Sigcum

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Exhibit in the US National Museum of Cryptology

Sigcum ( proper spelling : SIGCUM ), also referred to as Converter M-228 , was an American rotor key machine developed during the Second World War and used for a short time . Due to cryptographic weaknesses recognized by the Americans themselves , it was replaced after a short time.

history

The trigger for the development of this key machine was the urgent need of the United States Army , i.e. the Army of the United States of America , to encrypt its worldwide telex communications , for example between the USA and Australia or the United Kingdom . The two renowned cryptologists William Friedman (1891–1969) and Frank Rowlett (1908–1998) were commissioned with the rapid development in 1942, shortly after the outbreak of war (December 7, 1941). Years before, both had developed the Sigaba , a rotor encryption machine for encrypting radio messages . These were usually using Morse code via radio broadcast. On the other hand, the Sigaba was not well suited for the efficient encryption of telex, which were transmitted using the Baudot code based on five bits .

The coding of telex is based on the 5 bits of the Baudot code, as can be seen here in the individual columns on the punched tape . It bears the plain text
"ZCZC WIKIPEDIA 1234567890 NNNN".

So instead of replacing letters with other letters, as happened with the Sigaba and also with the German Enigma machine , it made sense to link the 5-bit characters with five key bits to encrypt telex . The German telex key typewriters from Lorenz and Siemens , which were built around the same time, worked according to this principle .

Friedman and Rowlett agreed to use five cipher drums (rotors) for the telex encryption add-on to be developed, as with the Sigaba . In addition, in order to simplify the production of the new machine, they decided to use exactly the same rollers as the Sigaba. Each roller has 26 electrical contacts on both sides , which are secretly connected in pairs by 26 insulated wires inside (at that time). The current entering the roller from one side via a contact plate leaves it again on the other side via another contact plate. In this way, the permutation (swapping) required for encryption is achieved. In contrast to the Sigaba, not only a single line was activated here (corresponding to one of the 26 capital letters of the Latin alphabet ), but exactly half of all lines, i.e. thirteen at the same time. The signals pass through the set of five rotors connected in series and result in thirteen “ scrambled ” active (live) and thirteen inactive (dead) contacts at the output . Five of these are used as " pseudo-random " key bits and linked with the plain text (in the case of encryption ) or the ciphertext character (in the case of decryption ). As with other machines, the XOR link was used as a mathematical link with the Sigcum and implemented with the help of a mixer .

As with a mechanical odometer , one of the reels continues to rotate with each character and after 26 characters the “carry-over” occurs and a second roller continues to rotate. After 26 × 26 characters, a third roller turns. After 26 × 26 × 26 characters a fourth roller turns, and finally after 26 × 26 × 26 × 26 characters the fifth roller turns. In this way it is achieved that the encryption changes within a period of 26 5 or 11,881,376 characters for each additional character ( polyalphabetic substitution ).

Each of the five rotors can be manually set to one of 26 starting positions. You can also set which rotor should turn as the “faster” rotor with each character, which as the “second fastest”, “third fastest” and so on. A total of ten different rollers were available for the Sigaba and thus also for the Sigcum, each of which could be used “directly” or “inversely”. In total, there were 20 × 18 × 16 × 14 × 12 possibilities for setting up the roller set. Multiplied by the 26 5 possible starting positions, the Sigcum has a key space of 20 × 18 × 16 × 14 × 12 × 26 × 26 × 26 × 26 × 26 or 11,497,369,927,680 possibilities, corresponding to a little more than 43 bits. This made it clear to Friedman and Rowlett that their machine was cryptographically sufficiently secure.

After presentation of the machine in the Signal Security Agency (SSA) , the former secret service of the US Army , based in Arlington Hall near Washington, DC , quickly became the serial production of the now as M-228 officially designated machine by the Teletype Corporation , a manufacturer recorded by teleprinters based near Chicago . At the beginning of January 1943 the first copies were delivered and immediately used for the route from Washington to Algiers . Cleverly, at Rowlett's suggestion, the Americans monitored their own radio communications. After no abnormalities whatsoever could be observed for several days and the radio traffic was handled smoothly and without errors, an incident occurred after just under a week: Algiers reported that the decryption had not been successful. The operator in Washington then entered the text to be encrypted into the machine a second time. In breach of regulations, however, he did not use a new (unused) key, but the old (used) one a second time.

The multiple use of a key is referred to as depth in English . The German technical term is "Klartext-Klartext-Kompromittierung" . Using a key more than once is a cardinal error for machines of this type , which can lead to the breaking of the ciphertext and even to the complete exposure of the encryption system. Therefore, multiple use of one and the same key was expressly prohibited. But prohibitions are not always obeyed. Rowlett tried to prevent the broadcast, but it was already too late, the broadcast had already taken place. He secured the corresponding logs and examined them cryptanalytically . Within just a few hours of the night he succeeded in fully deciphering the text and also in reconstructing the drum wiring. He then immediately informed his superior, General Frank E. Stoner (1894–1966), and made sure that the M-228 was immediately withdrawn from service.

As a result, the machine was modified and cryptographically improved. This is how the M-228-M (also known as SIGHUAD ) was born. The SIGTOT, which is based on the cryptographically secure one-time key method ( English one-time pad , or OTP for short ), was used for messages with the highest level of confidentiality .

literature

Web links

Commons : SIGCUM  - collection of images, videos and audio files

Individual evidence

  1. ↑ Stephen J. Kelley: The SIGCUM Story - Cryptographic Failure, Cryptographic Success. Cryptologia 1997, 21: 4, pp. 289-316, doi: 10.1080 / 0161-119791885940