Caesar cipher

Shifts each letter along the alphabet by a fixed amount, wrapping round at Z. A shift of 3 turns A into D — the shift Suetonius records Caesar himself using.

A Caesar cipher has 25 possible keys, so breaking it by trying all of them takes seconds by hand. It is a puzzle and a teaching tool, not security, and the page says so because people do occasionally reach for it in earnest. Two special cases have their own names: ROT13 is a shift of 13, which is its own inverse because 13 is half of 26 — encoding twice returns the original, which is why it caught on for hiding spoilers. Atbash maps the alphabet onto its reverse, A to Z and B to Y, and is also self-inverse. Non-letters pass through untouched in all of them, which incidentally leaks word lengths and punctuation to anyone looking.