All of the cypher systems we have looked at so far have been single-key character cyphers. By this, we mean that the same key which is used to encrypt the plaintext into the ciphertext is used to recover the plaintext from . . .
All of the cypher systems we have looked at so far have been single-key character cyphers. By this, we mean that the same key which is used to encrypt the plaintext into the ciphertext is used to recover the plaintext from the ciphertext, and the cypher operates on only one character at a time. In these discussions the plaintext character space is taken to be the ASCII character set, as is the ciphertext character space. The key space is not necessarily the ASCII character set, but is a function, such that p=D(C(p)), where p is the plaintext, C is the encryption algorithm and key, and D is the decryption algorithm and key. Clearly, D is the inverse operation of C.

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