Today, apps sell themselves on offering secure communications. But 40 years ago, one man was given a secret mission to build an encryption device to send intelligence to top officials. Now he breaks his silence over the assignment for the first time.
The mission began in a Cheltenham department store, busy with shoppers. It was 1980 and Mike - first name only as his identity is still hidden - worked at nearby GCHQ. He purchased two normal briefcases.
They were taken back to his lab where he began tearing out the insides and stuffing them with the latest technology.
What emerged was given the codename Brahms. Inside the regular-looking briefcase was the UK's first portable encrypted communications system designed to allow high-ranking officials to communicate securely.
Secure speech was not new. During World War Two, if the leaders of the US and the UK wanted to talk securely, it took a massive machine, which in London sat permanently in the basement of another department store - Selfridges.
But by 1980, new technology meant a device could be made small enough to be portable, and therefore more useful in a crisis for a wider group of people.