Free Spycatcher by Peter Wright Page A

opened the Post Office technicians were amused to discover that it was addressed to MI5 and contained a typewritten message, which read: "To MI5, if you steam this open you are dirty buggers." Denman classified it as "obscene post," which meant that legally he had no duty to send it on to the cover address.

    In fact, Denman was very particular about warrants. He was prepared to install a tap or intercept an address without a warrant only on the strict understanding that one was obtained as soon as possible. MI5 were, however, allowed to request a form of letter check without a warrant. We could record everything on an envelope, such as its origin and destination and the date it was sent, as long as we did not actually open it. Denman, like everyone in the Post Office who knew of the activity, was terrified in case the Post Office role in telephone and mail intercepts was discovered. They were not so worried about overseas mail, because that could be held up for days at a time without arousing suspicion. But they were always anxious to get domestic mail on its way to the receiver as soon as possible.

    Responsibility for warrants lay with the Deputy Director-General of MI5. If an officer wanted a tap or an interception, he had to write out a short case for the DDG, who then approached the Home Office Deputy Secretary responsible for MI5. The Deputy Secretary would advise as to whether the application presented any problem. Once a month the Home Secretary vetted all applications. Like the Post Office, the Home Office was always highly sensitive on the issue of interceptions, and they were always strictly controlled.

    As well as St. Paul's, there was also Dollis Hill, the rather ugly Victorian building in North London where the Post Office had its research headquarters in the 1950s. John Taylor ran his small experimental laboratory for MI5 and MI6 in the basement behind a door marked "Post Office Special Investigations Unit Research." The rooms were dark and overcrowded, and thoroughly unsuitable for the work that was being attempted inside.

    When I joined MI5, Taylor's laboratory was overrun with work for the Berlin Tunnel Operation. A joint MI6/CIA team had tunneled under the Russian sector of Berlin in February 1955, and placed taps on the central communications of the Soviet Military Command. The actual electrical taps were done by Post Office personnel. Both the CIA and MI6 were reeling under the sheer volume of material being gathered from the Tunnel. So much raw intelligence was flowing out from the East that it was literally swamping the resources available to transcribe and analyze it. MI6 had a special transcription center set up in Earl's Court, but they were still transcribing material seven years later when they discovered that George Blake had betrayed the Tunnel to the Russians from the outset. There were technical problems too, which Taylor was desperately trying to resolve, the principal one being the ingress of moisture into the circuits.

    Taylor's laboratory was also busy working on a new modification to SF (Special Facilities), called CABMAN. It was designed to activate a telephone without even entering the premises by radiating the telephone with a powerful radio beam. It worked, but only over short distances.

    They were also in the early stages of developing a device called a MOP. A MOP made a cable do two jobs at once - transmit captured sound and receive power. It was in its early stages, but it promised to revolutionize MI6 activity by removing the extra leads which were always likely to betray a covert microphoning operation. I spent a lot of time in my first years in MI5 ensuring the correct specifications for MOP, and it was eventually successfully manufactured at the MI6 factory at Boreham Wood.

    Soon after the Philby interview I began to look into ways of improving and modernizing the seventh floor. The method of processing a tap followed a set pattern. A case officer responsible for a tap or