The 1944 Bletchley Park Cryptographic Dictionary
Codes and Ciphers
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RECIPROCAL a :
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 :
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 :
RECOGNITION GROUP :
2. A subtractor or other key or any part thereof obtained by solving or reconstruction.
3. A code-group identification.
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