generators are usually implemented in software.
(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."
(O) "(In a public key cryptosystem) that key of a user's key pair which is publicly known." [X509]
(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]
(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.
(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.
(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.
(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.
(C) This term is *not* a synonym for "Internet Standard".
(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]
(C) True random number generators are hardware-based devices that depend on the output of a "noisy diode" or other physical phenomena. [R1750]
(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).
(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.
(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]
(C) For example, rekey is required at the end of a cryptoperiod or key lifetime.
(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.
(O) "Denial by one of the entities involved in a communication of having participated in all or part of the communication." [I7498 Part 2]
(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]
(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]
(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.
(I) Hierarchical PKI usage: The self-signed public-key certificate at the top of a certification hierarchy.
(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.)
(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.
(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.
(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.
(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.
(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.
(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.
(C) Salt protects a password-based access control system against a dictionary attack.
(C) This term applies to symmetric keys, private keys, and passwords.
(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.
(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.
(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.
(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.
(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.
(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.
(C) That is, a security kernel is an implementation of a reference monitor for a given hardware base.
(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]
(C) Some examples of security mechanisms are authentication exchange, checksum, digital signature, encryption, and traffic padding.
(C) An example is the Bell-LaPadula 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?
(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.
(C) In a self-signed X.509 public-key certificate, the issuer's DN is the same as the subject's DN.
(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.
(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.
(O) "Authentication by means of simple password arrangements." [X509]
(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.
(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.
(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.
(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.
(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.
(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]
(C) An example of a steganographic method is "invisible" ink. (See: digital watermark.)
(O) "Authentication by means of cryptographically derived credentials." [X509]
(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.
(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.
(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.
(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.
(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.
(C) The TLS protocol is misnamed, because it operates well above the transport layer (OSI layer 4).
(C) TLSP evolved directly from the SP4 protocol of SDNS.
(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.
(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.
(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]
(C) A URL is a URI that provides explicit instructions on how to access the named object. For example, "ftp://bbnarchive.bbn.com/foo/bar/picture/cambridge.zip" 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.
(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.
(C) Often verified by a password in an authentication process.
(C) ISDs should use this abbreviation only after using the full term at its first occurrence and defining the abbreviation.
(C) ISDs should use this abbreviation only after using the full term at its first occurrence and defining the abbreviation.
(C) ISDs should use this abbreviation only after using the full term at its first occurrence and defining the abbreviation.
(C) A VAN may also provide additional services, ranging from EDI format translation, to EDI-to-FAX conversion, to integrated business systems.
(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.
(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.
(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.)
(I) Cryptography usage: The estimated amount of computing time and power needed to break a cryptographic system.
(O) Erase electronically stored data by altering the contents of the data storage so as to prevent the recovery of the data. [FP140]