Polybius Square Cipher
23-15-31-31-34 52-34-42-31-14
Encode letters into row+column coordinates
Every letter maps to its two-digit grid coordinate (row then column). J is treated as I, so both become 24. Codes in a word are joined with hyphens, e.g. “Hello World” becomes “23-15-31-31-34 52-34-42-31-14”. Digits, punctuation, and other non-letters are dropped and simply break one word from the next.
When to use this tool
Reach for the Polybius square when a puzzle, CTF challenge, escape room, or intro-cryptography lesson encodes letters as row/column coordinate pairs in a 5×5 grid. It is one of the oldest substitution ciphers and the basis for related schemes like the tap code and the Nihilist cipher, so it rounds out the classical family here (Caesar, Vigenère, Atbash, Affine, A1Z26) with a coordinate-based mapping instead of a letter-to-letter or letter-to-single-number swap.
Privacy and limitations
Everything runs locally in your browser; nothing is sent anywhere. This is a classical cipher, not real encryption — the fixed 5×5 square is trivial to reverse. It uses the standard I/J-merged grid, so I and J both encode to 24 and always decode back to I; encoding is case-insensitive and drops any non-letter, treating runs of them as word breaks. Decoding returns letters in uppercase, so the original case, spacing, and punctuation are not recovered. Codes that are not exactly two digits, or whose digits fall outside 1–5, are reported as invalid input. Use the encode/decode toggle to switch directions, since the transform is not self-inverse.