M-209

In cryptography, the M-209 (also C-48) was a portable, mechanical cipher machine used by the US military in World War II. 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 was about the size of a lunchbox, and represented a brilliant achievement for pre-electronic technology. It used a wheel scheme similar to that of a telecipher machine, such as the Lorenz cypher and the Geheimfernschreiber.

User operation

From the code clerk's point of view, the M-209 was simple to operate. There were six wheels on top of the box that each displayed a letter of the alphabet through a window, and he could set these six wheels to provide the key for enciphering a message. This only gave a limited number of possible keys, but as will be discussed momentarily there were internal settings as well that could provide a much greater range of key settings.

To actually encipher a message, the code clerk first set an "encipher-decipher" switch to the "encipher" position, rotated an "indicator" knob labeled with the letters of the alphabet on the left of the box to the first letter to encipher, and then pushed down on a lever on the right side of the box. This printed an enciphered letter on a paper strip that reeled out the top of the box. The code clerk then enciphered all the other letters in the message in the same way, resulting in a listing of the ciphertext on the paper strip, which could then be read off for transmission by Morse code.

The lever on the right was called the "power handle", though the only power provided to the M-209 was muscle power. One suspects that code clerks using the machine acquired a disproportionately strong right arm. There was a "letter counter" indicator on top of the box, something like a automobile's odometer, to let the code clerk know how many letters he had enciphered in a message so he didn't lose place.

Deciphering the message was just as simple. The code clerk on the receiving end set the six wheels to the same initial positions, turned the encipher-decipher switch to "decipher", entered the first letter of the ciphertext on the knob on the left, and pressed the power handle to print out the first letter of the deciphered text on the paper strip. He then went through all the ciphertext letters to print out the deciphered message on the paper strip. It typically took about two to four seconds to encipher or decipher each letter, so the scheme was reasonably fast.

Encryption mechanism

The M-209 was unsurprisingly more complicated under the hood. In broad outline, when enciphering a message, pressing the power handle activated a mechanical enciphering mechanism that advanced a printwheel a certain number of letters ahead of the plaintext letter entered on the indicator knob, and then printed the appropriate ciphertext letter on the paper strip.

The printwheel was linked to the indicator knob, so that its position was always set to the plaintext letter before pressing the power handle and moving the printwheel through the offset. Of course, the offset changed with each plaintext letter entered, otherwise this scheme would have simply implemented a totally insecure Ceasar shift cipher.

The mechanical enciphering mechanism included two assemblies. One was an assembly of six wheels. Each wheel had pins that could be pushed through the wheel in either direction. A pin was pushed to the right to make it operational and pushed to the left to disable it. All six wheels were advanced one position with each push of the power handle, but the wheels had different numbers of pins, with 26, 25, 23, 21, 19, and 17 positions, meaning that the arrangement of wheels would only repeat itself after a period of 101,405,850 letters.

This assembly was very much like that found in a telecipher machine. The other assembly, the "cage", was much more unusual. It essentially translated the positions of the wheel assembly to the cipher offset of the printwheel. The cage looked exactly like its namesake, resembling a cylindrical birdcage with 27 bars held between two disks. The cage was laid on its side in parallel and behind the wheel assembly, and with the printwheel mechanism to the left.

The bars of the cage could slide and protrude out of the cage assembly on the left. When the power handle was pressed, the cage was spun around through a complete rotation, and the bars were selectively pushed out the left side as governed by the pin settings of the wheel assembly, forming a "variable toothed gear". The number of bars pushed out gave the printwheel offset, with an intermediate gear advancing one position for each protruding bar to move the printwheel forward. At the end of the power handle stroke, the letter was printed, and all the bars pulled back into the cage in preparation for enciphering the next character.

There were two moveable "lugs" on each of the 27 bars, and the lugs could be set to engage any of the six wheels. For example, one lug on a bar might be set to engage wheel 2, while the other might be set to engage wheel 5. There were also two "neutral" positions on each bar, allowing one or both of the lugs to be set so they never interacted with a wheel at all.

The wheels interacted with the cage through a set of six "guide arms", one for each wheel. When the code clerk pushed the power handle, each wheel with the current pin set to the right would activate its corresponding guide arm, while the guide arms of wheels with the current pin set to the left and disabled would remain inactive. The cage would then spin around through all 27 bars. Those that had lugs in the same position as a wheel with active guide arm would be deflected to the left to form the variable toothed gear, while those bars where either lug didn't hit a guide arm would stay in place.

The mechanism worked much the same way for deciphering a message, except that the direction of the printwheel's offset was reversed, moving it back to translate from the ciphertext letter to the plaintext letter.

A complete key for the M-209 amounted to not only setting the wheel positions, but also the pins on the wheels and the positions of the lugs on the bars. Performing such a full configuration was unsurprisingly very time-consuming, and so it was usually only done at intervals, and not with every message sent.

Security

The M-209 was good for its time but it was by no means perfect. As with Tunny (the Lorenz Electric teletype cypher machine), if a codebreaker got hold of two overlapping sequences, he would have a fingerhold into the M-209, and its operation had some distinctive quirks that could be exploited. It was, however, perfectly adequate for tactical use and was still used by the US Army during the Korean War.

Production and usage

Hagelin built about 140,000 M-209 / C-48s and became quite wealthy. A WWII German cryptographer named Fritz Menzer actually built cipher machines based on Hagelin's designs, though no doubt Hagelin never received royalties from them. Menzer's "Schluesselgeraet 1941 / Cipher Device 1941 / SG-41" was a purely mechanical device, with an internal organization along the lines of the M-209 but larger, with a real keyboard. It was actually put into limited production, with about a thousand built for use by the Abwehr, the German intelligence service, which operated them from 1944.

The SG-41 was supposed to have been a standard tactical cipher machine, but the Germans had only limited supplies of lightweight metals such as magnesium and aluminum, and it was simply too heavy for tactical use. Menzer also worked on two other cipher machines based on Hagelin technology, including a follow-on to the Enigma, the "SG-39", and a simple but fairly strong handheld cipher machine, the "Schluesselkasten (Cipher Box)". Neither of these machines reached production. Had the Menzer devices been put into service, they would have certainly caused trouble for Allied cryptanalysts, though they were certainly no more uncrackable than the M-209.

After the war, Hagelin came up with an improved model of the M-209, designated the "C-52". The C-52 featured a period of 2,756,205,443; wheels that could be removed and reinserted in a different order; and a printwheel with a mixed alphabet. However, the C-52 was one of the last generation of the classic cipher machines, as by that time new digital technology was permitting the development of ciphers that were far more powerful.

This article, or an earlier version of it, incorporates material from Greg Goebel's Codes, Ciphers, & Codebreaking.