"Stalin's tyranny was far from well-known and did not become so until 1956 when his crimes were exposed by Khrushchev at the 20th Congress"

Now that is odd Jim - Fitzroy Maclean was present during Stalin's great purges in the late 1930s and he most certainly knew and told people about what was going on. I think what Khrushchev wittered on about at the 20th Congress was purely face saving and for inter-party domestic consumption - the Russian people after all could see what was going on before their eyes plainly enough.