The Soviets send coded traffic to their embassy in Washington in WWII. Those messages were encrypted using a one-time pad system – in principle unbreakable, as long as you follow the rules. But the Soviet company making the pads, under from invasion, had cut corners, had produced a significant number of duplicate key numbers. Other methods may also have played a part in recovering keys, such as black-bag jobs and a partially burned codebook recovered from a battlefield in Finland.
Out of some hundreds of thousands of intercepted encrypted texts, maybe a couple of percent were ever decrypted – quite a few of the GRU messages in 1943, almost half of the NKVD messages in 1944, but generally far fewer than that.
Venona exposed Soviet Espionage in the Manhattan project, the OSS, the White House. It played a role in exposing the Cambridge Five.
But we never decrypted most of the messages. We have learned quite a bit about Soviet espionage from other approaches and individuals, but much is still unknown.
It strikes me that the Russians could do the US and themselves a big favor by releasing all the messages and one-time pads used in that era. Let it all hang out. No actual former spies would be endangered: all of them must be dead by now. The US would have a better understanding of its own history, a better understanding what kind of people can be trusted. Worth knowing.The Russians would be hated by leftist types, but then they are anyhow, apparently for dropping the Commie torch. The underlying reason is that the true strategic threat to both Russia and the United States is China (not the casbah, for God’s sake): the cowboys and the Cossacks should be friends.