Some months before German Reunification in October 1990, a secure communication channel was established between East Germany (GDR: German Democratic Republic) and West Germany (FRG: Federal Republic of Germany) by using T-310, the most widely used teletype cipher machine in GDR (p.181). This is something I haven't heard (because I know little about modern crypto-history) until I read Winfried Stephan, "Use of T-310 Encryption During German Reunification in 1990" (HystoCrypt 2022).
In May 1990, six months after the fall of the Berlin Wall, the two German states signed a treaty for monetary, economic, and social union, which came into effect in July.
In the meantime, T-310 devices were handed over by the GDR and installed in Marienthal/Ahrweiler in the FRG in June (p.182). They were connected to their counterpart in Prenden in the north of Berlin, thus establishing a secure communication link between the GDR and the FRG (p.183). From these nodes, almost every place could be reached in secure communication in the GDR and the FRG. The amount of telexes exchanged between the two nodes was more than 800 to 900 telexes per day. (p.184)
The link between the two German states employed the East Germany device T-310 probably because it was exclusively used in the GDR, rather than used between member states of the Warsaw Treaty Organization. The device used in the FRG could not be provided to the GDR because of the NATO regulations. (p.184, p.181) (The GDR withdrew from the Warsaw Treaty only in September.)
When a direct connection to the GDR Ministry of the Interior was esbalished, the teletype link between Marienthal and Prenden ceased to operate around mid-August. In the direct link, the NATO cipher device Elcrotel 4 was used (p.184). This was made possible by rapid political developments (p.189). The Reunification Treaty was signed on 31 August, which came into effect on 3 October 1990.
