SIGSALY: Churchill's top secret hotline to the president
iwm.org.uk
iwm.org.uk
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.
(I see now that it was already linked elsewhere in the the thread.)
Pretty impressive for cross atlantic one-time pad voice comms in an analogue era, I think. It's amazing what money and smart people can accomplish.
Also found this fantastic page: https://www.cryptomuseum.com/crypto/usa/sigsaly/
If they were classified, they can’t have worked as patents. Patents, by their nature, must be open and available for others to read.
Invention Secrecy Act of 1952. https://en.wikipedia.org/wiki/Invention_Secrecy_Act
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...
Splitting audio into bands and voiced/unvoiced content is basically how many "modern" low-bandwidth voice coding techniques like LPC, MELP, or CLEP work (see https://en.wikipedia.org/wiki/Linear_predictive_coding). The point of that processing is to compress, not encipher, the voice data.
Analog scramblers, like A-3, by contrast, shift the frequency content of the speech as the means of encipherment.
The point of the SIGSALLY voice coder is simply to keep the amount of key material for the one time pad reasonable. Even with that compression, each OTP record lasted only 12 minutes. MELP, which is a current DoD vocoder standard for use in secure voice channels, produces intelligible speech rates as low as 600 bps.
The part that is definitely a kluge is the key synchronization method. It seems that they did not use a PLL (which had already been invented by that point) to keep both ends in sync and instead relied on highly precise frequency references and turntables driven by synchronous motors with hundreds of poles.
> 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?
This was at the dawn of the age of modern information theory, built on the shoulders of giants, but there are often some subtleties.
An example: if we imagine the encryption is perturbing a waveform +/- 30% of its frequency and amplitude at any one time, the encrypted bitstream is likely totally intelligible to the ear when replayed, even if reconstructing the exact bitstream is difficult.
Another example - if you make a block cypher that relies on prior blocks to seed randomness, then encrypting say 1,000 sequential blocks of zeros is very likely going to have some structure to it unless you've specifically designed it not to.
I'd love to hear some waveforms of what these calls sounded like encrypted -- it would be a fun project to work on decrypting them with modern tech.
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.
So, depending on how you count, these were $19-150m each in today's dollars, or between 1/10 and 1/3 of an ASML EUV machine.
My lesson for today is not to trust my ability to count comma separated digit groups.
I also think the headline as is would be good enough even for people who might not have learned about ww2 yet. A headline is not meant to be a complete summary after all...
There is an illustration for the encryption process in Appendix B here: https://www.nsa.gov/portals/75/documents/about/cryptologic-h...
It's a slightly alternate universe. It's not quite our world, it just looks kind of like it from a distance. Qwghlm isn't really Skye and Lewis, it's like if you kind of wodged the Falklands and the Isle of Man into the same space.
Some of the people are kind of the same but not quite.
But from way over here, you'd be hard pushed to spot the differences, unless you looked annoyingly closely.
A little bit of analog buffer would help. It wouldn't be a problem for transistor era electronics. You need something like an analog shift register (see "bucket-brigade device"[1]) Those were not developed until the 1970s, and, for a while, appeared in guitar pedals, reverbs, and other simple audio processing devices. If you had to do that with tubes, you'd have a whole rack of tube delay line.
Fun retro electronics project for someone.
Besides, even with the automated start system, the actual spin-up time of the turntables would have been a little chaotic. I suspect they were only aiming to get the four turntables within about 100ms of each other. The decrypted signal itself could then be used for the final fine-tuning of the timings (which also included adjusting for any speed-of-light delays)
If you weren't within 20ms, the operator would only hear noise (like a radio). Once they were within ~20ms, the operator would start hearing silence and would tune the phase until all noise went away.
You don't really need exact synchronisation because like a radio, the operator could just scrub the tuning to find the silence.
The synchronised clocks meant that once the operators had locked on, it would stay locked for a long time.
[1] https://www.nsa.gov/portals/75/documents/about/cryptologic-h...
> It came to be known as SIGSALY