Earl Clark Jr. testified in the espionage trial of Jerry A. Whitworth.
From the New York Times 1986 article about the trial…
Earl D. Clark, an expert in communication security for the National Security Agency, which has the sole responsibility for developing coding material for the United States military, testified that the circuit boards of a cryptography machine were its ”guts.”
”If a hostile intelligence organization” gains the information on the boards, he said, ”they can reconstruct a device to assist in reading our most sensitive communications.”
Additional information about the Clark’s testimony during the trial (source UPI.com):
Earl D. Clark Jr., testified the secret code key cards Whitworth allegedly stole were one half of an elaborate two-part system. The heart of the system is a logic machine used to scramble English into codes. The key cards, resembling computer punch cards, are inserted into the machines daily to change the code. The code logic contained in the various machines’ transistors or tiny computer chips is the heart of the code system, Clark said.
That makes the key cards, as the second half of the puzzle, extremely valuable, he said.
Whitworth allegedly photographed them and passed them to Walker for sale to the Soviets.
Because the logic is contained in often bulky machines it is rarely lost, but from time to time hostile forces get possession of the logic that forms the foundation of the codes. Just such a loss of the highly prized logic portion is believed to have resulted in 1968 in the seizure of the USS Pueblo by North Koreans, Clark said.
Under defense questioning, Clark said the logic in the machines was changed after the Pueblo incident and after another loss during the Vietnam War.
Repair manuals for the machines contain diagrams for the logic portion of the encryption equipment that amount to blueprints for the construction of the machine. Clark testified that the repair manual has the lowest level of security classification — confidential.
According to Clark, that is necessary to allow widespread distribution of the repair manual to service centers and ships around the world.
U.S. District Judge John P. Vukasin rejected a defense request to exclude Clark’s claim that the manual could enable someone to reconstruct the entire logic function of the machine.
Clark said, ‘If the key and logic, if the integrity of the two systems is maintained,’ the American code is not subject to cracking.
A small gray box filled with knobs, plugs and switches was displayed to jurors as a 20-year-old encryption machine used to scramble messages from English into undecipherable codes.
From Earl’s wife Janet’s 2019 obituary:
Janet and Earl met while employed at the National Security Agency and both continued to work for the NSA throughout the Cold War era when the international information gathering capabilities of the agency were very critical to the national defense of the United States.
During this period Janet and Earl made their home and raised their daughters in the Maryland suburbs, nearby Washington DC, and later in the Northern Virginia area. Upon Earl’s retirement from NSA in 1986, the couple left Virginia and moved to a new home they built together in Sun City West, AZ. Earl passed away in 2000. Janet resided in AZ until her death in 2019.