Affine Cipher

The Affine cipher is a substitution cipher that combines multiplication and addition modulo 26. Encryption is E(x) = (ax + b) mod 26, where a must be coprime with 26 so the mapping is reversible. Only 12 valid values of a exist (1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25) — multiply that by the 26 choices for b and you get 312 possible keys, trivially brute-forceable.

Multiplier a (coprime with 26)

Offset b (0–25)

Encrypt: E(x) = (5 · x + 8) mod 26 · Decrypt: D(y) = 21(y − 8) mod 26

Plain text

Affine cipher

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Substitution Table

Brute Force All Keys

Click any row to load that key. 312 possible combinations — instant crack for short ciphertext.