ROT13
The cipher that has hidden spoilers for forty years.
Where it came from
ROT13 is a Caesar cipher with shift 13, popularized on 80s Usenet: movie spoilers and puzzle answers were hidden behind it — 'don't read below, it's ROT13'. Nobody took it seriously as encryption; it was a cultural contract meaning 'you were warned'.
Choosing exactly 13 was laziness at its finest: Latin has 26 letters, so applying ROT13 twice returns the original text — encode and decode are the same operation.
How it works
Each letter shifts 13 positions through the alphabet: A→N, B→O, … N→A. Case is preserved; digits and punctuation untouched.
The math: shifting by 13 twice = shifting by 26 ≡ 0 mod 26. Hence self-inversion — one function does both directions.
UWC applies ROT13 to Latin letters in your browser; for Cyrillic there's keyboard-layout conversion and translit in neighboring converters.