A cipher meaning an algorithm used as a means of performing encryption or decryption is a series of well-defined steps that can be followed as a procedure. A less commonly used term used for this is encipherment. To encipher or encode means to convert a certain set of information into cipher or code. In common parlance, the word cipher is synonymous with code, since they are both a set of steps that encrypt a message. However, these concepts are distinct in cryptography, especially classical cryptography.
Codes are used to typically substitute different length strings of character in the output, while ciphers generally substitute the same number of characters as the input. There are exceptions to this and some cipher systems may also use slightly more, or fewer, characters when output versus the number that was input.
The operation of a cipher depends on a piece of auxiliary information, known as a key or, in traditional NSA parlance, a crypto variable. The encrypting procedure varies depending on the key, which in turn changes the operation of the algorithm. A key needs to be selected before the use of a cipher to encrypt a message. If the key is not known, it should be extremely difficult, if not impossible, to decrypt the resulting ciphertext into readable plain text.
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