Foundations
Squares and pairs
How Polybius and Playfair use a letter square in different ways.
Also found as alphabet square · digraph · coordinate square
Two ways to use a square
Polybius replaces one letter with the labels of its row and column. Playfair takes two letters at a time and uses the geometry between their cells: same row, same column, or opposite corners of a rectangle. Both often use a 5×5 alphabet with I and J in one cell, but they use the square in different ways.Military Cryptanalysis, Part IVElements of Cryptanalysis
| Square method | Input unit | Output |
|---|---|---|
| Polybius | one letter | one coordinate pair |
| Playfair | one letter pair | another letter pair |
The finished square is the Key
A keyword is often just a way to build the square. Once the square is finished, its complete cell arrangement determines the result. Two construction methods that produce the same square will produce the same cipher.