IV. Case Studies‎ > ‎

FISH and the Jellyfish Convoy

            The set of German teletype machines codenamed “Fish” were a major target of TICOM.


            The use of Radio Teletype (RTTY) by military forces had numerous advantages in speed and accuracy for the transmission of large amounts of text. RTTY used a five bit Bardot code to communicate text, and in 1918 American AT&T engineer, Gilbert Vernam, developed an automatic means to encrypt the Bardot code punched onto 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.


By the beginning of World War II, military communications services were replacing the paper tape with a Hagelin style rotor machine to produce the key used in encrypted teletype messages. The Germans deployed two types of machines, the Lorenz SZ-40/42 and the Siemens T-52 Geheimschreiber.


        The Lorenz machine was an encryption device added to a standard teletype terminal, and was widely used by the German Army. It had twelve pinwheels of different sizes, which could have their pins adjusted by the user, creating an irregular stepping procedure. To break this cipher, the British invented two devices, the “Heath Robinson” and the “Colossus”, the first proto-electronic computer. The traffic they generated was codenamed “Tunny” by the British.


The SZ-42 was developed for the Army and was intended to be used on the air; it had a better-designed receiver and was able to overcome much of the fading and Doppler shift inherent to HF signals. Since the German Army was often on the move and could not count on an established telecommunications system, especially in areas like North Africa or Russia, they made more use of this type of communications. Then the military situation deteriorated for the Germans and they were forced to retreat; they took to the air more often with RTTY. Therefore, as teletype systems were used by higher headquarters, FISH traffic was strategic in nature.


The Geheimschreiber was a more sophisticated machine that had the cryptologic machinery integrated into the device. It added a

permutation to the substitution of the cipher text making decryption far more difficult. TICOM concluded that the latest version of the Geheimschreiber, the T-52e was “probably secure”. The T-52, used primarily by the Luftwaffe and the Navy, was codenamed “Sturgeon”. The T-52 machines were used by naval ships in port, connected to the landline system, and a similar set up was used for the Air Force, therefore limiting the amount of Geheimschreiber material that could be intercepted.


The first FISH traffic (Tunny) was intercepted by GC&CS in mid-1941, with the first Sturgeon circuit detected later that year. By January 1942 Bletchley Park understood the design and operation of Tunny, and by mid-year had analyzed the Sturgeon system. The decision was made to concentrate on the Tunny traffic, which promised a better intelligence yield.


            GC&CS’s knowledge of the SZ-42 was derived purely from cryptanalysis. TICOM officer Lt Arthur J. Levenson recalled “… (GC&CS) wanted to capture these … machines. Enigma machines we had tons of.  But Tunny machines we'd never seen and we were most anxious to see them.”


Therefore, TICOM Team 1 kept their eyes peeled for FISH machines as they investigated Bavaria. At the Luftwaffe comm center in Kaufbeauren, the team found a number of T-52s, but to their disappointment the machines were smashed and their rotors missing. Later a side group of Team1 under the famous British cryptologist Major Ralph Tester found an intact T-52 machine in the town of Pfunds, 40 miles southwest of Innsbruck.


A few days later near Berchtesgaden, an Army task force captured Field Marshal Kesselring’s communications train, a convoy of six German signal trucks complete with SZ-42 Tunny machines and their operating personnel. Each truck had radio transmitters, antenna, radio receivers, and encription devices.  Each vehicle included two bunks, designed to be lived in by the driver and his helper, who also set up and operated the equipment. This mobile station was the field end of the communications circuit between Berlin and OB West, which had been codenamed JELLYFISH by BP.


Turned over to TICOM, Major Tester and Lt. Levenson were ordered to accompany this convoy on the long drive back to the channel ports, making slow progress, guarded by their own prisoners.




    They left for Augsburg on May 16 with 6 Diesel trucks and 12 prisoners. Crossing Europe just a few days after the surrender was problematic, finding fuel and getting permissions to cross various army areas delayed the trip. Then they ran into problems in Brussels. As Levenson later explained:  


   “When we got to Belgium….These trucks had no cap(tured) numbers and we looked like German troops. And the people were very mad at the Germans and they were throwing things at the truck. I got hit once when they threw a tin can or something…. Because there were hard feelings, and it was so obvious. We never bothered to write capture numbers on the trucks or give any identification. We did put a big star, big white star, but for all purposes it looked just like the German trucks that the natives had seen during the war. They thought, "well, if the Wehrmacht is back, what's going on here?" They didn't ask questions either. They were just waiting for something, as I say. When I walked off, (the prisoners) got scared to death, they said, "No, no, stay around."


            They loaded the trucks onto an LST at Ostend at the end of the week, and by the next day they were back in England. There, the trucks were set up and the Germans demonstrated their use to the Bletchley Park experts.