Syko Cipher Device
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The SYKO cipher device was a compact British cipher apparatus in use during World War II. It was invented and patented by Morgan O'Brien before the war. O'Brien developed a sophisticated cipher typewriter, which was complex and delicate, but then came up with the relatively simple device adopted by the military that was known the SYKO. Although the German cryptanalysts managed to break the SYKO code, it remained in use for much of the war as a quick method for low-level encoding of radio traffic, particularly from aircraft. Code breaking took considerable time plus a sufficient sample of messages, and for certain classes of radio traffic (such as an aircraft's position) decoded information that became available many hours later was of little value.
The SYKO machine is a purely mechanical device. Details of its mechanical construction can be found in GB Patent 534615 (Morgan O'Brien and O'Brien Cipher Machines Ltd, London)[1] Robust, small, and lightweight, it fitted in a brown hardcover case, with an optional canvas outer bag. A few machines survive, and operating instructions issued by the Royal Australian Air Force in November 1940 have been made available on-line.[2]
When the grey top cover is opened up it reveals 32 vertical columns of letters. Each column has the characters reading downwards from A to Z then 9 to 0 followed by a pause character (37 characters in all). These columns are moveable on an endless band. The SYKO case contains a propelling pencil with a small brass pin at the other end, which is inserted in a notch at each letter allowing the columns to be moved as required. Use of the device requires the code card for that day. This is placed within the machine and is revealed as the columns are moved down.
The code card contains 32 columns of symbols, each column having the same 37 characters as in the moveable columns but in a random order. Code cards were different for each day, and usually aircraft only carried the card for the day. In the event that events dictated a flight unexpectedly went past midnight there was provision when sending code to indicate you were using an earlier days card. The characters on the code card were paired in each column (18 pairs) leaving a single character unpaired. The pairing means that, for example, if 'C' codes as 'R', then 'R' codes as 'C'. There is a 1 in 37 chance that a character will be the un-paired character and code as itself.
The bottom cover of the device can be slid in to reset all the columns to 'A' at the top. Encoding involves taking the special pencil and, for each column in turn, selecting the corresponding letter in the message using the pencil brass pin to draw down the moveable column so that letter is at the bottom bar thus revealing the code card underneath. The encoded message is then the lower-most symbol visible on the code card (i.e. above the 'A' in each moveable column). The original message is still visible along the bottom bar, so you can check the column placement hasn't moved. Due to the pairing of characters outlined above, the decoding of messages follows the same process, with it essential that both parties are using the same coding card. To avoid confusion between similar letters on the code card, the 'i' was in lower case, the zero had a line through it, and the 'e' was lower case.
The use of the reciprocal codes (e.g. A codes as Z, and Z codes as A) limited the variations possible, so in July 1942 non-reciprocal codes for SYKO and NYKO were introduced. This meant turning the code card over when changing from coding to decoding, but it did make the code significantly harder to break.[3]