About this tool
The Caesar Cipher Encoder / Decoder is a free, in-browser tool for the oldest and most famous substitution cipher. Each letter is shifted a fixed number of places along the alphabet: encoding maps a letter at position x to (x + shift) mod 26, and decoding reverses it with (x − shift + 26) mod 26. Upper- and lower-case bands are shifted independently and any non-letter — spaces, digits, punctuation — passes through unchanged. A shift of 3 is the shift Julius Caesar himself is said to have used; a shift of 13 is the self-inverse ROT13.
Everything runs locally in your browser, so nothing you type is uploaded or stored. Pick encode or decode and set a shift from 0 to 25, or switch to brute-force mode to see all 26 possible decryptions at once. Because there are only 25 non-trivial keys, a Caesar cipher is trivially breakable by trying them all — which is exactly what brute-force mode does.
To save you scanning, brute-force mode scores each candidate with a chi-square test against standard English letter frequencies and marks the most likely plaintext with a star (★). The lower the chi-square value, the closer the letter distribution matches ordinary English, so the starred row is usually the correct decryption. This makes the tool handy for CTF challenges, escape rooms, and classroom cryptography without needing to know the key in advance.