Ok so, MI5 essentially hacking a manual encryption machine.
In 1956 there was the beginnings of the crisis in Egypt which ended with the Suez Crisis (rather embarrassing for the UK). It was deemed important to spy on the Egyptian embassy and critically the encrypted messages being sent by General Nasser to the embassy. We (the UK) knew that like most Third World Countries (as Egypt was classed at the time) they used a Hagelin machine. The Hagelin worked on the principal of seven wheels inside it that were operated by switched currents, before the machine could be used each day the cipher clerk had to manually set the wheels, so a microphone positioned near the machine could detect the movement and MI5 could probably work out where the wheels were set.
The plan was very straight forward, there was a telephone in the cipher room so the Post Office created a fault on the embassy line and when their engineers were called to fix it an MI5 officer accompanied them. The Officer was able to get a listening device into the cipher room phone and as luck would have it the Egyptians wanted the phone nearer the encryption machine so he obliged. The plan worked and MI5 were able to listen in on most of the communications, and found out Nasser was working with the Soviets who were planning to provide air support so we were able to pull back before that happened.
The operation was known as 'Engulf'.
