Playfair cipher decoder
Decrypt Playfair with a known keyword, using the 5x5 digraph square. Recognisable by its even length and total absence of doubled letters.
Open in ctfpalPlayfair encrypts pairs of letters using a 5x5 square built from a keyword, with I and J sharing a cell. Because it works on digraphs rather than single letters, single-letter frequency analysis is useless against it - which is what made it a genuine field cipher for decades.
Recognising it
- The ciphertext length is always even.
- The letter
Jnever appears (it was merged intoI). - No digraph ever contains a doubled letter -
LLcannot occur in a pair, because the encoder inserts anXto split them. - Stray
Xcharacters at odd positions in the recovered plaintext are the padding, not part of the message.
The three encryption rules - same row, same column, rectangle - all reverse cleanly, so decryption with a known keyword is mechanical. Without the keyword, Playfair yields to simulated annealing on digraph statistics, which needs a good deal more ciphertext than a monoalphabetic break.
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