Caesar Cipher

Caesar Cipher

Caesar Cipher shifts each letter by a fixed number of positions, with a ROT13 preset. It encodes and decodes, since a shift cipher is reversed by shifting the other way.

How to use it

1. Enter your text and set the rotation amount.
2. Press ROT13 for the classic 13-position shift.
3. Copy either the encoded or decoded output.

Why ROT13 is its own inverse

A Caesar cipher shifts every letter a fixed distance through the alphabet, wrapping from Z back to A. Julius Caesar reportedly used a shift of three.

ROT13 shifts by exactly half of the 26-letter alphabet, so applying it twice returns the original text. That self-inverse property is why it became the convention for hiding spoilers and punchlines in early internet forums: one function both encodes and decodes.

When it helps

• Hiding a spoiler or a puzzle answer from casual reading.
• Teaching how substitution ciphers work and why they fail.
• Solving a Caesar-shifted puzzle by trying each rotation.
• Light obfuscation where the goal is preventing accidental reading, not security.

Common mistakes

• Using it for anything that needs to stay secret. With only 25 possible shifts, brute force takes a fraction of a second, and letter frequency analysis breaks it instantly.
• Assuming it obscures non-letter characters. Numbers, punctuation and spaces pass through unchanged, which leaks the structure of the message.