Fish ( cryptography ) Fish ( sometimes FISH ) was the UK ' s GC & CS Bletchley Park codename for any of several German teleprinter stream cipher s used during World War II . Enciphered teleprinter traffic was used between German High Command and Army Group commanders in the field , so its intelligence value ( Ultra ) was of the highest strategic value to the Allies . This traffic normally passed over landlines , but as German forces extended their geographic reach beyond western Europe , they had to resort to wireless transmission . History . In June 1941 , the British " Y " wireless intercept stations , as well as receiving Enigma - enciphered Morse code traffic , started to receive non - Morse traffic which was initially called NoMo . NoMo1 was a German army link between Berlin and Athens , and NoMo2 a temporary air force link between Berlin and Königsberg . The parallel Enigma - enciphered link to NoMo2 , which was being read by Government Code and Cypher School at Bletchley Park , revealed that the Germans called the wireless teleprinter transmission systems " " Sägefisch " " ( sawfish ) , so the name " Fish " was adopted for this traffic . The enciphering / deciphering equipment was called a " Geheimschreiber " ( secret writer ) which , like Enigma , used a symmetrical substitution alphabet . The teleprinter code used was the International Telegraph Alphabet No . 2 ( ITA2 ) — Murray ' s modification of the 5 - bit Baudot code . When the Germans invaded Russia , during World War II , they began to use a new type of enciphered transmission between central headquarters and headquarters in the field . The transmissions were known as Fish at Bletchley Park . The German army used Fish for communications between the highest authorities in Berlin and the high ranking officials of the German Army on the field . The Fish traffic which the personnel at Bletchley Park intercepted , contained discussions , orders , situation reports and many more details about the intentions of the German Army . However , these transmissions were so challenging to decrypt that even with the assistance of decryption machines , such as Colossus , the messages could not be read until several days later . “ “ Vital intelligence was obtained about Hitler ’ s intentions in the run up to D - Day 1944 . ” Traffic Code names . Tunny . The NoMo1 link was initially named Tunny , a name which later went on to be used both for the Lorenz SZ40 / 42 " Geheimschreiber " machines , and also for the Bletchley Park analogues of them . The NoMo1 link was subsequently renamed Codfish . A large number of Tunny links were monitored by the Y - station at Knockholt and given names of fish . Most of these were between between the " Oberkommando der Wehrmacht " ( German High Command , OKW ) in Berlin and German army commands throughout occupied Europe . Cryptanalysis of the Lorenz cipher at Bletchley Park , assisted initially by a machine called Heath Robinson and later by the Colossus computer s yielded a great deal of valuable high - level intelligence . Tunny decrypts provided high - grade intelligence in an unprecedented quality . Walter Jacobs , a US Army codebreaker who worked at Bletchley Park , wrote in an official report on the operation to break Tunny that in March 1945 alone ' upward of five million letters of current transmission , containing intelligence of the highest order , were deciphered ' . Sturgeon . This was the name given to traffic encoded with the Siemens and Halske T52 " Geheimschreiber " . In May 1940 , after the German invasion of Norway , the Swedish mathematician and cryptographer Arne Beurling used traffic intercepted from telegraph lines that passed through Sweden to break this cipher . Although Bletchley Park eventually diagnosed and broke Sturgeon , the relatively low value of the intelligence gained , compared to the effort involved , meant that they did not read much of its traffic . Thrasher . This was the name used for traffic enciphered on a " Geheimschreiber " that was probably the Siemens T43 one - time tape machine . This was used only on a few circuits , in the later stages of the war and was diagnosed at Bletchley Park , but considered to be unbreakable . List of senior involved staff at Bletchley Park . Including both executives and cryptographers on FISH ( Tunny ) in the Testery .