Basic Classic Symmetric Intermediate
The Four-Square Cipher
Learn about the four-square cipher, a digraph substitution cipher that uses four 5x5 grids to encrypt letter pairs without Playfair's awkward same-row and same-column special cases.
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#Digraph Cipher
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Basic Classic Symmetric Breakers Advanced
Breaking the Playfair Cipher
Playfair defeated single-letter frequency analysis by encrypting pairs of letters through a 5×5 key square. But the same simulated-annealing idea that cracks the substitution cipher still works here too, once you swap in quadgram statistics and a richer set of moves.
Basic Classic Symmetric Intermediate
The Two-Square Cipher
Learn about the two-square cipher, a digraph substitution cipher that sits between Playfair's single grid and four-square's four grids: two independently keyed squares, one uniform encryption rule.