This is digraphic, bipartite substitution (see substitutions and permutations) V 25 × V 25 → V 25 × V 25, using a 5 × 5 Polybios square and a ‘crossing step’ \((\alpha\beta)(\gamma\delta)\mapsto(\alpha\delta)(\gamma\beta)\). In the example below, the key Palmerstone has been used to make the Polybios square (see mixed, alphabet).
If a bigram is found in the same row (or the same column), the ‘crossing step’ degenerates: it takes the cyclically right neighbor: am ↦ LE; ae ↦ LP; aa ↦ LL (or the neighbor cyclically below: dl ↦ KT; dx ↦ KL). Modified rules are common, especially the one concerning the doubles: perhaps one letter of the pair will be omitted or replaced with a null; in cases like ‘less seven’ this will lead to encrypt ‘le s& s& se ve n&’, where & is a null.
A modified Playfair uses two Polybios squares, one for the first and one for the second character of the bigrams, e.g.,
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References
Bauer, F.L. (1997). “Decrypted secrets.” Methods and Maxims of Cryptology. Springer-Verlag, Berlin.
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Bauer, F.L. (2005). Playfair Cipher. In: van Tilborg, H.C.A. (eds) Encyclopedia of Cryptography and Security. Springer, Boston, MA . https://doi.org/10.1007/0-387-23483-7_303
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