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In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing the inverse substitution process to extract the original message.
Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the units of the plaintext are rearranged in a different and usually quite complex order, but the units themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered.
There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of letters is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa.
The first ever published description of how to crack simple substitution ciphers was given by Al-Kindi in A Manuscript on Deciphering Cryptographic Messages written around 850 CE. The method he described is now known as frequency analysis.
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different types of substitutioncipher. If the cipher operates on single letters, it is termed a simple substitutioncipher; a cipher that operates on larger...
message, so the cipher is classed as a type of monoalphabetic substitution, as opposed to polyalphabetic substitution. The Caesar cipher is named after...
polyalphabetic cipher is a substitution, using multiple substitution alphabets. The Vigenère cipher is probably the best-known example of a polyalphabetic cipher, though...
ciphers used in the past are sometimes known as classical ciphers. They include simple substitutionciphers (such as ROT13) and transposition ciphers...
Napoleon cipher, and tic-tac-toe cipher) is a geometric simple substitutioncipher, which exchanges letters for symbols which are fragments of a grid...
Playfair cipher or Playfair square or Wheatstone–Playfair cipher is a manual symmetric encryption technique and was the first literal digram substitution cipher...
Hill cipher is a polygraphic substitutioncipher based on linear algebra. Invented by Lester S. Hill in 1929, it was the first polygraphic cipher in which...
between transposition and substitution operations, they are often combined, as in historical ciphers like the ADFGVX cipher or complex high-quality encryption...
in Sweden, found the cipher to be an encrypted German text. The manuscript is a homophonic cipher that uses a complex substitution code, including symbols...
one of three categories of cipher used in classical cryptography along with substitutionciphers and transposition ciphers. In classical cryptography...
The affine cipher is a type of monoalphabetic substitutioncipher, where each letter in an alphabet is mapped to its numeric equivalent, encrypted using...
Classical ciphers are often divided into transposition ciphers and substitutionciphers, but there are also concealment ciphers. In a substitutioncipher, letters...
Caesar cipher and one-time pad) Polyalphabetic substitutioncipher: a substitutioncipher using multiple substitution alphabets (e.g., Vigenère cipher and...
Beaufort cipher, invented by some Giovanni Sestri in early 18th century but widely attributed to Sir Francis Beaufort, is a substitutioncipher similar...
In classical cryptography, the running key cipher is a type of polyalphabetic substitutioncipher in which a text, typically from a book, is used to provide...
The Enigma machine is a cipher device developed and used in the early- to mid-20th century to protect commercial, diplomatic, and military communication...
Atbash (Hebrew: אתבש; also transliterated Atbaš) is a monoalphabetic substitutioncipher originally used to encrypt the Hebrew alphabet. It can be modified...
potential to be exploited in a ciphertext-only attack. In a simple substitutioncipher, each letter of the plaintext is replaced with another, and any particular...
provide him with an important clue—he realizes that they form a substitutioncipher and cracks the code by frequency analysis. The last of the messages...
cryptography, a block cipher is a deterministic algorithm that operates on fixed-length groups of bits, called blocks. Block ciphers are the elementary building...
are used: for instance, a cipher in which pairs of letters are substituted is bigraphic. As a concept, polygraphic substitution contrasts with monoalphabetic...
This method was used in the second Beale cipher. This variant is more properly called a substitutioncipher, specifically a homophonic one. Both methods...
advance positions, changing the substitution. By this means, a rotor machine produces a complex polyalphabetic substitutioncipher, which changes with every...
(or secret decoder) is a device that allows one to decode a simple substitutioncipher—or to encrypt a message by working in the opposite direction. As...
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