Rainer Rupp was a top spy who worked under the pseudonyms Mosel and later Topas for the GDR in the NATO headquarters in Brussels from 1977 until 1989, releasing documents of the highest importance to the Eastern Block.

Rupp grew up in West Germany with strong leftist political leanings. In 1968, as a student in Mainz, work as a spy for the GDR was suggested to him, and he agreed out of conviction. He continued his studies in Brussels, was trained as a spy in East Berlin and was hired by NATO in 1977. He rose quickly in the ranks and provided photographs of some 10,000 pages to his bosses, including the precise location plans for the deployment of cruise missiles and Pershing II rockets in Western Europe, as well as the central MC 161 document (Cosmic Top Secret) which summarized the NATO strategy as well as NATO's analysis of the Warsaw pact and its intentions. These documents were promptly transferred to the KGB.

He would photograph documents in his office, or take them home and photograph them in his wine cellar. He met contact persons all over Europe and received instructions via number stations, radio programs broadcasting messages encrypted as number sequences. His British wife knew about his activities and tried to persuade him to stop. He later said "At the time I did it, I believed it to be my moral duty."

NATO did not have any knowledge of the existence of Topas until GDR officer Heinz Busch defected in 1990. Busch however did not know the identity of Topas. Several meetings of the secret services of a number of countries ensued with the aim of identifying Topas, who took part in some of those meetings. With the help of GDR files that had fallen into the hands of the CIA after the dissolution of the GDR, Rupp was caught in 1993, while on vacation in Germany. He confessed and received a prison sentence of 12 years in 1994. He was released early in July 2000.