Siemens and Halske T52 The Siemens & Halske T52 , also known as the Geheimfernschreiber ( " secret teleprinter " ) , or " Schlüsselfernschreibmaschine " ( SFM ) , was a World War II German cipher machine and teleprinter produced by the electrical engineering firm Siemens & Halske . The instrument and its traffic were codenamed " Sturgeon " by British cryptanalyst s . While the Enigma machine was generally used by field units , the T52 was an online machine used by Luftwaffe and German Navy units , which could support the heavy machine , teletypewriter and attendant fixed circuits . It fulfilled a similar role to the Lorenz cipher machines in the German Army . The British cryptanalysts of Bletchley Park codenamed the German teleprinter ciphers Fish , with individual cipher - systems being given further codenames : just as the T52 was called " Sturgeon " , the Lorenz machine was codenamed " Tunny " . Operation . The teleprinters of the day emitted each character as five parallel bit s on five lines , typically encoded in the Baudot code or something similar . The T52 had ten pinwheel s , which were stepped in a complex nonlinear way , based in later models on their positions from various delays in the past , but in such a way that they could never stall . Each of the five plaintext bits was then XOR ed with the XOR sum of 3 taps from the pinwheels , and then cyclically adjacent pairs of plaintext bits were swapped or not , according to XOR sums of three ( different ) output bit s . The numbers of pins on all the wheels were coprime , and the triplets of bits that controlled each XOR or swap were selectable through a plugboard . This produced a much more complex cipher than the Lorenz machine , and also means that the T52 is not just a pseudorandom number generator - and - XOR cipher . For example , if a cipher clerk erred and sent two different messages using exactly the same settings — a " depth of two " in Bletchley jargon — this could be detected statistically but was not immediately and trivially solvable as it would be with the Lorenz . Models . Siemens produced several and mostly incompatible versions of the T52 : the T52a and T52b , which differed only in their electrical noise suppression , and the T52c , T52d and T52e . While the T52a / b and T52c were cryptologically weak , the last two were more advanced devices ; the movement of the wheels was intermittent , the decision on whether or not to advance them being controlled by logic circuits which took as input data from the wheels themselves . In addition , a number of conceptual flaws , including very subtle ones , had been eliminated . One such flaw was the ability to reset the keystream to a fixed point , which led to key reuse by undisciplined machine operators . Cryptanalysis . Following the occupation of Denmark and Norway the Germans started to use a teleprinter circuit which ran through Sweden . The Swedes immediately tapped the line , in May 1940 , and the mathematician and cryptographer Arne Beurling cracked the two earliest models in two weeks , using just pen and paper . The telephone company Ericsson manufactured a number of T52 analogue machines that could decode the messages once the key settings had been found by hand . The Swedes then read traffic in the system for three years , not only between Berlin and Oslo , but also between Germany and the German forces in Finland , and of course the German embassy in Stockholm . In total , the Swedes intercepted 500 , 000 German messages and decrypted 350 , 000 . However , poor security meant the Germans eventually became aware of this . An improvement in T52 security in 1942 was defeated by the Swedes . However , a second upgrade in mid - 1943 was not , and the flow of decrypted messages came to an end . The British at Bletchley Park later also broke into Sturgeon , although they did not break it as regularly as they broke Enigma or Tunny . This was partly because the T52 was by far the most complex cipher of the three , but also because the Luftwaffe very often retransmitted Sturgeon messages using easier - to - attack ( or already broken ) ciphers ; thus , attacking Sturgeon was not the most economical way to get the plaintext . The British first detected T52 traffic in summer and autumn of 1942 . One link was between Sicily and Libya , codenamed " Sturgeon " , and another from the Aegean to Sicily , codenamed " Mackerel " . Operators of both links were in the habit of enciphering several messages with the same machine settings , producing large numbers of depth s . These depths were analysed by Michael Crum .