ROT13 and Caesar Cipher
Shift every letter through the alphabet by a fixed amount. A shift of 13 is ROT13, the classic spoiler-hiding cipher; any other value gives you a general Caesar cipher. Numbers, spaces, and punctuation pass through untouched.
Why ROT13 is its own inverse
The alphabet has 26 letters, and 13 is exactly half of that. Shifting a letter forward 13 places and then forward 13 more brings you back where you started. That means a single ROT13 operation both encodes and decodes — which is precisely why it became the convention for hiding spoilers and punchlines on Usenet and later on forums. One function, no direction to remember.
The Caesar cipher
Named for Julius Caesar, who according to Suetonius shifted letters by three to protect military messages. It is among the oldest known ciphers and offers essentially no security today — there are only 25 possible shifts, so trying them all takes seconds. Even without brute force, letter frequency analysis breaks it immediately: in English, whichever letter appears most often in the ciphertext is almost certainly e.
Legitimate uses
- Hiding spoilers in forum posts and comments, the original purpose of ROT13.
- Puzzles and escape rooms, where a simple, solvable cipher is exactly what you want.
- Teaching cryptography — it is the standard first example for explaining substitution, keys, and frequency analysis.
- Obscuring answers in quizzes so readers do not see them accidentally.
Never use this for anything real
ROT13 and Caesar ciphers provide zero security. They are obfuscation, not encryption — the equivalent of writing backwards. Anyone who wants to read your text can, in seconds, with no key. For genuine confidentiality you need modern cryptography such as AES, and for transmitting data securely, TLS. Treat this tool as a toy and a teaching aid, which is exactly what it is.