ROT13

ROT13 is the Caesar cipher with a shift of 13 — the one shift that is its own inverse, because 13 doubled wraps the 26-letter alphabet exactly. That symmetry made it the standard way to hide spoilers, puzzle answers, and punchlines on Usenet in the 1980s, a role it still plays in forums and geocaching hints today.

It was never meant to keep secrets: everyone knows the shift, so decoding needs no key and no skill. Think of it as a "look away if you don't want to know" marker rather than encryption. Anything genuinely secret needs at least a key — and, for real protection, modern cryptography.