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Cipher |
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It has been suggested that Encipherment be merged into this article or section. (Discuss) |
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. An alternative term is encipherment. 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. 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; it should resemble random gibberish to those not intended to read it.
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 difficult, if not nearly impossible, to decrypt the resulting cipher into readable plaintext.
Most modern ciphers can be categorized in several ways:
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“Cipher” (Middle French as cifre and Medieval Latin as cifra, from the Arabic صفر ṣifr = zero) is alternatively spelled “cypher” (however, this variant is now uncommon); similarly “ciphertext” and “cyphertext”, and so forth.
The word “cipher” in former times meant “zero” and had the same origin (see Zero — Etymology), and later was used for any decimal digit, even any number. There are these theories about how the word “cipher” may have come to mean encoding:
Dr. Al-Kadi (ref-3) concluded that the Arabic word sifr, for the digit zero, developed into the European technical term for encryption.
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 meaning — that is, words or phrases are converted into something else and this chunking generally shortens the message.
An example of this is the Telegraph Code which were used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of Telegrams.
Ciphers, on the other hand, work at a lower level: the level of individual letters, small groups of letters, or, in modern schemes, individual bits. Some systems used both codes and ciphers in one system, using superencipherment to increase the security. In some cases the terms codes and ciphers are also used synonymously to substitution and transposition.
Historically, cryptography was split into a dichotomy of codes and ciphers; and coding had its own terminology, analogous to that for ciphers: “encoding, codetext, decoding” and so on.
However, codes have a variety of drawbacks, including susceptibility to cryptanalysis and the difficulty of managing a cumbersome codebook. Because of this, codes have fallen into disuse in modern cryptography, and ciphers are the dominant technique.
There are a variety of different types of encryption. Algorithms used earlier in the history of cryptography are substantially different from modern methods, and modern ciphers can be classified according to how they operate and whether they use one or two keys.
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include simple substitution ciphers and transposition ciphers. For example “GOOD DOG” can be encrypted as “PLLX XLP” where “L” substitutes for “O”, “P” for “G”, and “X” for “D” in the message. Transposition of the letters “GOOD DOG” can result in “DGOGDOO”. These simple ciphers and examples are easy to crack, even without plaintext-ciphertext pairs.
Simple ciphers were replaced by polyalphabetic substitution ciphers which changed the substitution alphabet for every letter. For example “GOOD DOG” can be encrypted as “PLSX TWF” where “L”, “S”, and “W” substitute for “O”. With even a small amount of known or estimated plaintext, simple polyalphabetic substitution ciphers and letter transposition ciphers designed for pen and paper encryption are easy to crack.
During the early twentieth century, electro-mechanical machines were invented to do encryption and decryption using transposition, polyalphabetic substitution, and a kind of “additive” substitution. In rotor machines, several rotor disks provided polyalphabetic substitution, while plug boards provided another substitution. Keys were easily changed by changing the rotor disks and the plugboard wires. Although these encryption methods were more complex than previous schemes and required machines to encrypt and decrypt, other machines such as the British Bombe were invented to crack these encryption methods.
Modern encryption methods can be divided by two criteria: by type of key used, and by type of input data.
By type of key used ciphers are divided into:
In a symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption. In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform correct decryption.
Type of input ciphers data can be distinguished into two types:
In a pure mathematical attack (i.e., lacking any other information to help break a cipher), three factors above all, count:
Since the desired effect is computational difficulty, in theory one would choose an algorithm and desired difficulty level, thus decide the key length accordingly.
An example of this process can be found at Key Length which uses multiple reports to suggest that a symmetric cipher with 128 bits, an asymmetric cipher with 3072 bit keys, and an elliptic curve cipher with 512 bits, all have similar difficulty at present.
Claude Shannon proved, using information theory considerations, that any theoretically unbreakable cipher must have keys which are at least as long as the plaintext, and used only once: one-time pad.
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