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Caesar / Shift cipher

Caesar cipher decoder

Decode a Caesar-shifted message with a known key, or compare every possible shift when the key is unknown.

Local processing: your text stays in this browser. This tool makes no network requests and stores nothing.

01 / Workspace

Decode ciphertext

Choose a workflow

A shift here means the forward A-to-Z shift originally used to encrypt the plaintext.

0 / 2,000 code points

Only ASCII letters A-Z and a-z are shifted. Case, spacing, line breaks, digits, punctuation, emoji, accents, and other scripts are preserved.

Enter an integer from 0 through 25. Decoding subtracts this key, so ciphertext D with key 3 becomes plaintext A.

Ready to decode.

02 / Result

Decoded text

No result yet

Known-shift mode fills this result directly. In unknown-shift mode, select a candidate below to load its full text.

03 / Guide

How decoding works

  1. Use the encryption key. If plaintext A was shifted forward by 3 to ciphertext D, enter 3. The decoder moves every ciphertext letter back by 3.
  2. Try every key when needed. Unknown-shift mode computes keys 0 through 25 in numeric order. Inspect the previews, then select the candidate that fits your context.
  3. Read modulo 26. With A as 0, plaintext is ((ciphertext - key) % 26 + 26) % 26. The added 26 makes the JavaScript remainder non-negative before the final modulo.

Known checks

Key 3
KHOOR decodes to HELLO.
Key 13
Uryyb, Jbeyq! decodes to Hello, World!.
Keys 0 and 25
Abc stays Abc with 0; Zab decodes to Abc with 25.

Limits and security

  • This tool implements the conventional 26-letter ASCII alphabet only. It preserves letter case and copies every non-ASCII character unchanged.
  • Browser text fields normalize CR and CRLF line endings to LF. The tool does not transliterate accented letters or shift digits.
  • Input is limited to 2,000 Unicode code points. Unknown-shift mode bounds its projected work to 52,000 code points and shows compact previews; selecting a row reveals that candidate in full.
  • The tool does not rank languages or guarantee that a readable candidate is correct. Very short text can have several plausible decodings, and no readable candidate may mean the input is not a Caesar cipher.
  • Classical ciphers are educational and not secure. Do not use Caesar cipher output to protect passwords, personal data, or other sensitive information.