Page:Preliminary Historical Report on the Solution of the "B" Machine.pdf/7

 11. A method for this conversion to the same base was developed and was termed "the identification of homologs." That is, an attempt was to be made to establish that a given letter on a certain day and another letter on a different day were treated in an absolutely identical or, more accurately speaking, manner by the machine when set to the same indicator. This conversion process is too involved to explain in this report but suffice it to say that difficult though it is, it was successful in two cases. One of these yielded a set of 6 messages, all in indicator 59173, which could all be reduced to the same base. These formed the crucial set of messages from the study of which success in solution of the machine was finally achieved.

12. Distribution tables of the letters constituting the text of these six messages were made. It should be stated that in four of these six crucial messages only fragments of plain text had been reconstructed, here and there; the complete or nearly complete plain texts of only two of these six messages had been reconstructed. However, enough data were accumulated from these two completely and the other partially reconstructed messages to yield distribution tables which, on careful examination, disclosed the presence of repeated sequences, here and there. This, on September 20, 1940, at about 2:00 P.M., was the very first indication that a successful attack might be possible. There was much excitement at this first glimmer of light upon a subject that had for so many months been shrouded in complete darkness and regarded occasionally with some discouragement. The nature of the distribution tables referred to is also too involved to explain in this report, but suffice it to indicate that they showed certain symmetries between the successive cipher equivalents of a given plain-text letter and the successive appearances of that plain-text letter in the cryptographic text.

13. As soon as the existence of cyclic or symmetric sequences became clear, attempts were made to uncover complete basic sequences of the type theoretically predicted. But many conflicts and inconsistencies soon developed, due to the fact that the cryptographic laws underlying the shifting from sequence to sequence were still unknown. Concurrently with the work connected with straightening out and removing inconsistencies in these reconstructed basic sequences ran the work of uncovering the cryptographic laws referred to, and very soon the general nature of the latter became quite clear. All efforts were concentrated upon the development of the specific laws and specific basic sequences applicable to the indicator under study, viz, 59173, with a view to uncovering all the cryptographic phenomena in this case and then searching for analogous phenomena in the case of other indicators. Certain qualified personnel from other sections were brought in to assist, and a considerable amount of night work was found desirable in order to push this study to completion at the earliest possible moment.

14. By September 27, just one week later, the work had progressed to a point where it became possible to hand in two translations representing the very first "" to the B-machine. Two messages of recent dates, both in the 59173 indicator, were available and were solved by