Page:Preliminary Historical Report on the Solution of the "B" Machine.pdf/2

 PRELIMINARY HISTORICAL REPORT ON THE SOLUTION OF THE "B" MACHINE Part I. - Technical

1. In the latter part of 1938, messages appeared in a special secret Japanese cipher giving the authorization for travel for a "Communications Expert" named Okamoto, in order that he might put into service certain cryptographic paraphenalia [sic] termed by the Japanese diplomatic offices as the Type "B" cipher machine. This machine was to replace the then currently used Type "A" machine for highly secret communications among the important Japanese embassies1 throughout the world and the Foreign Office in Tokyo. On February 19, 1939, a message bearing the date of origin as February 18, 1939, in superenciphered code (K-1 transposed and enciphered by special A-machine procedure) was intercepted and was found to give the effective date of the initiation of the B-machine as February 20, 1939. The A-machine was still to be used by all holders for certain classes of communication.

2. Among the first messages received after the effective date of the B-machine were three messages originating in Warsaw,which had a new type of indicator instead of the normal "A" type indicator. Examination of these messages showed that they were definitely not "A" type messages, but due to the fact that six of the twenty-six letters appearing in the text of the messages were abnormally high (as they would have been had the A-machine been used for their encipherment) it was assumed that the messages were prepared by the B-machine and that it was a modification of the basic A-machine. Further intercepts tended to corroborate this theory. The A-machine was continued in regular use at Hsinking and Shanghai and very occasionally (apparently when the B-machine was out of commission) the A-machine continued to be employed at the places which had been provided with B-machines.

3. After a brief study it was confirmed that the division of the letters into two categories (one group of six letters and another group of twenty letters) which was the basis of the cryptographic treatment in the A-machine was retained in the B-machine but with a very important change. Whereas in the A-machine the 6 letters comprising the "6's" as well as the 20 comprising the "20's" were enciphered by means of what had been deduced as being a rotating commutator, whose stepping was controlled by a break wheel of 47 positions with certain skips in the cycle (the commutator could advance 1, 2 or 3 steps at a time), in the B-machine the "6's" were enciphered by means of a series of 25 hetergeneous [sic] and differently mixed alphabets which were constant in their nature and cyclic repetition. These 25 alphabets were merely a carefully selected set of 25 of the possible 720 permutations or transpositions of 6 elements taken 6 at a time, and a

1Washington, Berlin, London, Paris, Moscow, Rome, Geneva, Brussels, Ankara, Shanghai, and Peking.