This week, the man responsible for what is probably the biggest cryptographic failure in military history died—just a few months before he was due to be released from prison. John Walker Jr. led a family spy ring that exposed a vast trove of classified data to the Soviet Union for 18 years by giving them access to Navy cryptographic materials. He passed away in a North Carolina federal prison hospital on Thursday after a long battle with cancer. He was 77 years old.

As a newly commissioned Navy officer, I was sent to a school at Newport, R.I. to be trained as a “CMS custodian,” and indoctrinated into the arts of managing the Communications Security Material System. It was 1986, just a year after Walker stood trial, and the instructors jokingly referred to the course as the “John Walker Jr. Memorial Stay Out Of Prison School.” A good portion of the course involved training videos detailing how Walker had managed to break the chain of custody created around the Navy’s single-use code sheets and secret them off base in his trunk—mostly to hammer into us how important that chain of custody was.

Walker’s Cold War spying for the Soviets started in 1967, when he was selling information about US Navy communications systems and the encryption codes used to configure Navy communications gear for secure transmissions over the Fleet Broadcasting System. The information he provided, some claim, led directly to the North Korean seizure of the US Navy intelligence collection ship USS Pueblo, as the Soviets apparently spurred the attack to gain access to the hardware used with the material Walker provided just a month earlier.

Back then, crypto codes were printed on cards or sheets of paper. There were single-use pads for transmitting encoded voice communications in the clear over unencrypted circuits, and cards used to configure the switches on crypto gear in the radio room. It was this second category of materials, which had the printed cryptographic keys used with the KW-7 Orestes teletype encryption system, that Walker sold in bulk to the Russians—first by stealing them himself, and then by enlisting family members. The cards initially gave the directions for how to manually configure the “plugboard” in the KW-7, until the Navy moved to a punched card reader in 1977 to configure the daily codes.

As a result, the Soviets were able to record encrypted radio broadcasts from the fleet, and then attempt to match them up after the fact with the crypto keys provided by Walker. It didn’t give them immediate real-time insight into fleet operations, but it did give them access after the fact to millions of Top Secret-classified messages over the years. It included “access to weapons and sensor data and naval tactics, terrorist threats, and surface, submarine, and airborne training, readiness and tactics," Defense Secretary Caspar Weinberger had said. That data, Weinberger claimed, helped the Soviet Union make huge leaps forward in the development of their own navy.

It’s hard to calculate the total damage done by Walker. The data he provided could have contributed to Vietnam wartime deaths; it most certainly escalated the conventional arms race between the Soviet Union and the United States over the last two decades of the Cold War. And it could have led to the exposure of US intelligence assets around the world. Also, Walker didn’t do what he did for principles. He did it for money—first to get out of debt from a failed bar he started while he was stationed aboard a ballistic missile sub in Charleston, S.C., and then just to support his wild 1970s lifestyle.

“Traitor” is a strong word to apply to someone. People throw it around when talking about Chelsea Manning and Edward Snowden. But measured against the yardstick of what John Walker Jr. did, the ethically directed actions of Manning and Snowden pale in comparison.