Operation STOCKADE

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The operation STOCKADE was from 1960 to 1963 an operation of the Security Service (MI5) and the Government Communications Headquarters (GCHQ), the secret communication of the French Embassy in London to record and decipher.

First investigation

The starting point of the enterprise was the order of the British Foreign Ministry to the British secret services to obtain information about the plans of the French government agencies with regard to the application of Great Britain to join the European Economic Community . The GCHQ's first investigations revealed that two types of transmission were used to send messages to the French Foreign Ministry. Transmission took place over a telex line using a less complex encryption ( encoding ) of the messages with the aid of an encryption machine . The other transmission concerned messages which the French ambassador in London transmitted and which were specially encrypted using a special procedure. GCHQ expert Hugh Alexander thought it possible to decipher the less complicated messages. The deputy director of the GCHQ research facilities Josh Cooper then gave permission for the operation.

Communications from the French embassy

The technical direction of the operation was in the hands of Peter Wright, an MI5 scientist. An examination of the construction documents at the municipal authority and the responsible department at the post office revealed the presumed space where the messages could be encrypted when determining the location of the post lines for telephone and telex. In order to get into the building and the encryption room, the Post staged a disruption of the telephone connections for the French embassy on Palmer Street. It was determined that there was no telephone in the encryption room that could have been used to transmit an acoustic signal. The technical devices such as the teleprinter and the encryption machine were in an adjacent room.

Signal recording

In order to be able to pick up the signals from the postal lines, a component for frequency transmission with a broader spectrum was installed in a cable distributor at Albert Gate , which transmitted the signals to a room in the Hyde Park Hotel in Hyde Park . As a precautionary measure, block capacitors were installed in the signal transmission so that the signal could not flow back into the French embassy. The first action was to check the intercepted frequencies and compare them with the radio signals from the French embassy. The recorded signals were printed out on a telex in the hotel. The encrypted character string was obtained by simply separating characters from the unencrypted messages. Upon carefully analyzing the string, Wright realized that there must be some more encoding in the signal string.

Signal separation

A check of the signal curve clearly showed that a second signal was superimposed on the stronger one. There was only one explanation for this overlay, which must have come from a local source. This signal could only be the transmission signal of the top secret, more complex encrypted signal of the French ambassador, which was transmitted through a wall onto the teletype line. By means of electronic fine control and signal amplification, it was possible to separate the two frequency overlaps and to arrive at the actual signal chains. This enabled the decryption of the signal chains to be achieved within an hour, which were then passed on directly to the British Foreign Office.

Results of the operation

For nearly three years, British intelligence was able to follow every step taken by the French ambassador and the French government, including reports from French President Charles de Gaulle . Information was also passed on to the United States concerning the French nuclear force, Force de frappe . After this extremely successful operation, British intelligence carried out similar operations at other embassies. Attempts were also made at the German embassy to get the news transmission. After various efforts, however, this operation was discontinued because no satisfactory results could be achieved. Apparently the German encryption machine was adequately shielded from signal radiation to the outside.

The FBI modeled this British operation on the French Embassy in Washington, DC. Wright later stated that such operations have a limited impact because they cannot adequately intervene in the course of a process.

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