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5 articles with this tag.
ADFGVX fractionates each letter into two symbols and then shuffles them with a columnar transposition, the combination that made it Germany's toughest field cipher of 1918. This guide runs the modern automated attack: recover the transposition with the index of coincidence, then solve the substitution with quadgrams, and read a 330-letter message back out.
14 min read
Bifid fractionates each letter into coordinates and shuffles them, which hides single-letter frequencies. Given the period, a hill climb over the Polybius square guided by English quadgram statistics peels the key apart anyway. This guide runs the real attack and recovers a 669-letter message.
13 min read
The four-square cipher hides letter frequencies behind two keyed grids, but it is still a pencil-and-paper cipher. Learn how simulated annealing with quadgram statistics recovers both secret squares from the ciphertext alone, with runnable Python.
14 min read
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.
20 min read
A monoalphabetic substitution key is one arbitrary permutation of 26 letters: 26! of them, far too many to brute force and immune to Kasiski or the Index of Coincidence since there's no repeating key. Here's how frequency analysis and a hill-climbing search crack it anyway.
26 min read
