ROT13 is a cyclic shift cipher that substitutes each English letter with one that is 13 away in the alphabet. It is used today not for security, but as a simple obfuscation device, because the same algorithm is used for both encryption and decryption. People wishing to encrypt or decrypt a message, M, just cut-and-paste M to a ROT13 converter and click a button "APPLY" to do the encryption or decryption. Given an example of another ROTi transformation that could be used for both encryption and decryption in a similar way. (When I was interviewed at Google HQ in 2015, I was asked to implement ROT13 in one round of the interview.) solution online