The 1944 Bletchley Park Cryptographic Dictionary

- 14 -

CHI-2 FUNCTION :

The addition (by a species of Boolean addition making two crosses add to a cross) of the pattern of the second chi wheel in the Tunny machine to the motor pattern produced by the two motor-wheels at the next (following) position; also, the addition (non-carrying) in the scale of 2 of the pattern of the second chi wheel to the fifth impulse of the preceding plain letter, followed by the addition of the reverse of this result by Boolean addition, to the pattern produced by the two motor-wheels one position later (of. KTF), the final result being the motor at this later position.

CHIASMIC DIFFERENCES :

Two-column differences produced from two adjacent groups in any one message and any two groups in the same columns whether in one or two different messages (usually in a shallow depth), used in searching for cliches whioh have the same good groups opposite corresponding groups of them at different points of a depth but which, as these two groups are not in both cases ( or in either case) in the same message, are not revealed by ordinary two-column differencing

For example, they would reveal the two groups cliche (denoted by CLI CHI) occurring in conjunction with two good groups (denoted GG1 and GG2) in such an arrangement as the following. (The maximum number of messages involved is six, though more would naturally be examined if available):

... ... GG1 ... ... ... ... ... ... ... ...
... ... ... ... ... ... ... ... CLI CHE ...
... ... ... GG2 ... ... ... ... ... ... ...
... ... ... ... ... ... ... ... GG1 ... ...
... ... CLI CHE ... ... ... ... ... ... ...
... ... ... ... ... ... ... ... ... GG2 ...