The History and Mathematics of Codes and Code Breaking

Expository Essay - The Purple Machine

Here's Jon's paper on the Purple machine [PDF].


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  1. Erin Baldwin

    I liked that this paper was written on a truly unique topic, one that we had not covered in any form during our class discussions. The introduction of the paper was great. It effectively set the stage with reference to previous codes in the history of cryptology while preparing the reader to hear about the main point of the paper, the Purple machine. I thought the parallel with the Mary Queen of Scotts decipherment was very intersting, though I would have liked to see the two subjects tied together in the end. Especially so since the Japanese received intelligence that their code had been broken, but were so confident in their cipher that they continued to send very important diplomatice messages in the code. It was similar to how Mary was so confident in her cipher and in her messenger that she wrote blatantly about their plans.
    One of the many strengths of the paper was the explanation of the inner-workings of the Purple machine itself. It was easy to follow and made such a complicated machine very conceivable for the reader. The technique of building upon the red machine, which had been previously described not only helped the reader to understand how the machine worked, but also added to the historical context of the cipher machine. This section of the paper was very well done.
    I think that the only area in which the reader was left wanting, in reading this paper was in the relationship between the American code units and Bletchley park. It was the Americans who eventually broke Puple, but Bletchley Park is abruptley mentioned in the description. Did they also have a team working on the code when the Americans beat them to it? It would seem that it was in their best interest to try to crack it, as the decipherment was so useful on the European front as well as the Pacific. Also, did the Americans only share the "magic" that they received from the decipherings? Or did they share their actual technology with Bletchley Park? Finally, I would have liked to know more about the key that was used by the Japanese for Purple, specifically how it related to the machine. Did it include numbers, letters, positions of switches? How exactly were the Americans able to guess each day's key?
    Despite the fact that some secondary topics were not explored to their fullest potential, I really enjoyed reading this paper. It was an interesting take on a cipher machine from the World War II era, a time period in which the Enigma usually takes center stage. It was also very enlightening to hear about some of the innerworkings of the American cryptanalysis units and their cooperation with the British.

  2. Sam Mallick

    I largely agree with Erin. I enjoyed learning about a code that we hadn't talked much about. I know we've mentioned Purple once or twice in our WWII discussions, so it was nice to hear about it more in-depth. The paper puts Purple in an historical context that allows the machine's importance to be well-understood.

    As someone who's studied the Japanese language, it is interesting to read about a cipher system they used, though it seems like you explained it as if it were an English cipher to make the paper more accessible. I am also curious as to how the American government dealt with the issue of translation–was it deciphered and then given to translators or did the cryptographers have a basic knowledge of Japanese? With languages like French or German, letters are very similar to English. Japanese, however, is an Asian language with a radically different, syllabic alphabet, so I assume intercepting these communications would be very challenging.

    I thought the parallels drawn between Purple and Enigma were very appropriate–not only were they similar cipher systems, but also the breaking and continued use of the Axis communication happened in the same way. By making connections to the Enigma machine, which is much more familiar within the context of this course as well as public awareness, the paper is able to educate the reader more substantially.

  3. Tyler Merrill

    This exploration of the Purple Machine and its connection to the Enigma effectively evinces its importance during World War Two. Because the topic was omitted from the class discussions, it appeared interesting to me. The beginning sets up the history and explains the origins of the machine soundly. The history is complete and of adequate depth. The connection to the Red machine provides a solid base for the amendments made to create the Purple machine.

    For me, the explanation of how the machine works was vague. I do not understand what a stepping switch is. You say it is different than a rotor from the enigma machine, and that is clear. However, how the stepping switch actually works is not apparent to me. Also, you mention fast, medium and slow speeds for each disc. I did not understand what was meant by this. There was also a minor error made in this section. You stated that "apples" could be enciphered "dfnqq." There are six letters in "apples" but only five in "dfnqq." I feel like this section could have been explored deeper.

    The explanation of the decipherment of the code was concise, but superficial. The importance of the cipher is clear as the paper finishes. The explanation of the usefulness of the code breaking efforts in the Battle at Midway and D-Day demonstrates the historical impact of this cipher.

    Though the explanation of the cipher was at times unclear, the history and importance of the machine are strong enough to support this paper. The depth of importance of Purple deserves the attention given. The decision to sacrifice depth of the explanation of the cipher in favor of the importance renders an intriguing paper that brilliantly captures the interest of the reader.

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