Caesar Cipher Encoder / Decoder

How to use

1. 1Pick a direction: Encode shifts plain text forward, Decode shifts cipher text backward, and Brute force tries every shift from 0 to 25 at once.
2. 2Set the shift with the slider, the number box, the quick preset buttons (1, 3, 5, 7, 13, 25), or the dedicated ROT13 button.
3. 3Paste your text into the input box. Letters shift, while digits, spaces, and punctuation pass through untouched.
4. 4Read the result panel for encode and decode, or read the ranked candidate list in brute force mode where the best English match is highlighted at the top.
5. 5Click Copy result, Copy on any brute-force row, or Use as input to round-trip the result back into the input box.

About this tool

Caesar Cipher Encoder / Decoder is a complete, browser-only toolkit for the classic shift cipher. Pick any shift from 0 through 25 and the tool maps every letter forward (encode) or backward (decode) by that many positions while leaving digits, spaces, and punctuation untouched. Case is preserved automatically, so 'Hello' becomes 'Khoor' at shift 3, not 'KHOOR'. A live alphabet map below the controls shows the exact A-to-Z translation that the current shift produces, with shifted letters highlighted, so you can sanity-check the wiring before you hit copy. Quick presets jump straight to the shifts you actually use in the wild: shift 1 (the gentlest puzzle), shift 3 (Caesar's original), shift 5 and 7 (common CTF and homework variants), shift 13 (ROT13, the symmetric encode-and-decode), and shift 25 (effectively a back-shift of 1). The dedicated ROT13 button does the same job in one click. Brute-force mode is the standout feature when you have ciphertext and no key. The tool computes all 26 candidate decodings at once and ranks them by chi-squared distance to the expected English-letter frequency distribution. Real plaintext lands at the top almost every time and is highlighted as the best guess, so you can read the answer instead of squinting at 26 alternative grids. Each candidate row shows its shift, its chi-squared score, the decoded text in a copyable mono-spaced block, and a copy button so you can grab the right one without selecting text by hand. The input box stays in place across modes, so switching between encode, decode, and brute force does not lose your work, and a Use as input button lets you round-trip the result back into the input to confirm encode/decode pairs cleanly. Sample buttons load known plaintext and known ciphertext so you can see the cipher in action in two clicks. Useful for school cryptography assignments, capture-the-flag puzzles, escape-room props, programming interview practice, geocaching clues, ARG and puzzle-hunt clue construction, ROT13-obfuscated spoilers and puzzle answers, and any time you encounter a string of jumbled letters and want a fast, dependable way to read it. Everything from the alphabet shifting to the frequency analysis runs locally in your browser, so cipher text and any sensitive notes you paste here never leave your device.

Free to use. Works in your browser. No signup, no login.