Playfair squares

: What is the best way to go about solving a Playfair word square(i.e. to
: find the keyword)? It seems trial and error works fine, but how does one
: speed up the process?

In the context of crosswords, to look beyond trial and error seems rather
pointless, as normally compilers will give you just enough information to
deduce the code square with as little redundancy as possible: so the keyword
will typically be fairly obvious once you have sufficient pairs of letters.

The only trick I know of is to look for sequences that have a high
probability of being in the alphabetical string that follows the keyword:
e.g. BCD, KLM, XYZ etc. The problem is that naughty compilers are liable
to use keywords with as many letters of the alphabet as possible, like
DERMATOGLYPHICS or even VOLKSWAGEN-TRIUMPH.

Long before it provided inspiration for crossword compilers, Playfair
was a code of practical importance: e.g. it was used by the British
army between 1900 and 1914. Derivative forms of the cipher were still
in use in the second world war, as is recorded in "Codebreakers -
The Inside Story of Bletchley Park" (OUP, ISBN 0-19-285304-X). Gaines's
"Cryptanalysis" (Dover, ISBN 0-486-20097-3) is another book with a
chapter on real-world Playfair decryption techniques. Here's an example
from it (the cleartext is known to start "enemy agents"):

OS CF WD OG DR AN PO AS OA DH SD EH XK FU CN DR MS UK SD

--
Ross Beresford Author of TEA - word search for crosswords
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