Expository Essay – SIGSALY

I found the entire method of enciphering a message with SIGSALY to be very interesting. It seems completely different from the other ciphers and cipher machines we have learned about, and you do a good job of highlighting its unique qualities. The way in which the original audio signal is broken up, assigned numerical values, and then resynthesized into an understandable audio clip is fascinating and complex. When you also consider the onetime pad method of assigning keys, it is easy to see why SIGSALY was never likely to be deciphered.

Your explanation of the complex method by which the message was broken up, transmitted, and recreated is a real strength of the paper. I was wary beginning the paper, thinking that this particular cipher may be particularly hard to understand given its unique nature. However, you provide a lucid of each stage of the process, breaking it up into manageable steps. Rather than just state what each step was, you go so far as to explain why each step took place. For example, you give the reason that the six levels did not progress linearly, instead of just stating that they did not. The diagram summing up the process serves as a simple, handy complement to the explanation. You also do an excellent job of describing other aspects of the cipher, such as the physical qualities of the machine (huge, heavy, and energy inefficient), its operation by the 805th Signal Service Company, and its drawbacks (size/maintenance and distribution of keys).

There are a few aspects of SIGSALY that I would like to know more about. One topic that you could cover in greater depth is the development of the machine. Some background information on Clark and his career could be helpful. This leads to the answers of other questions, such as why Clark decided to use vocoder technology instead of using a more traditional cipher method. I would also be interested in knowing how closely the government/military was involved with SIGSALY. It seems that many ciphers were created by military employees. How did Bell Telephone, as a private company, get involved with the military? Did the government approach Bell about the issue, or did Bell independently decide to attack the problem? Was the project funded by the government, or did Bell develop SIGSALY for free out of patriotism? On a separate note, did any similar technology exist on the Axis side of the war? These are some of the questions that popped into my head as I read.

You make some good connections between SIGSALY and other topics in the course. You mention the involvement of Alan Turing in developing SIGSALY, and Turing’s work on the Naval Enigma. You also explain the modular arithmetic that is used in SIGSALY, which is used in many other ciphers we have looked at. Additionally, you describe SIGSALY’s use of the onetime pad key method, another technique that we have previously discussed. Furthermore, you address the difficulty of key distribution as a drawback of this cipher, which is an important issue ranging from the earliest substitution ciphers to RSA encryption. You hit upon another theme we discussed, the value of cryptography in war, by noting the significance of the “unparalleled security” provided by SIGSALY. Finally, (though it has nothing to do with connections to other topics) I must say I enjoyed your clever concluding line. Overall, my compliments to you on a well-written paper.