Substitution cipher
A substitution cipher replaces plaintext units with other symbols while preserving their order.
Definition
A substitution cipher replaces each plaintext unit—usually a letter, but sometimes a pair or block—with another symbol according to a key. The units keep their positions; their identities change.
Main types
Monoalphabetic ciphers use one fixed replacement alphabet. Polyalphabetic ciphers switch among several alphabets, and polygraphic ciphers substitute groups of letters. Caesar and affine ciphers are monoalphabetic examples.
Cryptanalysis
Simple substitution preserves language patterns such as letter frequencies, repeated words, and common letter combinations. Enough ciphertext can therefore be attacked with frequency analysis and pattern matching. Classical substitution ciphers are educational, not secure for modern data.
Substitution changes the symbols but keeps their order; transposition rearranges the existing symbols without replacing them.
No. Polyalphabetic and polygraphic systems are also substitution ciphers, but their mapping varies or operates on groups.