substitution cipher
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Key facts
- Field
- Cryptography
- First published
- Around 850 CE
- 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
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Article
In cryptography, a substitution cipher is a method of encrypting that creates the ciphertext (its output) by replacing units of the plaintext (its input) 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.