Atbash cipher encoder and decoder

How it works

The Atbash cipher pairs the first letter of the alphabet with the last, the second with the second-to-last, and so on. In the English alphabet, A maps to Z, B maps to Y, C maps to X, and M maps to N. The remaining letters use the same mirror pattern in reverse.

Atbash does not shift every letter by one constant amount. Instead, the amount changes across the alphabet: A travels 25 positions, B travels 23, and the center pair M and N exchange places. You can picture the normal alphabet written above a reversed alphabet and substitute vertically between the two rows.

The mapping is self-inverse. If H maps to S during encoding, S maps back to H during decoding. That is why the Encode and Decode buttons use the same calculation. They remain visible so the interface matches the other cipher tools and makes the user's intention clear.

This implementation normalizes letters to uppercase A–Z. It removes spaces and punctuation by default. Enable Preserve spaces and punctuation when you want the original sentence structure to remain visible while only the letters are replaced.

Worked example

Encode HELLO by finding each letter's partner in the reversed alphabet. A has position 1 and maps to position 26; position 8 therefore maps to position 19.

1. Write the normal alphabet: Use A through Z as the source row.
2. Reverse the alphabet: Place Z through A beneath it to create mirror pairs.
3. Substitute each letter: Map H to S, E to V, L to O, and O to L.
4. Read the result: The mapped letters combine to form SVOOL.

The final letters form SVOOL. Decode with the same table: S maps to H, V maps to E, O maps to L, and L maps to O. The result returns to HELLO without a key.

History and origin

The name Atbash comes from the first and last pairs of the Hebrew alphabet: Aleph with Tav, then Bet with Shin. Examples of this reversed-alphabet substitution are traditionally identified in the Hebrew Bible, including coded names in the Book of Jeremiah. The historical system operates on Hebrew letters rather than the modern Latin alphabet shown here.

Later writers applied the same mirror principle to other alphabets. The English version is now common in recreational cryptography, puzzle hunts, classroom exercises, and geocaching. Its appeal comes from the perfectly symmetrical substitution table: no keyword or arithmetic equipment is required.

How to break it

Atbash is not secure by modern standards. It has no key and only one possible mapping for a given alphabet. Anyone who suspects Atbash can apply the reversed alphabet once and recover the message. The symmetry that makes it convenient also removes any uncertainty for an attacker.

Even when the method is not immediately recognized, it behaves like a monoalphabetic substitution. The same plaintext letter always becomes the same ciphertext letter, repeated patterns remain visible, and common English letter frequencies survive under different labels. Word shapes are especially revealing when spaces are preserved.

Atbash is appropriate for historical demonstrations, games, riddles, and visual explanations of substitution. It should never protect passwords, financial details, personal records, or confidential messages. Modern encryption uses secret keys and far larger mathematical key spaces.

Related ciphers

• The Caesar cipher uses a constant rotation, while Atbash mirrors positions across the center of the alphabet.
• ROT13 is another self-inverse substitution, created by rotating every letter exactly halfway around the alphabet.
• The cipher tools directory compares these classical substitution methods and their relationships.

Frequently asked questions

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