SIGSALY: The 50-Ton Machine That Let Churchill Call Roosevelt in Secret
Sigsaly: Churchill's top secret hotline to the President
Before the US entered World War II, Churchill and Roosevelt relied on scrambler phones that censors could cut off and encrypted telegrams. In 1940 Bell Telephone Laboratories began Project X, later SIGSALY: a system that digitized the human voice, mixed it with random noise from synchronized records, and sent it by radio across the Atlantic. Each terminal cost $1 million and weighed 50 tons; one sat beneath Selfridges in London, with an extension hidden in a fake toilet cupboard at the Cabinet War Rooms.
Even within this secret site, the existence of the Transatlantic Telephone Room was kept secret from the people who worked there.
- Merrill
A 1983 review of this remarkable system for the Institute of Electrical and Electronic Engineers (IEEE) attributes no fewer than eight "firsts" to SIGSALY. They are as follows:
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.
- NitpickLawyer
> a pair of vinyl phonograph records, each containing identical-but-random noise.
> These were the (single-use) keys to the encryption and decryption process.
> 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?
- Animats
Here's the technical description, from NSA.[1]
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 anal […]
- ahmedfromtunis
I know this is a bit offtopic, but shouldn't the title say "the President of the United States," since 1) Churchill is UK's Prime Minister, and Roosevelt is not his President and 2) the publication on which this article appears is from the UK?
- djmips
Two turntables and a microphone...