|John A. Walker
In 1968, the naval communications specialist walked into the Soviet Embassy in Washington and offered Navy secrets to the Russians in return for cash. It was the start of a 17 years spying career and probably the largest breach of U.S. military communications security in history. After Walker's apprehension in 1985, it became clear that he provided the Soviets for almost two decades with most sensitive information about cryptographic systems and communications security.
|KW-7 Teletype Crypto
He also provided the Soviets with complete technical drawings and repair manuals of crypto equipment. During a search of his house after his arrest, the FBI discovered a special device, provide by the KGB, to read the internal wiring of KL-7 rotors (to obtain the highest level of security, the rotor wirings were changed on a regular basis). Together with the technical information and daily key sheets, the Soviets had all they needed to read U.S. communications. The damage that Walker caused was enormous.
|KL-7 Off-line Cipher Machine
It was without doubt the biggest Soviet supervised SIGINT coup of the Cold War. His spy game ended in 1985 when his wife (who else, of course) tipped off the FBI. During a stake-out, the FBI observed Walker making a deaddrop to covertly exchange secret documents for cash.
The subsequent damage assessment by U.S. intelligence showed the devastating consequences of Walker's betrayal. The compromised communications channels provided the Soviets with invaluable information about the location of U.S. ships and submarines, running operations and exercises, naval tactics, operational procedures and war plans, the technical capabilities and specifications of various weapons systems, performance of satellite imagery and information about the technology and capabilities of anti-submarine warfare. A true treasure trove for the Red Army. John Walker paid for his treason with life imprisonment.
Robert Derenčin wrote USS Pueblo, John Walker and KGB (pdf), a good overview of Walker's spying and the damage he caused. U.S. Major Laura Heat wrote how John Walker exploited weaknesses in U.S. naval communications systems. NARA has the documentary The Walker Spy Ring - Lessons Learned (1989).
KGB General Boris Solomatin gave an interesting interview about supervising John Walker. I also wrote about the capture of USS Pueblo, another major SIGINT incident.
On my website you can find more detailed information about the famous Cold War TSEC/KL-7 cipher machine, compromised by John Walker, and a realistic software simulation of the KL-7. If you're interested in Cold War spy stories, then you should visit Operation Tinker Bell, a most realistic Cold War spy game where you can use crypto machines and spy techniques to decrypt messages and unveil the story of a KGB defector.