Polybius Square Cipher
NewEncrypt and decrypt text with the Polybius square, an ancient Greek coordinate cipher that maps each letter to a row/column pair on a 5x5 grid.
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The Polybius square arranges the alphabet (with I and J sharing a cell) into a 5x5 grid, and encrypts each letter as its row/column coordinate pair. It's one of the oldest known ciphers, described by the Greek historian Polybius around the 2nd century BC, and it's the direct ancestor of the ADFGVX cipher, the bifid cipher, and the Tap Code.
Frequently Asked Questions
How old is the Polybius square cipher?
It dates back to the Greek historian and cryptographer Polybius around the 2nd century BC, who originally devised it as a signaling system using torches to transmit letters over long distances, making it one of the oldest documented ciphers in history.
Why do I and J share the same grid cell?
A 5x5 grid has 25 cells but the alphabet has 26 letters, so the two least-distinguishable letters (I and J) are conventionally merged into one cell — this is the standard convention also used in the ADFGVX and bifid ciphers that descend from Polybius.
What can I customize about the coordinate output?
You can change the separator placed between each letter's coordinate pair — a space by default, but you could use a comma, a dash, or no separator at all depending on how you want to present the ciphertext.
What other ciphers are derived from the Polybius square?
The German ADFGVX cipher used in World War I, the bifid and trifid ciphers (which add a fractionation step for extra security), and the Tap Code (also on this site) all use the same 5x5 coordinate-grid idea that Polybius originated over two thousand years ago.
Is the Polybius square secure for real use?
No — it's a simple substitution cipher with no key, so anyone who recognizes the coordinate-pair pattern can decode it instantly; it's included here for its historical importance and because it's the foundation several more complex classical ciphers build on.