Shift letters by any amount from 1 to 25, or break an unknown Caesar cipher instantly by showing all 25 possible shifts at once.
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Shift every letter
A Caesar cipher moves each letter a fixed number of places along the alphabet. With 25 useful shifts it can be broken by trying all of them, which takes seconds by hand — so it is a teaching example and a puzzle tool, never a security measure.
Worked example
shift 3: HELLO → KHOOR shift 13: HELLO → URYYB (this is ROT13)
ROT13 is a Caesar cipher
A shift of 13 is its own inverse in a 26-letter alphabet, which is why ROT13 is used for spoilers: one operation both hides and reveals. It is the only shift with that property.
How it is broken
Two ways, both easy. Brute force tries 25 shifts and reads which output is English. Frequency analysis notes that the most common ciphertext letter is probably E and derives the shift directly. Neither needs a computer.
What it is good for
Teaching what a key is and why key space matters, puzzle and escape-room work, and obscuring an answer so it is not read accidentally. Everything is transformed in your browser.
How to use the Caesar Cipher Encoder and Decoder
- Paste your message.
- Set the shift amount, or choose "Break it" to see all 25 possibilities at once.
- Pick encrypt or decrypt.
- Click "Apply Caesar shift".
Frequently asked questions
How was the Caesar cipher used historically?
Suetonius records Julius Caesar shifting letters by three places in his private correspondence. Against opponents who were largely illiterate, and with no concept of cryptanalysis, it was adequate – which says more about the era than about the cipher.
How is it broken?
By exhaustion or by frequency analysis. There are only 25 useful keys, so trying them all takes seconds – the "Break it" mode does exactly this and you simply read down the list for the line that makes sense.
What is the relationship to ROT13?
ROT13 is the Caesar cipher with a shift of 13. That shift is special only because it makes the operation self-inverse.
Are numbers and punctuation shifted?
No. Only the 26 letters are rotated; digits, spaces and punctuation pass through unchanged. That is faithful to the classical cipher, though it does leak the shape of the message.
Break or check a simple cipher
Plaintext from a classical cipher, or confirmation of which one was used.
- ROT13 Encoder and DecoderTry ROT13 first: it is its own inverse, so one pass tells you immediately.
- Caesar Cipher Encoder and Decoder you are hereTry every Caesar shift, since ROT13 is only the shift-13 case.
- Atbash CipherTry Atbash, which reverses the alphabet rather than rotating it.
- Vigenere Cipher Encoder and DecoderIf a single shift never works, the key is probably a word rather than a number.
