ROT13 / Caesar Cipher cipher
Shift letters by a fixed number of positions. ROT13 is its own inverse, so applying it twice restores the text.
What this tool does
The Caesar cipher is the oldest substitution cipher: every letter moves a fixed number of positions through the alphabet, so a shift of three turns A into D and B into E. This tool accepts any shift from 0 to 25 and passes digits, spaces, and punctuation through unchanged.
The most common setting is a shift of 13, known as ROT13. Because the alphabet has 26 letters, shifting by 13 twice returns exactly to the start, which makes encoding and decoding the same operation and lets one tool handle both directions.
ROT13 is used as a convention rather than as security. Message boards use it to hide quiz answers or film spoilers so nobody reads them by accident, and anyone who wants to read the text can reverse it trivially. That is the intended behaviour.
When to use it
Use it to share spoilers or puzzle answers in a lightly obscured form, to read ROT13 text in older forum and Usenet archives, to work through classical cipher exercises in CTF or security training, or to sanity-check a character substitution routine.
Input and output examples
Hello, World!
Uryyb, Jbeyq!
ROT13 output; punctuation is preserved.
Uryyb, Jbeyq!
Hello, World!
Applying ROT13 again returns the original text.
abc with shift 1
bcd
A shift of one; reverse it with a shift of 25.
Notes and limitations
The Caesar cipher offers no security whatsoever. With only 25 possible keys an exhaustive search takes seconds, and letter frequency analysis breaks it instantly. Never use it to protect real data. This tool also shifts Latin letters only, so text in other scripts, digits, and symbols pass through unchanged and may not be obscured at all.
Frequently asked questions
Why is there no decode button for ROT13?
Thirteen is half of twenty-six, so applying the shift twice returns the original letter. Encoding and decoding are the same operation.
Why is non-Latin text unchanged?
The cipher rotates a 26-letter alphabet, so it does not apply to other scripts. This tool shifts Latin letters and leaves everything else alone.