Monday, August 1, 2016

The US military attaché double transposition cipher

Apart from the standard cryptosystems (Military Intelligence Code, War Department Confidential Code. M-138-A strip cipher) US attaches also had an emergency double transposition cipher. According to the instructions for this system, found in the files of Pers Z (decryption department of the German Foreign Ministry) (2):

Use of cipher. To enable M/As to exchange safely secret or confidential messages with other attaches or with assistants or agents acting under their direction, the double transposition cipher is prescribed.

Keys. The keys will be determined by the M/A. They will consist of short phrases consisting of from five to twenty letters. They will be changed at frequent intervals.

However the same numerical sequence was used for both cages, which means that this system would have been vulnerable to cryptanalysis.

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