Polybius Cipher
Use the classic I/J-merged Polybius square locally in your browser.
A QUICK WALKTHROUGH
How to use this tool
- Enter text or digits.
- Choose encode or decode.
- Review and copy the result.
The classic 5×5 square
I and J share one cell in a 5×5 A–Z square. During encoding, other characters pass through unchanged.
Educational cipher
Decoding reads digit pairs from 1 to 5; other non-digits are ignored, invalid pairs become question marks, and a final unmatched digit is ignored. This historical cipher is not secure encryption.
GOOD TO KNOW
Common questions
Is this secure encryption?
No. The Polybius square is an educational historical cipher and should not protect sensitive information.
How are I and J handled?
They share one square cell. Encoding J uses the same coordinate as I, and decoding that cell returns I.
What happens to invalid digits?
During decoding, digit pairs outside 1–5 become question marks; non-digits and a final unmatched digit are ignored.