Atbash cipher

What the Atbash cipher is

Atbash is a monoalphabetic substitution cipher: each letter always maps to the same other letter. The mapping is the alphabet folded in half, which is why it is commonly called the mirror cipher. A pairs with Z, B with Y, C with X, and the pairs meet in the middle at M and N. Thirteen pairs cover all twenty-six letters, and every pair works in both directions, so the same table encodes a message and decodes it.

The cipher is ancient. The name comes from the Hebrew alphabet, where the first letter, aleph, is swapped with the last, tav, and the second, beth, with the second-to-last, shin: A-T-B-Sh gives “atbash.” Scholars commonly point to a few words in the Hebrew Bible as early examples of it. Applied to the Latin alphabet, it became a staple of puzzle books and of introductions to cryptography, usually as the first example of a cipher a reader can break with no key at all.

Because there is nothing to guess, the work is pure lookup. The reliable way to do it is to write the alphabet forwards on one line and backwards directly beneath it, so each ciphertext letter sits above its plaintext letter. Kids usually lose time in two ways: counting letters from the end of the alphabet in their head instead of writing the reversed line down, and losing their place in the middle of the alphabet, where the pairs L–O and M–N are easy to skip past. A written table removes both problems.

How to solve an Atbash cipher

Maps A↔Z, B↔Y, C↔X, etc. — every letter is mirrored across the middle of the alphabet. Often called the "mirror cipher" because "it doesn't matter which way you look at it" — encoding and decoding are the same operation.

1. Look up each ciphertext letter in the Reverse Alphabet table.
2. A↔Z · B↔Y · C↔X · D↔W · E↔V · F↔U · G↔T · H↔S · I↔R · J↔Q · K↔P · L↔O · M↔N.
3. Symmetric: applying Atbash twice gets you back to the original.

A worked example

Decode HXRVMXV

H → S (H is 8th from start, S is 8th from end)
X → C
R → I
V → E
M → N
X → C
V → E

Plaintext: SCIENCE

Atbash in Codebusters

On a Codebusters paper, Atbash is commonly the lowest-value question, and the organizers of some tournaments print the reversed alphabet on the resource sheet. Either way it is the question to answer first: there is no key to find and no inference to do, so a prepared student can decode a full sentence in under a minute and bank those points before moving on to the ciphers that take thought.

The scoring is usually by letter or by the whole message, often with a small allowance for errors. Mistakes on Atbash are almost always slips of the eye rather than of understanding, so the habit that pays is to decode in one steady pass with the table in view, then read the sentence back as English. If a word does not read, the most likely cause is a letter looked up from the wrong line of the table.

Atbash also serves as a warm-up for the Aristocrat, the larger substitution puzzle on most Codebusters papers. Both replace each letter with the same other letter every time; Atbash just tells you the mapping in advance. A student who is comfortable reading a substitution table quickly is halfway to the frequency-analysis work the Aristocrat asks for.

Practice the Atbash cipher

The lab below is set to the Atbash cipher alone. Pick a difficulty, generate a challenge, decode it, then check your answer or reveal the solution. It runs in your browser, needs no account, and is unlimited on every OlyPrep plan.

Where the Atbash cipher is tested

The Science Olympiad rule sets OlyPrep has read that name this cipher:

• Bay Area Elementary Science Olympiad — on the Codebusters list alongside Caesar, the two warm-up ciphers
• Science Olympiad Division B — on the Codebusters list
• Virginia Science Olympiad, Division A — on the Division A Codebusters list
• Macomb Elementary Science Olympiad (Michigan) — on the Codebusters list
• Milpitas Elementary Science Olympiad (MESO) — on the Codebusters list; the reversed alphabet is on the resource sheet

Bay Area ESO never asks teams to encode; five to ten of its twenty questions are multiple choice or fill-in on how the ciphers work.

Cipher lists change with each season’s rules; a tournament’s own rulebook is the authority on what it tests.