Background
ROT13 is a simple encryption technique that replaces each letter in a message with the letter 13 positions ahead of it in the alphabet. For example, "A" becomes "N", "B" becomes "O", and so on.
To calculate ROT13 for a given message, you simply need to shift each letter 13 positions ahead in the alphabet. If a letter is near the end of the alphabet and there are not enough letters left to shift it 13 positions ahead, you would wrap around to the beginning of the alphabet and continue counting.