Grid Ciphers

Bifid, Trifid, ADFGVX/ADFGX, Hill, Nihilist and the straddling checkerboard, both ways: fractionation through a 5×5 square or a 3×3×3 cube step by step, Hill's key matrix with its determinant mod 26, Nihilist sums of square and key numbers, a checkerboard with an optional digit key. Delastelle (1902) describes bifid and trifid; the German army adopted ADFGX and ADFGVX in 1918; Hill published his cipher in 1929.

• Félix Delastelle, Traité élémentaire de cryptographie (Paris: Gauthier-Villars, 1902)
• Lester S. Hill, “Cryptography in an Algebraic Alphabet”, The American Mathematical Monthly 36 (6), 1929, pp. 306–312
• David Kahn, The Codebreakers: The Story of Secret Writing (New York: Macmillan, 1967)
• American Cryptogram Association, “Nihilist Substitution”
• Dirk Rijmenants, “Straddling Checkerboards”, Cipher Machines and Cryptology