Caesar Cipher Encoder / Decoder

Encodes and decodes text with a Caesar shift, any value from 1–25, with ROT-13 as a one-click preset since it's the variant that actually shows up in the wild. All done in the browser.

How it actually works

Each letter shifts a fixed number of places through the alphabet — shift 3 turns A into D, B into E, and wraps Z back around to C. Numbers, punctuation and spaces are left untouched. Named after Julius Caesar, who reportedly used it for military dispatches, which places it somewhere north of two thousand years old — genuinely one of the oldest documented encryption techniques still taught today.

ROT-13 specifically

A shift of exactly 13 has a neat property: applying it twice returns the original text, because 13 is exactly half of the 26-letter alphabet. That means the same operation both encodes and decodes — no separate "decrypt" function needed. It's why forums and puzzle sites use it to hide spoilers and answers from a casual glance: it's not security, it's just enough friction that you have to deliberately choose to reveal it rather than seeing it by accident.

Don't mistake this for actual security

With only 25 possible shift values, a Caesar cipher falls to brute force in microseconds on any modern computer, and frequency analysis makes it even faster by hand — in English, E, T, A, O, I and N appear at predictable rates that give the shift away almost immediately once you've got enough ciphertext to work with. Its real value today is entirely educational: it's the simplest possible introduction to the concepts — substitution, key space, cryptanalysis — that scale up into the actual cryptography (AES, RSA) securing HTTPS, SSH and VPNs. Understanding why this cipher is trivially breakable is a genuinely useful stepping stone to understanding why modern algorithms aren't.