Playfair

Enciphers and deciphers Playfair (a keyed 5×5 square, I and J sharing a cell; each pair by row, column or rectangle, highlighted) and Four-Square, and solves Playfair without the key by simulated annealing (Cowan 2008) from about 200 letters. Lyon Playfair recalled explaining “Wheatstone's newly-discovered symmetrical cipher” to Palmerston in January 1854; Delastelle set out the four-square in 1902.

• T. Wemyss Reid, Memoirs and Correspondence of Lyon Playfair (London: Cassell, 1899), pp. 158–159
• Michael J. Cowan, “Breaking Short Playfair Ciphers with the Simulated Annealing Algorithm”, Cryptologia 32 (1), 2008, pp. 71–83
• Félix Delastelle, Traité élémentaire de cryptographie (Paris: Gauthier-Villars, 1902)