Page:Preliminary Historical Report on the Solution of the "B" Machine.pdf/3

 deciphering chart or "development" was constructed to correspond with these 25 permutations. This chart was revised and corrected from day to day until it became certain that all its elements were absolutely correct.

4. This having been accomplished (by April 10, 1939), it became possible, as a result of cryptanalytic technique elaborated for the purpose, to decipher the "6's" in practically every message of any considerable length in the B-machine. It was found that so far as the "6's" between two messages with unlike indicators were concerned, the only difference between one indicator and another was the starting point in the cycle of 25 alphabets. There were 120 different indicators but only 25 different starting points, so that four (in certain cases, five) different indicators represented the same starting point.

5. When the "6's" in a given message were deciphered, the plain text values of cipher letters scattered here and there throughout the text became available, so that the skeletons of words and phrases offered themselves for completion by the ingenuity and the imagination of the cryptanalyst. For example, suppose that on a given day the 6 letters forming the "6's" were E Q A D R H and the following text was at hand:

It is not difficult to imagine that the missing letters are those shown below:

In this process of filling in the plain text values of the "20's" the cryptanalyst could be guided only by two things: (1) the positions and identities of the deciphered "6's" and (2) the context. For it speedily became apparent that any cryptographic relationship between the plain text and the constantly-shifting cipher text values in the case of the letters constituting the group of "20's" had been most carefully eliminated, disguised, or suppressed. This fact corroborated the conclusion drawn from all statistical and analytical tests made on the cipher texts of the various messages studied.

6. The process of filling in the plain text values of the "20's" was therefore,as a rule, a very difficult matter, depending usually upon the particular assortment of letters constituting the "6's". If the text was in Japanese there was, in addition to the difficulty inherent in that language itself, the added perturbation occasioned by the fact that the Japanese Foreign Office had, on May 1, 1939, instituted a species of "Phillips Code" in connection with their use of the B-machine, with a long series of arbitrary letters and abbreviations standing for numbers, punctuation signs, and frequently used combinations of letters, syllables, words, and sometimes complete phrases. For instance, the combination C F C represents ; C C F represents ; the single letter L (not normally used in Japanese)