VENONA Decrypts
National Security Agency
1995
1943–1980
Thousands of partially decrypted Soviet intelligence cables, cracked over decades by exploiting a one-time-pad reuse error by a wartime Soviet cipher factory.
Why it was classified
Revealing the break would have told Moscow which of its wartime traffic had been read, and which agents were compromised.
Why it matters
VENONA settled decades of argument about Soviet penetration of the Manhattan Project — the evidence existed the whole time and could not be shown.