Rot 13

Rot 13 is a cipher that is never used for real security. It simply replaces every letter with the letter that comes 13 later in the alphabet, so the encryption is:

input    abcdefghijklmnopqrstuvwxyz
output   nopqrstuvwxyzabcdefghijklm

The alphabet on the lower line is rotated by 13 places.

Rot 13 is never used for real security, since breaking it is trivial. However, it is fairly commonly used in Usenet news postings or email. If a post gives away something - the ending of a movie or the punchline of a joke - it is said to contain a spoiler. Some readers might wish not to read it, because it would spoil their enjoyment of the film. On the other hand, the writer may consider mentioning it essential to the discussion. The solution is to rot-13 encrypt that part of the message and let readers decide whether to decrypt and read it.

Mathematically, the operation is $x=(x+13)$ modulo 26. The decryption uses exactly the same code. Rot 13 is its own inverse, since $(x+13)+13=x$ modulo 26.

Rot 13 is one example of a Caesar cipher, a type of cipher once used by Julius Caesar. A Caesar cipher may rotate the alphabet by any amount. Caesar used three, so a was replaced by d, b by e and so on. Rot 13 uses 13.