• Used by British army in World War I • Type of digraph substitution cipher
• Construct a 5 x 5 matrix. • Insert the English alphabets into the matrix, starting from the rightbottom corner, going upwards in a spiral manner.
Plaintext Preprocessing:
Example: Plaintext: Loss Fight Back L O S S F I G H T B A C K L O S X F I G H T B A C K X plaintext: LO SX FI GH TB AC KX
For a pair of characters, if they appear on the same row on the play fair square, then get the character to right of each. Wrap around if needed.
Examples:
IG → HF XB → QL UT → TS
For a pair of characters, if they appear on the same column on the play fair square, then get the character below each. Wrap around if needed.
FQ → SP UN → VH FS → SR
For a pair of characters, if they appear at the corner of a rectangle, select the two characters in the opposite corners of the rectangle.