The portable cipher machine M-209
In cryptography, the M-209, designated CSP-1500 by the United States Navy (C-38 by the manufacturer) is a portable, mechanical cipher machine used by the US military primarily in World War II, though it remained in active use through the Korean War.
The M-209 was designed by Swedish cryptographer Boris Hagelin in response to a request for such a portable cipher machine, and was an improvement of an earlier machine, the C-36.
The M-209 bout the size of a lunchbox, in its final form measuring 3.25 by 5.5 by 7 inches (83 mm × 140 mm × 178 mm) and weighing 6 pounds (2.7 kg) (plus 1 pound (0.45 kg) for the case).
It esented a brilliant achievement for pre-electronic technology.
It used a wheel scheme similar to that of a telecipher machine, such as the Lorenz cipher and the Geheimfernschreiber.
Security of the M-209 was good for its time, but it was by no means perfect.
As with the Lorenz Electric teletypewriter cipher machine (codenamed Tunny by the Allies), if a codebreaker got hold of two overlapping sequences, he would have a fingerhold into the M-209 settings, and its operation had some distinctive quirks that could be exploited.
As of early 1943, German cryptanalysts were able to read 10-30% of M-209 messages.
It was, however, considered adequate for tactical use and was still used by the US Army during the Korean War.
US researcher Dennis Ritchie has described a 1970s collaboration with James Reeds and Robert Morris on a ciphertext-only attack on the M-209 that could solve messages of at least 2000–2500 letters.
Ritchie relates that, after discussions with the NSA, the authors decided not to publish it, as they were told the principle was applicable to machines then still in use by foreign governments.
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