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+1⁄4 by 5+1⁄2 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 was a rotor machine similar to a telecipher machine, such as the Lorenz cipher and the Geheimfernschreiber.
Basic operation
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. Printed ciphertext is automatically spaced into groups of five by the M-209 for ease of readability. A letter counter on top of the machine indicated the total number of encoded letters, and could be used as a point of reference if a mistake was made in enciphering or deciphering. The deciphering procedure is nearly the same as for enciphering; the operator sets the enciphering-deciphering knob to "decipher", and aligns the key wheels to the same sequence as was used in enciphering. The first letter of the ciphertext is entered via the indicator disk, and the power handle is operated, advancing the key wheels and printing the decoded letter on the paper tape. When the letter "Z" is encountered, a cam causes a blank space to appear in the message, thus reconstituting the original message with spaces. Absent "Z"s can typically be interpreted by the operator, based on context. An experienced M-209 operator might spend two to four seconds enciphering or deciphering each letter.
Internal elements
Overview Inside the casing of the M-209, a much more complicated picture emerges. The six key wheels each have a small movable pin aligned with each letter on the wheel. These pins may each be positioned to the left or right; the positioning of these pins affects the operation of the machine. The left position is ineffective, while the right position is effective.
Each key wheel contains a different number of letters, and a correspondingly different number of pins. From left to right, the wheels have:
26 letters, from A to Z 25 letters, from A to Z, excepting W 23 letters, from A to X, excepting W 21 letters, from A to U 19 letters, from A to S 17 letters, from A to Q This discrepancy is chosen to give the wheel sizes a coprime nature; the end result is that the wheels only align the same way once every 26×25×23×21×19×17 = 101,405,850 enciphered letters (also known as the period). Each key wheel is associated with a slanted metal guide arm that is activated by any pins in the "effective" position. The positions of the pins on each key wheel comprise the first part of the internal keying mechanism of the M-209. Behind the row of six key wheels is a cylindrical drum consisting of 27 horizontal bars. Each drum bar is affixed with two movable lugs; the lugs can be aligned with any of the six key wheels, or may be placed in one of two "neutral" positions. An effective pin causes its guide arm to tilt forward, contacting the drum. The positioning of the lugs comprises the second part of the internal keying mechanism. Owing to the complexity of setting the internal keying mechanism, it was altered relatively infrequently; changing internal keys once a day was common in practice. When the operator turns the power handle, the cylindrical drum makes a complete revolution through all 27 bars. If a lug on one of the bars contacts the guide arm of an active key wheel, that bar is slid to the left; lugs in neutral positions, or which do not contact a guide arm, do not affect the position of the bar. All bars that are slid to the left comprise a variable-toothed gear, which in turn shifts the letter to be encoded; the shift is equal to the number of bars protruding to the left. The resulting ciphertext letter is printed onto the paper tape. After the rotation is complete, a retractor pushes the protruding bars back into place. A set of intermediate gears advances the key wheels by one position, and a locking arm latches into the drum to prevent a second encoding until the indicator disk is adjusted for the next letter. This system allowed the offset to change for each enciphered letter; without this facility, the enciphering scheme would resemble a very insecure Caesar shift cipher.
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