The Polybius square cipher first distributes the letters of a chosen alphabet into a grid (typically 5×5). Here’s the original square used by the Greeks who invented the cipher.
For the Latin alphabet to fit into a 5×5 square, two letters must be combined (usually
K). For this, we first replace one letter by the other before encrypting. This is why the letter
J is not present in the following square.
To encode a message, each letter is translated to its coordinates in the grid – typically first row, then column. For instance, the letter
X is at row 5 and column 3. Thus, the plaintext letter
X translates to the ciphertext
For the typical Latin alphabet square above we get the following map:
By applying a Polybius cipher encryption you shrink the set of symbols necessary to represent a message from the original alphabet (typically 26 symbols) to the set of symbols you need to denote the coordinates of each letter in the ciphertext (typically 5 symbols). This can be very useful for telegraphy, steganography, and cryptography.