The 1944 Bletchley Park Cryptographic Dictionary

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2. Playfair using two letter-squares and two distinct operations in enciphering each bigram;

DOUBLET :

A bigram, especially in Playfair, consisting of a repeated letter.

DOUBLE TRANSPOSITION :

A cipher system in which the letters of the plain language are successively transposed twice either on the same or on different keys. (see transposition).

D.R. :

Short for Decoding Room.

DRAG :

1. To try (a likely clear word or crib) in successive positions in a message in order to find: (a) in a polyalphabetic cipher a position that gives a likely key-sequence, and so reading the message; (b) in a machine cipher depth a position that yields good sense in the other message or messages, and so finding the key and setting the messages. See stencil-search table; (c) in Enigma, to stagger (a crib); see Stagger, v.2.

2. To try (a good group) in successive positions in a reciphered code message with a view to finding a previously ascertained subtractor group which may enable the message to be set in depth with others; also to seek (a synthetic group) in successive positions.

DRAW INTO :

To include (dummies) in the columns of text taken out of a transposition cage.

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