ROT13 is a simple letter substitution cipher that replaces each letter with the letter 13 positions after it in the alphabet, wrapping around from Z back to A.
Because ROT13 shifts by exactly half the alphabet (13 of 26 letters), applying it twice returns the original text, so the same operation both encodes and decodes.
No, ROT13 only shifts letters; numbers, spaces, and punctuation are left unchanged.
No, ROT13 is not intended for security. It is a simple, well-known obfuscation method historically used to hide spoilers or puzzle answers from casual viewing.
Yes, uppercase letters are shifted to other uppercase letters, and lowercase letters are shifted to other lowercase letters.