The 1944 Bletchley Park Cryptographic Dictionary

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DIFFERENCE, v :

1. To obtain the differences, usually only the minor differences, of every possible pair in a number of (good) code-groups or in a column of cipher-groups in a depth of reciphered code.

2. To subtract a length of subtractor key, esp. of letter-subtractor machine key, from itself at the interval of one of the basic components (or wheels) in order to eliminate the component whose period is that interval, as a step in key-analysis or key-breaking.

3. To subtract two cipher texts (produced by a letter-subtractor machine) which are in depth with each other, the one from the other, thereby eliminating the key and obtaining the difference of the two clear texts, as a step in reading a depth of two messages, by the stencil-search method.

4. To subtract a cipher text from itself, usually at an interval of one, as e.g. in Tutte's method.

DIFFERENCE BOOK :

A book containing in numerical (or alphabetical) order usually the minor (but sometimes the major or also the major) differences between every pair of a convenient number of 'good groups' in a code-book, together with the two good groups from which each such difference arises.

DIFFERENCE PAPER :

Paper ruled in columns of suitable width to contain a group of cipher, and in sets of three lines, so that the cipher groups, code-groups, and plain-language equivalents of messages can be written below each other.

DIGRAPHIC :

Consisting of, or having as unit, two figures (or letters).

DILLYISMUS :

(in Enigma). A method of determining stecker when wheel-order, ring- stellung, and message-setting have been found or probably inferred, consisting in deciphering the message on an unsteckered machine, assuming each letter self-steckered in turn, and making twenty-six counts or dotteries on the same counting sheet, one for each assumption.
Self-steckered letters are usually discernible from the fact that they yield similar counts and the stecker of the commonest letter will normally be shown by the best group in these. cf. Dottery.

DINGY, a :

(of a crib). That has repeatedly proved, and is consequently considered likely to prove, unsuccessful.

DINGY n :

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