The First-Ever Recreation of the Iconic SIGSALY WWII Unbreakable Speech Scrambler

The logistics of military communications during World War II were imperative to a successful campaign, to put it lightly. Everything from troop movements, to supply chain information, to tactics all had to be relayed, but would result in disaster if they fell into enemy hands. That’s why a top secret and unbreakable speech scrambling device called SIGSALY was developed, and Jon D. Paul has built the first-ever recreation of the device for the NSA’s National Cryptologic Museum.

You’ve probably heard of similar systems like Germany’s Enigma machines and the United State’s Native American code talkers. Both were implemented for the express purpose of keeping military communications from prying enemy ears. But, SIGSALY was uniquely unbreakable, and pioneered many technologies that we still use today. As a top secret project, SIGSALY’s inner workings have largely remained a mystery, but through decades of studious research, Paul was able to recreate the key ingredient in its operation, the quantizer.

SIGSALY was able to scramble speech with a one-time digital encryption key, which was why it couldn’t be broken. But, this was the 1940s, and “digital” barely existed — especially not for audio recordings. For it to work, SIGSALY’s creators had to develop an entirely new method of analog-to-digital voice conversion. That ADC was state-of-the-art at the time, and its what Paul has painstakingly recreated over the past three years.

While modern ADC chips can easily digitize the full audio spectrum, the SIGSALY had to make due with just 12 slices of speech spectrum. That was all done with primitive, discrete components. To build his replica, Paul had to source those antique parts. They were combined with a few modern components, such as transformers, to complete the project. Paul built this recreation specifically for others to learn about SIGSALY, which is why it’s now in a museum where it can be enjoyed by everyone.