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The Affine cipher uses E(x) = (a × x + b) mod 26 on A–Z letters. The multiplier a must be coprime with 26 for decryption. Case is preserved.
Affine cipher tool
The Affine cipher extends Caesar substitution. Each A–Z letter becomes a number from 0 to 25, transformed by E(x) = (a × x + b) mod 26. Decryption uses the modular inverse of a.
There are 12 valid multipliers and 26 shifts, giving 312 possible keys. With a = 5 and b = 8, AFFINE CIPHER becomes IHHWVC SWFRCP. Brute force ranks potential plaintexts heuristically.