1) The first realization of enciphered telephony

2) The first quantized speech transmission

3) The first transmission of speech by Pulse Code Modulation (PCM)

4) The first use of companded PCM

5) The first examples of multilevel Frequency Shift Keying (FSK)

6) The first useful realization of speech bandwidth compression

7) The first use of FSK - FDM (Frequency Shift Keying-Frequency Division Multiplex) as a viable transmission method over a fading medium

8) The first use of a multilevel "eye pattern" to adjust the sampling intervals (a new, and important, instrumentation technique)

From https://www.jproc.ca/crypto/sigsaly1.html

I think that the relevant patents remained classified until 1975.

> Then they set their identical records spinning at the exact same moment. On each end, the voices of these two world leaders are threaded through the SIGSALY terminal, mixed with a layer of noise, sent across the ocean via shortwave radio, then re-synthesized in a reverse process.

> Anyone attempting to listen in on the calls would just hear a buzzing sound, which is how the system earned one of its nicknames: “The Green Hornet,”

> At the end of each conversation, the records were destroyed to protect against potential future decryption.

I wonder if with todays tech we could decrypt this. This sounds really close to a one-time pad (if the conversation was kept shorter than each record) so .. probably not? Unless their random noise wasn't as random, or if the mixing of random + voice would leave other tell-tale signs?

It was really an awful kludge. Today, you'd think, digitize audio into a bit stream, encrypt bitstream. That's not how it worked. It was somewhat similar to the older A-3 analog scrambler, which split up audio into a number of bands and rearranged them, changing the rearrangement occasionally. In SIGSALY, incoming audio was filtered out into 10 frequency bands. There was also a dominant pitch detector and a "hiss" detector for unvoiced content. That created 12 analog signals to encrypt. Those signals were much lower bandwidth than the original speech. The patent seems to indicate that the frequency band signals were merely encoded as above or below a threshold. Not a full A/D, just a 1-bit threshold. As late as 1973, people were still trying vocoder-like technologies, but it became easier to work on audio in the digital domain and that died out.[2]

The keying system changed the key every 20ms. It's not clear from the description whether the digitization of the 12 signals was also one sample for 20ms, but it probably was. If you used the same key bits for more than one analog sample, that would create a vulnerability. It's a one time key system, and those only work against serious opposition if the key is only used once. See VENONA.

The articles indicate that synchronizing the key stream, which was on phonograph records, was a huge synchronization headache. Keeping signals in sync was really hard, and was part of the curse of analog TV, with horizontal and vertical hold controls.

What this looks like is a system which had to work regardless of cost. So it was put together out of known building blocks. Straight digitization of audio was probably considered, especially since Alec Reeves, who had demoed pulse code modulation in 1937, was involved. But it was a high-risk technology. They would have had to invent a good A/D converter and a decent modem, which would have added technical risk to the project.

It's one of those systems of historical interest because it was too soon. You couldn't really do the job in the straightforward way, but it needed to be done, resulting in a magnificent kludge.

[1] https://www.nsa.gov/portals/75/documents/about/cryptologic-h...

[2] https://www.youtube.com/watch?v=UfpUexWJqb4

I've learned that I tend to underestimate how large inflation is for historical values, so I looked this up. $1 million in 1943 is apparently just under $19.5 billion today, which means the full set cost $234 billion. I'm not sure if the US foot the bill for all of them or not, but to make it more concrete, 44 of these would have cost more than entire budget for the Department of Defense for this year.

[0] https://99percentinvisible.org/episode/vox-ex-machina/

There is an illustration for the encryption process in Appendix B here: https://www.nsa.gov/portals/75/documents/about/cryptologic-h...