# Substitution Cipher

- ==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)
- [[!Beale ciphers]] - A [[!Homophonic substitution]] as a Book cipher variant based on the Declaration of Independence.
- [[!Enigma machine]] - A [[!Rotor cipher machine]] making use of a changing [[!Polyalphabetic cipher]] substitution.

#### References

#### Explore

- Tags: #cryptography #bud
- Linked: Types of ciphers
- Root: Book cipher

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