Combined Crypto, Anglo-American Style | Hackaday

If you think about military crypto machines, you probably think about the infamous Enigma machine. However, as [Christos T.] reminds us, there were many others and, in particular, the production of a “combined cipher” machine for the US and the UK to use for a variety of purposes.

The story opens in 1941 when ships from the United States and the United Kingdom were crossing the Atlantic together in convoys. The US wanted to use the M-138A and M-209 machines, but the British were unimpressed. They were interested in the M-134C, but it was too secret to share, so they reached a compromise.

Starting with a British Typex, a US Navy officer developed an attachment with additional rotors and converted the Typex into a CCM or Combined Cipher Machine. Two earlier verisons of the attachment worked with the M-134C. However the CSP 1800 (or CCM Mark III) was essentially the same unit made to attach to the Typex. Development cost about $6 million — a huge sum for the middle of last century.

By the end of 1943, there were enough machines to work with the North Atlantic convoys. [Christos] says at least 8,631 machines left the factory line. While the machine was a marvel, it did have a problem. With certain settings, the machine had a very low cipher period (338 compared to 16,900 for Enigma). This wasn’t just theoretical, either. A study showed that bad settings showed up seven times in about two months on just one secure circuit.

This led to operational changes to forbid certain settings and restrict the maximum message length. The machine saw service at the Department of State until 1959. There were several variations in use within NATO as late as 1962. It appears the Germans didn’t break CCM during the war, but the Soviets may have been able to decode traffic from it in the post-war period.

You can see a CCM/Typex combo in the video below from the Cryptomuseum. Of course, the Enigma is perhaps the most famous of these machines. These days, you can reproduce one easily.