Playfair Cipher

Related Concepts

Encryption; Symmetric Cryptosystem

This is digraphic, bipartite substitution (Substitutions and Permutations) ${V }_{25} \times {V }_{25} \rightarrow {V }_{25} \times {V }_{25}$, using a 5 × 5 Polybios square and a “crossing step” $(\alpha \beta )(\gamma \delta )\mapsto (\alpha \delta )(\gamma \beta )$. In the example given 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,...