Polybius Square Cipher
Encrypt and decrypt messages with the Polybius square, which turns each letter into a pair of digits on a 5×5 grid (with I and J combined).
Cifrado
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Turning every letter into two digits
Someone handed you a string like 34 31 11 and you want to know what it says, or you are building an escape room clue and need to convert a pile of words fast. The Polybius square is the oldest trick of this kind: instead of swapping one letter for another, it swaps a letter for its position in a grid, which doubles the message length and leaves you with nothing but digits.
The grid has 25 cells for 26 letters, so I and J share one. Text is uppercased and every J becomes an I before conversion. The arithmetic is plain: take the letter position in the J-less alphabet, the row is that divided by five plus one, and the column is the remainder plus one. O lands on 34, L on 31 and A on 11, so OLA comes out as 34 31 11. Anything that is not a letter, spaces and punctuation included, passes through untouched.
The tool description mentions decryption, but this page only encodes: there is no reverse mode, field or toggle. To decode by hand, read the digits in pairs, first the row and then the column, and pick between I and J from context. Do not treat this as protection for anything real. It is monoalphabetic substitution, and frequency counting on the pairs cracks a message in minutes. The conversion happens entirely in your browser.
Frequently asked questions
Why does the J disappear from the grid?
Can I decode a message here?
Is the Polybius square any good as security?
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