Venona project
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The VENONA project was a long-running and highly secret collaboration between the the NSA and England's MI5 during the Cold War. The NSA intercepted encrypted high-level Soviet diplomatic traffic and stored it for later analysis. This traffic was known to be encrypted with a one-time pad system, which is theoretically unbreakable when used correctly. However, mathematical analysis revealed that some of the one-time pads had incorrectly been re-used, which allowed the codebreakers to decrypt some of this traffic. Over time, using techniques ranging from statistical text analysis (analyzing traffic by looking at the frequency of Russian phrases) to physical theft of encryption pads, to bugging the embassy rooms where the encrypted text was entered into the encrypting devices (and analyzing the keystrokes by listening to them being punched in), more and more traffic was able to be decrypted.
This decryption and cryptanalysis project continued for decades, long after new traffic was being encrypted with this system.
References
- The Hidden Hand: Britain, America, and Cold War Secret Intelligence'; by Richard J. Aldrich. New York: Overlook Press, 2002. ISBN 1585672742.
- Body of Secrets: Anatomy of the Ultra-Secret National Security Agency'; by James Bamford. Anchor Books. ISBN 0385499086.