Caesar decode
Caesar decode. A classical cipher, computed on your device.
Runs on your device. The file is never uploaded.
Caesar decode subtracts the shift you give from every letter of a text file, defaulting to 3, and hands back digits, spacing and case exactly as they arrived. Only whole numbers from 1 to 25 are accepted. A shift that does not match the encoder still produces letters, so nothing on the page tells you the number was wrong.
Questions
What if I do not know the shift?
Use Caesar brute force. It prints all 25 possibilities, one line per shift, so you can spot the readable one by eye. This tool needs the exact shift up front, and a wrong shift gives you fluent-looking nonsense rather than an error, because every shift produces valid letters.
How does the shift option work here?
You give it the same number you encoded with and it applies that shift backwards. Enter 3 to undo a Caesar encode with shift 3. The value must be a whole number from 1 to 25, and anything else is refused with "shift must be a whole number between 1 and 25". The default is 3.
Will it recover my spacing and punctuation?
They were never changed. Only A to Z and a to z are shifted, and case is preserved, so everything else came through the encode step untouched and is still in place. This is a letter-by-letter substitution, so the decoded text lines up character for character with the original.
Can it decode ROT13 or Atbash?
ROT13, yes, with a shift of 13, though the ROT13 tool does the same thing with no setting to get wrong. Atbash is not a shift at all, it mirrors the alphabet, so use the Atbash cipher tool for that. For any other single-alphabet substitution, use Substitution cipher decode with the full 26-letter key.
Is Caesar decoding a form of decryption?
Only in name. The cipher offers no security, since anyone can try all 25 shifts in seconds. Treat this as a puzzle tool. If you need real decryption, AES-GCM decrypt and RSA-OAEP decrypt on this site are the tools for that job.
Where does the text go?
Nowhere. Your file is read in a Web Worker in this tab, transformed in memory, and shown back to you on the page. Nothing is uploaded and nothing is saved between runs, and it all works with the network off. The cipher itself gives you no privacy, so where the code runs is the only privacy on offer.