Editing Substitution cipher (section)

{{Short description|System to replace plaintext with ciphertext}}
{{More citations needed|date=March 2009}}
{{Infobox cryptographic technique
| name            = Substitution cipher
| caption         = [[ROT13]] is a [[Caesar cipher]], a type of substitution cipher. In ROT13, the alphabet is rotated 13 steps.
| field           = [[Cryptography]]
| first_published = Around 850 CE
| inventor        = [[Al-Kindi]]
| related_methods = [[Transposition cipher]], [[polyalphabetic cipher]], [[homophonic substitution cipher]], [[one-time pad]]
| key_size        = Varies (typically 88 bits for mixed alphabet simple substitution)
| cryptanalysis   = [[Frequency analysis]]
}}
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 cipher]]s. 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 AD. The method he described is now known as [[frequency analysis]].

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