Does Atbash need a key?

No. The alphabet is simply reversed, so the mapping is fixed and identical for everyone — and applying it twice restores the original text.

Why must a be coprime with 26?

Because otherwise the multiplication isn't invertible modulo 26: two different letters would map to the same ciphertext letter, making the cipher impossible to decode unambiguously.

How does the affine cipher relate to Caesar?

Caesar is the affine cipher with a = 1 — only the additive offset is used. Atbash is a = 25, b = 25. Both are special cases of the same formula.

Atbash & Affine Cipher Decoder

Decode Atbash and affine substitution ciphers with your choice of parameters.

live · client-sideClassical & Obfuscated Ciphersfree · no sign-up

RAW INPUT

Cipherab

DECODED

Output appears here as you type — decoded on your device.

Input is processed on your device. Nothing you paste is sent anywhere.

What is the Atbash & Affine Cipher Decoder?

Atbash maps each letter to its mirror image in the alphabet (A↔Z, B↔Y), while an affine cipher applies the formula (a × position + b) mod 26, generalising both Atbash and Caesar into one arithmetic substitution.

Atbash is one of the oldest ciphers known, originally applied to the Hebrew alphabet and appearing in biblical texts. It has no key at all: the alphabet is simply reversed, so the transformation is fixed and self-inverse — apply it twice and you are back where you started.

The affine cipher generalises the family. Each letter's position is multiplied by a and offset by b, modulo 26. The multiplier a must be coprime with 26 (1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25) or the mapping isn't reversible — two letters would collide. Set a = 1 and you have a Caesar cipher; set a = 25, b = 25 and you have Atbash.

With 12 valid multipliers and 26 offsets there are only 312 affine keys, so this cipher is brute-forceable in a fraction of a second and survives only in puzzles.

Atbash & Affine Cipher Decoder at a glance

Atbash: A↔Z, B↔Y — keyless and self-inverse
Affine formula: E(x) = (a·x + b) mod 26
Valid a values: Must be coprime with 26 — 12 choices
Total affine keys: 312 — trivially brute-forced

How to use the Atbash & Affine Cipher Decoder

1Choose Atbash for the reversed alphabet, or Affine to set your own a and b.
2For affine, enter a (coprime with 26) and b, then paste the ciphertext.
3Read the plaintext and adjust the parameters if it doesn't resolve.

When you would reach for it

Solving an Atbash-encoded clue in a puzzle hunt or CTF.
Exploring how modular arithmetic underlies classical substitution ciphers.
Testing a handful of affine keys against a short ciphertext.

Atbash & Affine Cipher Decoder — frequently asked questions

Last reviewed 25 July 2026 · questions or corrections: contact@decoder.tools