How the Caesar and Vigenère Ciphers Work

The Caesar cipher, named for Julius Caesar, shifts every letter a fixed number of places down the alphabet. With a shift of 3, A becomes D, B becomes E, and so on, wrapping around so X, Y, Z become A, B, C. To decode, you shift back by the same amount.

ROT13 is simply a Caesar cipher with a shift of 13. Because the alphabet has 26 letters, shifting by 13 twice returns you to the start — so the same operation both encodes and decodes, which is why ROT13 is popular for hiding spoilers online.

Atbash reverses the alphabet: A maps to Z, B to Y, C to X, and so on. It's its own inverse, so encoding and decoding are the same step. It's one of the oldest known ciphers, originally used for the Hebrew alphabet.

The Vigenère cipher is much stronger. Instead of one fixed shift, it uses a keyword, and each letter of the message is shifted by the corresponding letter of the repeated keyword. Because the shift changes from letter to letter, simple frequency analysis doesn't crack it the way it cracks a Caesar cipher.

A Caesar cipher has only 25 possible shifts, so if you intercept one without knowing the key, you can just try them all — this is called brute force. Decode the text with every shift from 1 to 25 and look for the version that reads as real words.

The cipher tool does this for you, listing all 25 decodings at once so you can spot the readable one instantly. It's a reminder that these classic ciphers are for puzzles and learning, not real security — modern encryption is what protects genuine secrets.