Reference: ROT13

ROT13(str) transforms the string str using ROT13, i.e. shifts all letters 13 positions to the right (modulo 26) in the English alphabet, preserving the case of each letter and all non-alphabetic characters such as spaces and punctuation marks.

Obviously, str ↦ ROT13(str) is an involution, i.e. str ↦ ROT13(ROT13(str)) is the identity operation on strings.