In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryptiona series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. 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. <\/p>\n
Codes generally substitute different length strings of characters in the output, while ciphers generally substitute the same number of characters as are input. There are exceptions and some cipher systems may use slightly more, or fewer, characters when output versus the number that were input. <\/p>\n
Codes operated by substituting according to a large codebook which linked a random string of characters or numbers to a word or phrase. For example, \"UQJHSE\" could be the code for \"Proceed to the following coordinates.\" When using a cipher the original information is known as plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it. <\/p>\n
The operation of a cipher usually depends on a piece of auxiliary information, called a key (or, in traditional NSA parlance, a cryptovariable). The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be extremely difficult, if not impossible, to decrypt the resulting ciphertext into readable plaintext. <\/p>\n
Most modern ciphers can be categorized in several ways <\/p>\n
\"Cipher\" is alternatively spelled \"cypher\"; similarly \"ciphertext\" and \"cyphertext\", and so forth. <\/p>\n
The word \"cipher\" in former times meant \"zero\" and had the same origin: Middle French as cifre and Medieval Latin as cifra, from the Arabic ifr = zero (see ZeroEtymology). \"Cipher\" was later used for any decimal digit, even any number. There are many theories about how the word \"cipher\" may have come to mean \"encoding\":It was firstly introduced by Ab Abdallh Muammad ibn Ms al-Khwrizm. <\/p>\n
Ibrahim Al-Kadi concluded that the Arabic word sifr, for the digit zero, developed into the European technical term for encryption.[1] <\/p>\n
In non-technical usage, a \"(secret) code\" typically means a \"cipher\". Within technical discussions, however, the words \"code\" and \"cipher\" refer to two different concepts. Codes work at the level of meaningthat is, words or phrases are converted into something else and this chunking generally shortens the message. <\/p>\n
An example of this is the Telegraph Code which was used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of Telegrams. <\/p>\n
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