⌘ K
atbash · mirror alphabet · self-inverse · live

Apply the Atbash cipher

Mirror the alphabet so A becomes Z, B becomes Y, and so on. One of the oldest known substitution ciphers, still common in puzzles. All processing runs locally.

Atbash Cipher
Input
11 B
1 lines
Output
11 B
1 lines
Status
ready
live
Ratio
1.00×
out/in
input · text
output · atbash
SVOOL DLIOW
Quick Start

How to use Atbash

Atbash reverses the alphabet. A maps to Z, B maps to Y, all the way through. Because the mapping is a reflection, encoding and decoding are identical.

  1. 01

    Type or paste text

    Only letters are transformed; digits and punctuation are kept verbatim.

    Bytes flow through TextEncoder/TextDecoder for predictable UTF-8 semantics — emoji, surrogate pairs, and combining marks all encode losslessly.

    inputUTF-8TextEncoderlosslesslive
    input · raw bytestext · UTF-8
    HELLO WORLD
    loaded · UTF-8MIME · ready
  2. 02

    Mirror the alphabet

    A↔Z, B↔Y, C↔X ... case is preserved.

    Hash and signature operations call crypto.subtle from the Web Crypto API, so output matches OpenSSL / Node bit-for-bit.

    mirrorWeb Cryptosubtledeterministiclive
    map · optionsA↔Z · live
    A B C ... X Y Z
Z Y X ... C B A
    options applied · livelive
  3. 03

    Live preview

    Output updates as you type. The same operation reverses it.

    Options re-run instantly through useEffect + useMemo — switching URL-safe mode or padding doesn’t re-encode from scratch.

    livememoisedoptionsinstantreactive
    out · encodedatbash · ready
    SVOOL DLIOW
    encoded · livelossless
  4. 04

    Copy or download

    One click to clipboard. Download as .txt.

    Copy uses the Async Clipboard API; downloads ship via Blob with the appropriate MIME (text/plain, image/png, …) so external tools recognise them.

    exportclipboardblobMIME-correctclient-side
    ok · copiedready · done
    Copied
    shipped · ready to shipsuccess
Pro tips

Encoding without surprises

Always UTF-8 first

Use TextEncoder / TextDecoder for bytes ↔ string conversion. btoa alone breaks on non-ASCII.

URL-safe vs standard Base64

Standard uses +, /, =; URL-safe swaps to -, _, no padding. Don’t mix them.

Hashes aren’t passwords

SHA-256 is fast; that’s good for fingerprints, bad for passwords. Use bcrypt / argon2 for credentials.

Watch padding rules

Some decoders are strict about = padding, others auto-pad. If round-trips fail at exactly 1 or 2 bytes, check padding.

Use Web Crypto for keys

Generate cryptographic keys via crypto.subtle, never Math.random. The latter is predictable.

JWT decode ≠ verify

Decoding a JWT just splits and base64-decodes. Verification needs the issuer’s public key / shared secret.

Related toolsROT13Caesar CipherSubstitution CipherAffine CipherVigenere CipherBacon Cipher
Use Cases

When to use Atbash

Puzzles, escape rooms, beginner crypto.

Crypto puzzles

Frequent in escape rooms and ARG-style hunts. Output round-trips losslessly with the matching decoder — both run client-side, so end-to-end testing happens in a single tab. Encoding flows through TextEncoder for predictable UTF-8 semantics.

RFC-compliant

CTF warmups

Capture-the-flag intros use Atbash for the first challenge. Web Crypto's subtle.digest produces output that matches OpenSSL, GPG, and Node's crypto module byte-for-byte. That makes the result trustworthy as a content fingerprint, ETag, or integrity check.

OpenSSL-matched

Historical study

Originally used for Hebrew scriptures — useful for analysis. Pairs cleanly with HMAC, JWT, and signed-URL workflows where bit-level correctness matters. The browser's crypto stack is FIPS 140-validated in Chrome and Firefox, so it's production-grade.

lossless

Learning crypto

Concrete example of a reflective substitution. Output uses canonical RFC 4648 / 7515 alphabets so curl, openssl, and any standards-compliant decoder accept it as input. URL-safe variants drop padding for tokens that travel in query strings.

byte-identical

Quick obfuscation

Hide a punchline or spoiler in plain sight. Heavy hashing runs through useMemo so large inputs don't block typing. PBKDF2, bcrypt, and Argon2 expose configurable cost factors — tune for your threat model, not the default.

spec-grade

Brute checks

Verify other Atbash implementations on the wire. Same input plus same options always yields identical bytes. That's the foundation for content-addressed storage (IPFS, Git, S3 ETags) and means the output is safe to commit to fixtures.

production-ready
FAQ

Frequently asked questions

Because the mapping reflects the alphabet, applying it twice maps each letter back to itself.
No — lowercase input produces lowercase output, uppercase produces uppercase. Other characters pass through.
ROT13 shifts every letter 13 positions. Atbash mirrors the alphabet — A↔Z, B↔Y — which is non-uniform.
No. Like all monoalphabetic substitutions, Atbash falls instantly to frequency analysis.
No. Encoding runs locally in your browser.
Suggest a tool

Something missing?

Tell us what you need. We ship the top requests every two weeks.

Sent straight to the maintainers · no account needed
47 in queue·8 shipped this month
Request queued. Thanks!