Cipher

In cryptography, a cipher (or cypher) is an algorithm for performing encryption and decryption — a series of well-defined steps that can be followed as a procedure. In most cases, that procedure is varied depending on a key which changes the detailed operation of the algorithm. In non-technical usage, a "cipher" is the same thing as a "code"; however, the concepts are distinct in cryptography. In classical cryptography, ciphers were distinguished from codes, which operated by substituting according to a large codebook. The modern day technical usage of "code" is also different, referring to any systematic way of representing information, such as ASCII or Error correcting codes.

Most modern ciphers can be categorized in several ways:

By whether they work on blocks of symbols usually of a fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers).
By whether the same key is used for both encryption and decryption (symmetric key algorithms), or if a different key is used for each (asymmetric key algorithms). If the algorithm is symmetric, the key must be known to the recipient and to no one else. If the algorithm is an asymmetric one, the encyphering key is different from, but closely related to, the decyphering key. If one key cannot be deduced from the other, the asymmetric key algorithm has the public/private key property and one of the keys may be made public without loss of confidentiality.