ROT13 / Caesar Cipher
The Caesar cipher is one of the oldest and simplest encryption techniques, attributed to Julius Caesar, who used it to protect military messages. It works by shifting every letter in the plaintext a fixed number of positions along the alphabet, preserving case and leaving non-letter characters untouched. ROT13 is the most common variant, using a shift of 13, which makes encoding and decoding identical operations since the alphabet has 26 letters.
Despite its simplicity, understanding the Caesar cipher is fundamental to learning about cryptography and substitution ciphers. ROT13 is widely used on the internet to obscure spoilers, puzzle answers, or offensive content without truly encrypting it. This tool lets you choose any shift value from 1 to 25 and toggle between encoding (forward shift) and decoding (reverse shift), making it easy to experiment with the full family of Caesar ciphers.
How it works
Encoded letter = ((original position + shift) mod 26). Positions are 0-indexed (A=0 … Z=25). For decode, use shift = 26 − shift_used. ROT13 is self-inverse: applying it twice returns the original text.
Use cases
- Hiding spoilers or puzzle answers on forums and social media
- Teaching basic cryptography and substitution cipher concepts
- Obscuring text in emails or documents without strong encryption
- Decoding ROT13-encoded text found online or in legacy systems
- Creating simple word games or escape-room puzzles