The 1944 Bletchley Park Cryptographic Dictionary

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RECIPROCAL a :

1. Producing zero or a series of zeros when added non-carrying to a given figure or series of figures.

2. (of substitution systems). Consisting of or involving pairs of letters or groups so related that when one is the cipher equivalent of the other, the other is also the cipher equivalent of the one.

RECIPROCAL n :

1. That figure (or letter) which when added non-carrying to a given figure (or letter) gives zero as the sum e.g. in the scale of 10, 9, is the reciprocal of 1, 8 of 2, etc.

2. (in two-box Playfair). A bigram having as its enciphered form the same two letters in the opposite order.

3. (in Enigma) A backward click.

RECIPROCAL IDENT :

An identification obtained from another by utilizing the fact that the trigram substitution tables concerned are reciprocal.

RECOGNITION GROUP :

(used for discriminant).

RECOVER :

To solve or reconstruct (a key or part of a key).

RECOVERY :

1. Action or process of solving or reconstructing subtractor or other keys.

2. A subtractor or other key or any part thereof obtained by solving or reconstruction.

3. A code-group identification.

REDUCE :

(in full 'reduce to the same basis'). To equate (columns of a depth of reciphered code messages).

REDUCTION :

Action or process of equating columns e.g. of a depth of reciphered code messages.

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