From the archives: Mercury

I had chance to spend a day at the UK National Archives a couple of weeks ago, and came across a file (AVIA 65/977) discussing cash awards to the designers of Mercury, an on-line cipher machine used by the RAF from 1950 to the 1960s. I wrote up all the information I could glean about this machine in a Wikipedia article.

Mercury embodies a similar principle to the US SIGABA (ECM Mk II) machine: some rotors control the movement of other rotors. It would appear that Mercury was an independent reinvention of the concept, as the ECM was around a decade older. The ECM concept had been kept secret from the UK by the US, and a Combined Cipher Machine (CCM) was used instead for inter-allied communication.

Another feature of Mercury appears to be that of "double rotors". Exactly how this worked wasn't clear from the PRO documents, but one plausible guess is that there were two independent wirings inside each rotor core, and that these wirings could be set in 26 offsets from each other, and optionally reversed. The current would pass once through the "inner" wiring, and then back again through the "outer" wiring. Ordinary Typex rotors had a "double" contact feature used to improve the reliability of the electrical contacts; this could have been pressed into service as a double rotor feature. Each pair of double rotors would, of course, step together.