==A method of [[!encryption]] where units of [[!plaintext]] are replaced with [[!Ciphertext]] according to a set of rules or fixed system.==
'Units' may be single letters, pairs, a mixture, or any combination that can be encoded by the system.
==Unlike a Transposition cipher, where [[!plaintext]] units are rearranged but unchanged, in a Substitution cipher units retain their order/sequence in [[!Ciphertext]] but are altered.==
Decode by performing the inverse substitution.
Substitution Method
Traditionally the [[!Ciphertext]] is written in blocks of fixed length with no spaces or punctuation to disguise word boundaries from the [[!plaintext]]. These blocks are called 'groups'.
[[!Ciphertext]] need not be the same as the [[!plaintext]] alphabet.
Rotated Alphabet: Simple systems rotate the alphabet a number of steps like the [[!Caesar cipher]].
Mixed Alphabet: ==A [[!keyword]] functions as the Cipher key.== Write the keyword at the start and then all the remaining letters in the alphabet in order after it.
Types of Substitution Ciphers
[[!Simple substitution cipher]]: operates on a single letter.
[[!Polygraphic substitution]]: operates on larger groups of letters.
Types of Substitution Systems
[[!Monoalphabetic cipher]]: uses the same fixed substitutions for the whole message.
[[!Polyalphabetic cipher]]: uses multiple cipher alphabets so different substitutions can be used at different points in the message. A unit of [[!plaintext]] is mapped to one of multiple possible options in [[!Ciphertext]] and vice versa.
[[!Nomenclator]]: Variant in 1400-1700s with a code table.
[[!Homophonic substitution]] disguise [[!plaintext]] frequency with [[!Homophony]]. [[!plaintext]] letters map to more than one [[!Ciphertext]] symbol (or even more for high frequency letters).
Security
==A weak [[!Cipher]] that is easily broken by frequency analysis==, even though there are many possible keywords.
In the [[!keyword]] method, less used letters at the end of the alphabet stay at the end.
Examples
[[!Caesar cipher]] - Classic 'ROT13' where each letter in the alphabet is replaced by the 13th letter after it.
[[!Great Cipher]] - A [[!Nomenclator]] developed by the Rossignols (read more)