Atbash Cipher
Pair the alphabet with itself backwards: A with Z, B with Y, and so on.
What is the Atbash Cipher?
Atbash uses one fixed substitution alphabet: the normal alphabet on top and the alphabet in reverse underneath.
It is reciprocal, which means the same operation encrypts and decrypts. If A becomes Z, then Z becomes A.
The cipher mechanics on this page are self-contained. Science Olympiad event formats, allowed variants, and tournament constraints can change by season; the current official Rules Manual and official clarifications take precedence.
No outside reference is assumed. Work through Foundations → Complete Reference → Encryption → Decryption in order, then use the competition and cryptanalysis sections.
What you need to know
- Memorize or reference the reversed alphabet row.
- The mapping never changes from problem to problem.
- Because it is monoalphabetic, repeated-letter and word patterns are preserved.
Beginner glossary
| Term | Meaning |
|---|---|
| Fixed substitution | A substitution whose mapping never changes from puzzle to puzzle. |
| Reciprocal | The same mapping performs both encryption and decryption. |
| Mirror pair | Two letters equally far from opposite ends of the alphabet, such as A/Z or B/Y. |
Keep this beside you while solving
| Plain | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Cipher | Z | Y | X | W | V | U | T | S | R | Q | P | O | N | M | L | K | J | I | H | G | F | E | D | C | B | A |
Useful pair shortcuts
| Pair | Pair | Pair | Pair |
|---|---|---|---|
| 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 | N ↔ M | O ↔ L | P ↔ K |
How encryption works
Write the normal alphabet
A through Z.
Write the reversed alphabet
Z through A directly underneath.
Replace each letter
Use the letter directly opposite the plaintext letter.
Keep separators
Spaces can remain exactly where they are.
How decryption works
Use the same table
There is no separate decryption key.
Find the ciphertext letter
Look in either row.
Read its partner
The opposite-row letter is the plaintext.
Check language
The entire result should form readable text.
How to approach it in Codebusters practice
- Atbash should be recognized quickly from its fixed alphabet relationship.
- If the cipher type is not named, pairs such as A↔Z and B↔Y are strong identifiers.
- Since there is no changing key, speed comes from familiarity with the pairings.
What the problem gives you vs. what you produce
| Part | What to expect |
|---|---|
| You may be given | Ciphertext identified as Atbash, or an unknown fixed substitution that reveals reversed-alphabet pairs. |
| You must find | The plaintext. |
| Fastest first move | Use the full reversed alphabet table; there is no key to calculate. |
How to attack an unknown or partially known key
- Atbash is not secure cryptographically because its mapping is fixed and public.
- A few known pairs are enough to identify it.
- Pattern analysis works exactly as it does for any other monoalphabetic substitution.
Follow one example from start to finish
GSV WZROB XRKSVI
G→T, S→H, V→E
gives THE.
THE DAILY CIPHER
Before moving on, make sure you can answer these without another site:
- Can you map A, H, M, and Z without guessing?
- Can you explain why applying Atbash twice returns the original message?
Common mistakes
Using a Caesar shift instead of a reversed alphabet.
Forgetting that Atbash is reciprocal.
Reversing the word order instead of substituting individual letters.
Competition speed strategies
Learn the outer pairs A/Z, B/Y, C/X and work inward.
Keep the two alphabet rows visible on a resource sheet.
Decode short words first to confirm the cipher type.
What to remember under time pressure
See every transformation
Use the lab to change inputs and keys, keep the relevant reference material visible, inspect each intermediate transformation, and then read the “How to reverse it” panel so encryption and decryption connect.