Atbash Cipher
A becomes Z, B becomes Y. Encoding and decoding are identical.
The alphabet, backwards
Atbash maps each letter to its mirror image: A to Z, B to Y, C to X, and so on to the middle, where M and N swap with each other.
Like ROT13, it is its own inverse — apply it twice and the text returns. Unlike ROT13, it has no key at all, not even a shift number. There is exactly one Atbash cipher.
Older than Caesar by several centuries
The name is Hebrew and describes the method: aleph-tav-beth-shin — the first letter paired with the last, the second with the second-last. It appears in the Hebrew Bible, in the Book of Jeremiah, where "Sheshach" is Atbash for "Babel".
Whether that was concealment or wordplay is debated by scholars. Given that the method was known to anyone literate in the tradition, it probably served more as a rhetorical device than as a secret.
It is a substitution cipher, so it breaks the same way
Atbash preserves letter frequencies exactly, just relabelled. In English, the commonest letter E always becomes V, so V appears about 12.7 % of the time — wildly unlike real English, where V is under 1 %.
That signature is unmistakable. Anyone doing frequency analysis spots Atbash immediately, and the fixed mapping means there is nothing to search for once recognised.
Where you will meet it
- Puzzle hunts, often as one layer among several.
- Biblical scholarship and the study of ancient texts.
- Teaching, as the simplest possible substitution cipher.
- Fiction, where it turns up regularly as a code to be cracked.
Common uses
- Decoding a puzzle that has resisted a Caesar shift.
- Reading an Atbash passage in a historical text.
- Making a quick code that anyone can reverse.
Questions people ask
How does Atbash work?
Each letter maps to its mirror in the alphabet: A to Z, B to Y, and so on. M and N swap in the middle.
How do I decode Atbash?
Apply it again. The mapping is symmetrical, so it is its own inverse.
How old is it?
Older than the Caesar cipher. It appears in the Hebrew Bible, where 'Sheshach' in Jeremiah is Atbash for 'Babel'.
Does it have a key?
No. There is exactly one Atbash cipher, which is part of why it offers no security.
How is it recognised?
By its frequency signature. E always becomes V, so a text with far too many Vs is almost certainly Atbash.
None of this is security
The ciphers on this site are historical. Every one of them was broken centuries ago, and a modern computer breaks them in microseconds — the Caesar page shows the break happening rather than just describing it.
They are worth knowing for puzzles, for teaching, for reading old texts and for understanding where cryptography came from. They are not worth using to protect anything. If you need something kept secret, you need modern encryption with a properly managed key, and you should not be building it yourself.
The encodings — Morse, binary, hex, NATO — make no secrecy claim at all. They exist to move information over a channel, and anyone can read them.
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