Shift letters by any amount from 1 to 25, or break an unknown Caesar cipher instantly by showing all 25 possible shifts at once.
Show calculation steps
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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.
