How does ROT13 work?

Applying ROT13 we replace each letter in a message by its partner 13 characters further along the alphabet. Thus, we replace a with n, b with o, c with p, and so on. Here’s the full mapping:

Original alphabet: abcdefghijklmnopqrstuvwxyz
Rotated alphabet:  nopqrstuvwxyzabcdefghijklm

ROT13 is a special case of Caesar cipher using the basic Latin alphabet (a-z) and a shift of 13 letters. As the alphabet consists of 26 letters (2132\cdot 13) we can undo the operation (decode) by applying the same algorithm. This makes ROT13 is its own inverse.

Where is ROT13 being used today?

Today, ROT13 is often found on social media platforms as a means of hiding spoilers and problem solutions from the casual glance. It has been described as the “Usenet equivalent of printing an answer to a quiz upside down” as it provides virtually no cryptographic security.