Use the Caesar cipher converter
The converter opens in Encrypt mode with Shift set to 3. It changes ASCII A-Z and a-z, keeps their case, and leaves all other characters unchanged.
Convert text
- Type or paste text into the left input box. To load a file, click Upload; its picker accepts
.txtandtext/plain. You can also drag a file onto the left input area. The Drop file overlay appears while dragging, and dropped files are accepted only when they match the text-file types shown above. - Choose Encrypt to shift letters forward or Decrypt to shift them backward.
- Set Shift from 1 to 25 with the slider or number field. The result updates whenever the text, mode, or shift changes.
- Review the converted text in the right box.
Use the available controls
- Click ROT13 preset to set Shift to 13. You can edit the shift afterward.
- Click Copy to copy the output to the clipboard. This uses
navigator.clipboard.writeText(); the tool keeps Copy unavailable until output exists and reports a clipboard failure beside the output actions. - Click Download .txt to create
caesar-cipher-output.txt.
Limits and privacy
The Caesar cipher has only 25 selectable shifts, so do not use it to protect sensitive information. Only ASCII A-Z and a-z are converted; accented and other Unicode letters remain unchanged. A blank or non-numeric Shift falls back to 3, while numeric values are constrained to 1-25. Copy and Download .txt do nothing when the output is empty.
The inspected conversion and file-reading logic runs in browser-side JavaScript. The inspected tool code contains no request that sends the entered text or file contents.
Frequently asked questions
What is a Caesar cipher?
It is a substitution cipher that shifts each letter by a fixed number of positions through the alphabet, wrapping from Z to A. The tool applies that shift to ASCII letters.
What is ROT13 and how is it related to the Caesar cipher?
ROT13 is a Caesar cipher with a shift of 13. Clicking ROT13 preset sets the shift to 13, and applying the same shift twice returns the original text.
Is the Caesar cipher secure?
No. With only 25 possible shifts, it is not appropriate for protecting sensitive information.
Does the shift value need to match to decrypt?
Yes. Select Decrypt and use the same shift that was used to encrypt. Equivalently, you can encrypt with 26 minus the original shift where applicable.
Is my text sent to a server?
The inspected conversion and file-reading logic runs in browser-side JavaScript, and the inspected tool code contains no request that sends the entered text or file contents.