ROT13 / Caesar Cipher

Apply ROT13 by default — the classic 13-position alphabet shift used to obscure spoilers, jokes, or simple ciphers. Or pick any shift from 1 to 25 for a general Caesar cipher. The cipher is symmetric at 13 (encode = decode), and at any shift, applying it twice with shift N and shift 26-N restores the original.

ROT13’s quiet usefulness

Real cryptography is for real secrets. ROT13 is for not-quite-secrets — text you want to obscure from casual scanning while keeping it trivially decodable. Spoilers in Reddit comments. Joke punchlines on Usenet. Easter-egg keys in puzzle games.

The cipher is so weak that “decoded” by hand is a few seconds of effort, which is the point — anyone determined gets the content; anyone scrolling sees gibberish.

How the cipher works

Each letter shifts forward by N positions in the alphabet. ROT13 = shift 13. A → N, B → O, M → Z, N → A (wrap around).

ABCDEFGHIJKLMNOPQRSTUVWXYZ
↓
NOPQRSTUVWXYZABCDEFGHIJKLM

Apply the same shift to ROT13 output and you get the original back — that is what makes 13 special. Other shifts need shift 26-N to invert.

Common workflows

Hide a spoiler. Wrap a sentence in ROT13 in a forum post. Readers who want to be spoiled can decode; others scroll past gibberish.

Easter eggs. Encode a hint in a project. ROT13 is enough to evade casual reading, easy enough that finders feel clever.

Caesar cipher puzzles. Set shift to N. Build crosswords or escape-room clues.

Educational example. First lesson in cryptanalysis. ROT13 falls to letter-frequency analysis instantly — a teaching exercise.

Don’t use for

• Confidentiality (use AES)
• Authentication (use HMAC)
• Password storage (use bcrypt)
• Anything where the contents matter

ROT13 is a reading filter, not a cipher.

Last updated: 2025-01-15