Polybius Square – A device used by the ancient Greeks
Polybius square is a method in which a chosen alphabet is layed out in a grid that is then being used to represent each letter by its coordinates.
How does the Polybius cipher work?
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.
┌───────────────┐ │ 1 2 3 4 5 │ ┌───┼───────────────┤ │ 1 │ Α Β Γ Δ Ε │ │ 2 │ Ζ Η Θ Ι Κ │ │ 3 │ Λ Μ Ν Ξ Ο │ │ 4 │ Π Ρ Σ Τ Υ │ │ 5 │ Φ Χ Ψ Ω │ └───┴───────────────┘
For the Latin alphabet to fit into a 5×5 square, two letters must be combined (usually I
and J
or C
and 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.
┌───────────────┐ │ 1 2 3 4 5 │ ┌───┼───────────────┤ │ 1 │ A B C D E │ │ 2 │ F G H I K │ │ 3 │ L M N O P │ │ 4 │ Q R S T U │ │ 5 │ V W X Y Z │ └───┴───────────────┘
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 53
.
For the typical Latin alphabet square above we get the following map:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 11 12 13 14 15 21 22 23 24 24 25 31 32 33 34 35 41 42 43 44 45 51 52 53 54 55
What makes the Polybius cipher special?
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.