The technology behind the interception of “Russian Fish” can be traced back to 1874 and the efforts of the French telegraph engineer Jean-Maurice-Émile Baudot. In a desire to increase the speed, amount and accuracy of transmitted text, Baudot adapted principles of the Hughes telegraphic printer and a five-unit code devised by Gauss and Weber to invent what would now be described as a synchronous time division multiplex system.
The heart of his system was a distributor, which rotated brushes over a set of contacts, which connected a series of transmitter and receiver circuits into a single line. This allowed up to four channels to operate simultaneously. The transmitted characters were interleaved so that the signal occurs in different time slots. Further development of multiplexing by Western Union allowed for the simultaneous transmission of eight channels by 1913. By 1936, further Western Union development of the Varioplex increased capacity to 72 terminals of transmission.
The code Baudot devised to drive this system represented letters of the alphabet with five electrical impulses, the unit representing either a pulse (mark) or its absence (space). This resulted in 32 combinations, 26 representing the letters of the alphabet and 6 that could be assigned as control characters, such as a shift or a number. In contemporary terms, it is described as a five-bit code.
The transmission was generated by a skilled operator manipulating a series of five piano like keys in the proper pattern to generate the character signal, which could be printed out at the receiving end. By 1902 a “start – stop” code sequence was added to the Baudot code that allowed automation of the transmission. Both the transmitter and the receiver could now be cued as to the start of the next 5-bit sequence, allowing a standard typewriter device to become the keyer. By the First World War this technology was being adopted by cable companies, railroads, and other corporations that had a need to communicate large amounts of textual data. Paper tape readers had also been devised which allowed the message to be punched out ahead of transmission and then run through a reader. The communication demands of the war led to military interest in teletype, but they also had to contend with the additional difficulty of security.
Gilbert Vernam, an AT&T engineer, developed an automatic means to encrypt the Baudot code punched onto the paper tapes. By creating another tape of randomly generated letters (a key), and running it in step with the plain text, the two message streams could be added together with Boolean “exclusive or” (XOR) function to create a cipher of the original message. Thus, a space + space = mark; a mark + space = mark; space + mark = mark; and mark + mark = space. By reversing the logic at the receiving end with the identical key, it would automatically recover the original plain text message.Teletype technology continued to develop throughout the 1920s and began to take to the air as radio teletype (RTTY). International services, well established for business and news use by the late 1930s, were also adapted by various nations’ military services. The use of Vernamencryption and multiplexing of teletype signals for military purposes became a common practice among the major combatants, so it was no surprise that the Soviets were engaged in the use of these modes.
The Baudwaurm traffic had an unusual aspect. The Russians had devised a method to break the message into pieces and to transmit these segments multiplexed on up to nine separate channels. Without knowledge of the signal characteristics and the proper equipment, interception was very difficult After the Army took over the project over from the FA they developed multiplex receivers to intercept this Russian traffic. The haul of gear captured at Rosenheim included three different intercept receivers:
1. A standard radio receiver fed the intercepted RF signal into the machine. The signal was similar to a standard carrier shift teletype circuit, except that the shift was of 5000 Hz rather than the standard 850 Hz.
2. An automatic Volume Control and a rectifier unit changed the voice frequency into direct current.
3. A double mechanical distributor on a single shaft, utilizing brush contacts, was synchronized to regenerate each channel. Synchronization was accomplished via the use of an oscilloscope. Once the machine was synchronized, it could be locked in with automatic circuits. With the HMFS, different distributors could be inserted into the machine to configure it for the different number of channels.
4. The output was sent into a pulse regenerator as a final stage that inserted a start-stop signal into the data flow and then stored it in a band of five relays that acts as short-term memory buffer ( a “Speicher”) before being transmitted to a corresponding teletype printer. This was necessary to expand the compressed Russian signal back to the standard 20-millisecond length.
The mission was assigned to the Baudot Reception Station of Section 4 of Horchleitatelle Ost, the Intercept Control Station East, (abbreviated HLS Ost), located at Loetzen, East Prussia. After the November 1944 reorganization, it moved to Potsdam and was redesigned as Group VI, Section 1b of the GdNA, under the command of Captain Rowder. Uffz. Karrenberg was the technician primarily responsible for this traffic. These targeted machine generated enciphered signals (codenamed ‘Bandwurm’ by the Germans), were determined to be high-level circuits from Moscow to the Front Armies. There were also one or two links to the Air Force, and one possible link to the Far East. Moscow acted as the net control station and traffic from one Front Army to the other was routed through this central point.