Decryption
This message traffic, which was encrypted with a one-time pad system, was stored and analyzed in relative secrecy by hundreds of cryptanalysts over a 40-year period starting in the early 1940s. Due to a serious blunder on the part of the Soviets, some of this traffic was vulnerable to cryptanalysis. Somebody who was working for the manufacturers of Soviet secret-communication materials had reused pages of some of the one-time pads in other pads, which were then used for other secret messages. This defeated the purpose of the one-time pad, which provides perfect security when each page is used exactly once and then disposed of. It is unclear as to why this mistake was made, or by whom.
Read more about this topic: Venona Project