The mechanics of the Double Playfair are involved enough that it is expected that
solving this cipher would be most readily achieved by a computer program that
recovers the first square by shifting letters around while testing for common
letter sequences and words in the resulting text. At Bletchley Park, this
process was performed by hand and was called "anagramming," since it moved
around the 25 letters of each key square, and it was guided by digraphs that
had been identified and by those that were found to be reversible.
The first square uses the keyphrase CHEAPSWORD, so that the total
C S B L U B R I G H
H W F M V T O N C K
E O G N X A D E F L
A R I Q Y M P Q S U
P D K T Z V W X Y Z
The plaintext is:
TO STRIKE GROUP WOTAN STOP
YOUR CONTACT IS PROPRIETOR OF NEWS STAND AT BUXLEIGH AND CHUDWORTH STOP
PREPARE TO EXECUTE PLAN WALKUERE ON SIX DECEMBER STOP
NO FURTHER WIRELESS CONTACT MESSAGE ENDS
While there would have been no way to directly identify members of the strike group
using this cipher, at least analysts would have had a contact they could have
watched. If the message was decrypted in time, they might have seen him or her
pass information to a member of the strike group, thereby enabling the analysts to make further progress.
The Double Playfair system is described in recently declassified documents in
the National Archives. They were formerly MOST SECRET and were produced by Bletchley Park.