ROT13 and ROT47

How it works

ROT13 is the Caesar shift by 13. Since $13 + 13 = 26$, doing it twice brings every letter back, so one operation both hides and reveals a text. It became the custom on Usenet in the 1980s for spoilers and punch lines: not secret, just not readable by accident.

ROT5 does the same for digits (shift 5 out of 10), and ROT18 combines the two. ROT47 extends the idea to the 94 visible ASCII characters from ! (code 33) to ~ (code 126): each moves 47 places in that range, $x \mapsto (x + 47) \bmod 94$, so letters, digits and punctuation all change.

None of these has a key, so they offer no security at all: anybody who knows the name of the trick reads the text.