Atbash & Affine Substitution Cipher
Mirror-alphabet and modular substitution ciphers.
About this tool
Atbash mirrors the alphabet (A⇄Z, B⇄Y) — a scheme old enough to appear in the Book of Jeremiah. The affine cipher generalizes it: each letter maps through ax + b (mod 26), where a must be coprime with 26 for the cipher to be reversible. Atbash is the special case a=25, b=25.
Like everything on encrypt.tools, this runs entirely in your browser — nothing you enter is transmitted or stored.
Frequently asked questions about Atbash & Affine
What is the Atbash cipher?+
A substitution that mirrors the alphabet — A becomes Z, B becomes Y, and so on. It's ancient, appearing in the Hebrew Bible, and is its own inverse.
What must 'a' be in the affine cipher?+
The multiplier 'a' must be coprime with 26 (1,3,5,7,9,11,15,17,19,21,23,25) so the cipher is reversible. The tool rejects invalid values.
How does Atbash relate to affine?+
Atbash is the affine cipher with a=25, b=25 — a special case of the general ax+b (mod 26) mapping.
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