Caesar Cipher
Named after Julius Caesar, the Caesar cipher is one of the earliest known encryption techniques. This tool lets you encrypt and decrypt messages by shifting each letter a chosen number of positions through the alphabet — with a shift of 3, A becomes D, B becomes E, and so on, wrapping around from Z back to A when needed. Enter your plaintext with a shift value between 1 and 25 to encrypt, or use the swap button to switch direction and decrypt ciphertext back to readable text using that same shift value. Uppercase and lowercase letters are shifted independently, and spaces, punctuation, and digits are left exactly where they are. Everything runs in your browser; nothing you type is ever sent to a server.
What is the Caesar cipher?
The Caesar cipher is one of the oldest and simplest encryption techniques, a type of substitution cipher in which each letter in the plaintext is shifted a fixed number of places through the alphabet. It's named after Julius Caesar, who is said to have used it to communicate with his generals — a shift of 3 was reportedly his usual choice, turning A into D throughout a message. It's one of two classic letter-substitution ciphers on this site: Caesar shifts each letter by a position, while the A1Z26 Cipher replaces each letter with its raw alphabet number instead of shifting it.
How do I decode a Caesar cipher if I don't know the shift?
Since there are only 25 possible shift values, you can simply try each one. This Caesar cipher decoder makes that fast: paste your ciphertext into the tool, click the swap button so it's set to decrypt, and step the shift value up from 1 to 25 — the correct one will be the shift that turns the text back into readable words. With only 25 options to check, brute-forcing an unknown shift takes seconds by hand and is instant with a tool like this one.
Is ROT13 a Caesar cipher?
Yes — ROT13 is just a Caesar cipher with the shift value fixed at 13. It's a special case worth knowing: because 13 is exactly half of 26, applying ROT13 twice returns the original text, and the same operation is used for both encoding and decoding. Every other shift value needs the opposite shift to reverse it, which is why this tool's swap button applies the same shift backward rather than asking for a second number.
Why do uppercase and lowercase letters shift separately?
Shifting them independently keeps the original capitalization intact, so a shifted message still reads with the same sentence structure and proper nouns still stand out — an uppercase letter always shifts to another uppercase letter, and lowercase always shifts to lowercase, rather than the case changing partway through the alphabet wrap-around.
Last reviewed: August 2026