The 1944 Bletchley Park Cryptographic Dictionary

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BOOK KRAC :

(in G.A.F. codes). A solution or identification of a group, especially in a three-figure alphabetical book.

BOOLEAN ADDITION :

A system of addition for units belonging to different, but not mutually exclusive, classes, which avoids counting more than once units which belong to more than one of these classes; e.g. Boolean addition is used to sum the kicks (i.e. activated bars) of individual wheels of a Hagelin machine using overlapping clips; any bar with two clips each opposite an active peg for the position in question is thus counted only once in the total.

BOUSTROPHEDON :

Taking each successive line or column in the direction opposite to that in which the previous line or column was taken.

BOX, n :

1. A Playfair square, especially one used in double Playfair systems.

2. A cage in transposition systems.

3. (in Enigma). A cipher alphabet so arranged and if necessary so divided into compartments that when one of the components is slid on one position (cyclically in each compartment) a second cipher alphabet is produced.

BOX, v :

(of Enigma indicators). To show repeats in the second three letters corresponding to repeats in the first three, thereby suggesting that the message settings are being enciphered twice on a Grundstellung.

BREAK :

1. (required codes). To ascertain or 'recover' (the key or subtractor) by placing a number of messages 'in depth' and identifying, usually from differences, already known 'good groups' in all the columns.

2. To establish the plain language equivalents of the groups of (a code-book).

3. (machine ciphers). To decipher simultaneously (two or more messages known to be in depth) by finding, usually by trial and error, key-letters which make reasonable sense of both or all the messages; to read (a depth).

4. To resolve (a length of key obtained by the above means) into its basic, short-period, components by analysis; to establish thus the essential structure and method of operation of (a cipher machine).

5. To reduce (any type of cipher) to plain language; to reconstruct (any cipher system).

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