Polybius Square Cipher

The Polybius Square cipher replaces letters with coordinates taken from a grid. Each symbol is identified by its row and column position, turning readable text into a sequence of number pairs.

In the classical English version, the alphabet is arranged inside a 5×5 square. Because 26 letters do not fit into 25 cells, the letters I and J traditionally share one position.

Unlike transposition ciphers that rearrange letters, Polybius Square is a substitution cipher. Each character is replaced with coordinates while the order of the message remains unchanged.

A classical 5×5 Polybius Square for the English alphabet looks like this:

Using this square, the word HELLO becomes 23 15 31 31 34.

The Polybius Square was described by the ancient Greek historian Polybius and is one of the earliest known coordinate-based ciphers. It was designed to transmit letters using pairs of numbers.

Today it is mainly used for education, puzzles, escape rooms, and learning the foundations of classical cryptography. Many later cipher systems were influenced by the idea of representing letters as coordinates.

Unlike Caesar, which shifts letters, or Vigenere, which changes letters using a keyword, Polybius Square converts each character into coordinates. This makes it easier to transmit messages using numbers rather than letters.

The cipher is simple to learn but offers little real security because the coordinate patterns can be analyzed and reversed.