EncryptCodecCaesar Cipher / ROT13
Shift letters by any amount (1–25). Set the shift to 13 for ROT13. Decode mode reverses the shift, and you can brute-force all 25 shifts at once.
Caesar cipher and ROT13
The Caesar cipher shifts each letter a fixed number of positions through the alphabet — the oldest known substitution cipher. ROT13 is simply a Caesar cipher with a shift of 13, which makes it its own inverse. With only 25 possible keys it offers no real security and is trivially broken by trying every shift. Use it for puzzles and learning, never to protect real data.
Plaintext read character by character
A fixed shift from 1 to 25 chosen — 13 gives ROT13
Each A–Z and a–z letter advanced by the shift, wrapping around the alphabet; case preserved
Non-letters (digits, spaces, punctuation) pass through unchanged
Encrypted text read character by character
The same key applied in reverse (shift backwards) to undo the rotation
Each letter moved back by the shift, wrapping around the alphabet
Original message recovered — or brute-force all 25 shifts when the key is unknown
Spec: Monoalphabetic substitution (shift cipher); ROT13 = shift 13