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Portex   BID/50/1
Electromechanical cipher machine - wanted item

Portex was a British electromechanical cipher machine that was used by secret services in the UK during the 1940's and 1950's. The machine is similar to the German Enigma machine, but is more advanced in that it features 8 cipher wheels with 26 contacts each, and has irregular stepping.

Plaintext is entered one character at a time by setting the alphabet ring at the left side to the desired letter and rotating the crank at the right. The letter then fires an electric current into the drum, starting with the leftmost wheel. The current that leaves the wheels at the right is used to drive the printer which is mounted at the left. The output is delivered on a (pre-gummed) strip that leaves the machine at the left rear.

Apart from similarities with the German Enigma, Portex also seems to have 'borrowed' features of the Swedish/Swiss Hagelin pin-wheel machines.
Portex cipher machine with top lid open

The design of the printer, the alphabet ring and the character counter (all mounted to the left of the cipher wheels) show great resemblance to portable Hagelin machines such as the M-209 and C-446. In fact, Portex can best be seen as a hybrid between a Hagelin M-209 and and Enigma G.

Portex was used by British secret services but also by the Canadian Army. The civil version was generally black, whilst the military variant was typically green and had some additional features, such as a push-button above/behind each cipher wheel. It is known that the military variant was still 1in use with the Canadian Army in the early 1960's [1].

Portex cipher machine with top lid open Portex cipher machine interior Close-up of the cipher wheels Close-up of the cipher wheels Close-up of the cipher wheels David White behind a Portex cipher machine (2009)

The image below shows a Portex cipher machine seen from the front left with its top lid open. At the front right are the eight electric cipher wheels that are moved mechanically on each key press. At the front left is the printer mechanism, the text entry knob (alphabet ring) and the counter.

Towards the rear, mounted behind the cipher wheels, are a couple of solenoids which are used to operate the machine's wheel stepping mechanism. The exact operation of the mechanism is currenly unknown. Any additional information about this machine, its operation and its use would be much appreciated.

Irregular stepping
Portex shows great resemblance to the Enigma machine that was used by the German Army throughout WWII. Whilst the Enigma I featured regular stepping, similar to the operation of an odometer, there was one class of Enigma machine that was driven by a stepping gear and had multiple turnovers on each wheel. This variant was known as the Zählwerksmachine or Enigma G.

The image on the right shows a close-up of the 8 cipher wheels inside a Portex machine as it was on public display at the Enigma Reunion in 2009, where it was presented by David White.

Machines with irregular stepping are generally more difficult to predict and hence are more difficult to break. The stepping of the British Portex machine is nearly identical to that of the Zählwerk Enigma and it is most likely that part of the design was 'borrowed' from it.

 More about Zählwerk Enigma
Close-up of the cipher wheels

Help required
We are currently looking for additional information about the Portex and ideally, a real machine for our collection. The exact operation of the machine is currently unknown and we do not know exactly how input and output were handled. If you have more information, please contact us.

  1. Jerry Proc's crypto pages, Portex - BID/50
    Retrieved January 2015.
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Crypto Museum. Created: Sunday 25 January 2015. Last changed: Thursday, 22 February 2018 - 22:14 CET.
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