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 is about 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 represented 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.

Basic operation of the M-209 is relatively straightforward. Six adjustable key wheels on top of the box each display a letter of the alphabet. These six wheels comprise the external key for the machine, providing an initial state, similar to an initialization vector, for the enciphering process.

To encipher a message, the operator sets the key wheels to a random sequence of letters. An enciphering-deciphering knob on the left side of the machine is set to "encipher". A dial known as the indicator disk, also on the left side, is turned to the first letter in the message. This letter is encoded by turning a hand crank or power handle on the right side of the machine; at the end of the cycle, the ciphertext letter is printed onto a paper tape, the key wheels each advance one letter, and the machine is ready for entry of the next character in the message. To indicate spaces between words in the message, the letter "Z" is enciphered. Repeating the process for the remainder of the message gives a complete ciphertext, which can then be transmitted using Morse code or another method. Since the initial key wheel setting is random, it is also necessary to send those settings to the receiving party; these may also be encrypted using a daily key or transmitted in the clear.

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