Caesar Cipher & ROT13
Encode or decode text with a classic educational substitution cipher. Your work stays on this device whenever possible.
How to use Caesar Cipher & ROT13
- Enter text in "Ciphertext or plaintext"; the box starts with "Khoor Zruog!".
- Set "Cipher mode" to Decode (shift backward) or Encode (shift forward) and enter the "Shift key (0 to 25)".
- Or press a preset: "ROT13 (Shift 13)", "Caesar Standard (+3)", "Atbash (A ↔ Z)" or "Auto-detect shift".
- Tick "Shift numbers (0-9)" if digits should rotate as well.
- Press "Process cipher", or open "Show all 25 shifts table" and click "Use" beside the row that reads as English.
Worked example
Enter "Hello World" and press ROT13: the output is "Uryyb Jbeyq", and pressing ROT13 again on that returns the original. Decoding "Khoor Zruog!" with shift 3 gives "Hello World!". The Atbash preset turns "Hello" into "Svool". The Show all 25 shifts table lists 25 candidate decodings of the input, one for each key from 1 to 25.
When to use this tool
- Solving a Caesar-style clue in a puzzle hunt using the 25-shift table.
- Demonstrating in class why a 26-key cipher is easy to brute-force.
- Hiding a spoiler in a forum post with ROT13.
Important limitations
Caesar cipher and ROT13 are classical substitution ciphers designed strictly for education, puzzles, and CTF challenges. They provide zero cryptographic security.
Frequently asked questions
Why does "Caesar Standard (+3)" decode rather than encode?
That preset sets shift 3 with Decode selected, which turns the sample "Khoor Zruog!" into "Hello World!". Switch Cipher mode to Encode and press Process cipher to produce the +3 ciphertext.
How reliable is Auto-detect shift?
It tries all 26 shifts and scores each by summing English letter frequencies. That works on a sentence or more of English, but short strings, names or other languages can select the wrong shift.
Are accented or non-English letters shifted?
No. Only unaccented A-Z and a-z rotate. Letters such as é, ß and Cyrillic, plus spaces and punctuation, pass through untouched. If you need to handle such letters, transliterate them to plain Latin first.
What does the "Preserve case" checkbox do?
Nothing at present. Uppercase and lowercase are always kept, whether the box is ticked or not. The control is a leftover, so use Atbash or the shift key for any other behaviour. Digits, by contrast, only rotate when the number box is ticked.
What if I type a shift above 25 or a negative number?
The tool takes the absolute value and reduces it modulo 26, so 29 acts as 3 and -3 also acts as 3. To go the other way, switch between Encode and Decode.