Adding another rotor to the old Enigma machine.
Modern computers would cut through the enigma machine like a hot knife through butter. Apparently this was nothing more subtle than listening in on unencrypted channels. The U.S. used to monitor Soviet microwave telemetry with satellites that the Soviets didn’t know existed. At the time encryption was expensive. They believed the channels were secure because they didn’t believe the transmitters could be observed by the U.S. They never considered the third dimension.
Whoever blabbed about the North Korean communications (assuming this isn’t all South Korean disinformation) should be shot.