The veil of state secrecy that surrounds the primarily known type of government agency is ubiquitous and enduring, meaning that this technology, which is otherwise in the public domain, has remained top secret for decades. Thus it is interesting to note that from time to time skirts are worn to reveal ankle prints, as apparently in the case of a BBC piece working with encrypted phones produced by GCHQ and used by Margaret Thatcher in the early 1980’s. Sadly, this is short for long and deep technology for human interest, but it can still be inferred enough to figure out how it probably worked.
We are told that it works through a standard phone line and transmits at a speed of 2.4 kilobytes per second, a digital data stream is encoded using a paper tape key that changes daily. If we were to present this design feature for application in a briefcase using 1980s elements, we would probably create an ADPCM (Adaptive Differential Pulse Code Modulation) system with XOR encryption opposite the key, which we think is well within its capabilities. Will be Digital logic and microprocessors in the early 1980s. We wonder if the BBC has typed and it should be KiloBit More than a kilobyte to work on a standard phone line.
There is no doubt that there are people in the comments who can tell us if they are willing to violate the Official Secrets Act, but we would suggest that they do not risk their freedom by doing so. It is noteworthy, however, that GCHQ is known for displaying some of their past glories, such as this 2019 exhibition at the Science Museum in London.