Siemens and Halske T52 - Cryptanalysis

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 depths. These depths were analysed by Michael Crum.