Affine Cipher

About this calculator

This tool encrypts and decrypts text with the Affine cipher, a substitution cipher defined by E(x) = (a·x + b) mod 26, where x is a letter's numeric position (A=0…Z=25), a and b are the encryption keys, and the result maps back to a letter. Because it needs modular division to decrypt, key a must be coprime with 26 (share no common factor besides 1) — valid choices are 1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25.

It's a classic teaching example in introductory cryptography courses because it generalizes both the Caesar cipher (when a=1) and simple multiplicative ciphers, while showing why modular arithmetic and coprimality matter for invertibility. Enter your text and the two keys a and b to encode or decode a message.