https://en.wikipedia.org/wiki/SIGSALY#Operation if anyone just wants to get a feeling for how it worked. Pretty good writeup IMO.
reply> I think that the relevant patents remained classified until 1975.
replyIf they were classified, they can’t have worked as patents. Patents, by their nature, must be open and available for others to read.
No they don't. There's an exemption in US law for the NSA/the government to file hidden patents which only become revealed when someone else files the same/a similar patent.
replyInvention Secrecy Act of 1952. https://en.wikipedia.org/wiki/Invention_Secrecy_Act