Before its termination in the 1980's the Venona project proved many things. It proved that Guy Burgess was a double agent and so on.

Venona

The venona project was a secret collaboration of the U.S and the U.K intelligence agencies involving cryptanalis of messages sent by the intelligence agencies of the the Soviet Union, mostly during world war II.

it cracked the soviet spy codes and provided strong evidence that the Rosenbergs were guilty

The papers confirmed that Julius Rosenberg was a spy.