Caesar Cipher Encoder / Decoder

The Caesar cipher shifts every letter a fixed number of places along the alphabet — the oldest trick in the book, named after Julius Caesar. This tool encodes, decodes and, when you do not know the shift, breaks it automatically by testing all 26 possibilities against how often each letter appears in English.

How to use this tool

1. Type or paste your text.
2. Choose Encode, Decode, or Break to crack an unknown shift.
3. Set the shift (A→D is a shift of 3).
4. Optionally rotate digits too. Case and punctuation are always kept.

Formula

Each letter at position x (0–25) becomes (x + shift) mod 26 to encode, and (x − shift) mod 26 to decode. Breaking ranks every shift by the chi-squared distance between the result’s letter frequencies and typical English.

The chi-squared score is lowest when the decrypted text’s letter mix looks most like ordinary English, so the correct shift usually sits at the top of the list.

Example

"HELLO" with a shift of 3 becomes KHOOR. Breaking the sentence "WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ DQG WKHQ UXQV KRPH" ranks shift 3 first, revealing "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG AND THEN RUNS HOME". On a fragment of only a few words the frequency ranking can pick the wrong shift, so scan the whole table.