Room 40

[1] Most notably the section intercepted and decoded the Zimmermann Telegram, a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico.

[3] Room 40 operations evolved from a captured German naval codebook, the Signalbuch der Kaiserlichen Marine (SKM), and maps (containing coded squares) that Britain's Russian allies had passed on to the Admiralty.

[8] Although Room 40 decrypted Imperial German communications throughout the First World War, its function was compromised by the Admiralty's insistence that all decoded information would only be analysed by Naval specialists.

However, installations belonging to the Post Office and the Marconi Company, as well as private individuals who had access to radio equipment, began recording messages from Germany.

Instead he turned to a friend, Sir Alfred Ewing, the Director of Naval Education (DNE), who previously had been a professor of engineering with a knowledge of radio communications and who he knew had an interest in ciphers.

It was not felt that education would be a priority during the expected few months duration of the war, so Ewing was asked to set up a group for decoding messages.

Ewing arranged for them to operate from the coastguard station at Hunstanton in Norfolk, where they were joined by another volunteer, Leslie Lambert (later becoming known as a BBC broadcaster under the name A. J. Alan).

Two light cruisers, Magdeburg and SMS Augsburg, and a group of destroyers all commanded by Rear-Admiral Behring were carrying out a reconnaissance of the Gulf of Finland, when the ships became separated in fog.

The commander, Korvettenkapitän Habenicht prepared to blow up the ship after it had been evacuated but the fog began to clear and two Russian cruisers Pallada and Bogatyr approached and opened fire.

Fleet paymaster C. J. E. Rotter, a German expert from the naval intelligence division, was tasked with using the SKM codebook to interpret intercepted messages, most of which decoded as nonsense since initially it was not appreciated that they were also enciphered.

An entry into solving the problem was found from a series of messages transmitted from the German Norddeich transmitter, which were all numbered sequentially and then re-enciphered.

The signals used four symbols not present in ordinary Morse code (given the names alpha, beta, gamma and rho), which caused some confusion until all those involved in interception learnt to recognise them and use a standardised way to write them.

Prince Heinrich of Prussia, commander in chief of Baltic operations, wrote to the C-in-C of the High Seas Fleet, that in his view it was a certainty that secret charts had fallen into the hands of the Russians, and a probability that the codebook and key had also.

[22] There was no immediate capture of the FFB codebook to help the Admiralty understand it, but instead a careful study was made of new and old messages, particularly from the Baltic, which allowed a new book to be reconstructed.

The Handelsverkehrsbuch (HVB) codebook which was captured contained the code used by the German navy to communicate with its merchant ships and also within the High Seas Fleet.

Its greatest importance during the war was that it allowed access to communications between naval attachés in Berlin, Madrid, Washington, Buenos Aires, Peking, and Constantinople.

Previous occupants of the room had complained that no one was ever able to find it, but it was on the same corridor as the Admiralty boardroom and the office of the First Sea Lord, Sir John Fisher, who was one of the few people allowed to know of its existence.

Rotter was initially suggested for the job, but it was felt preferable to retain him in code breaking and Commander Herbert Hope was chosen, who had previously been working on plotting the movements of enemy ships.

Hope was initially placed in a small office in the west wing of the Admiralty in the intelligence section, and waited patiently for the few messages which were approved for him to see.

On 16 November, after a chance meeting with Fisher where he explained his difficulties, Hope was granted full access to the information together with instructions to make twice daily reports to the First Sea Lord.

In the interests of secrecy the intention to give a separate copy of messages to the DID was dispensed with so that only the Chief of Staff received one, and he was to show it to the First Sea Lord and Arthur Wilson.

[32] Jellicoe, commanding the Grand Fleet, on three occasions requested from the Admiralty that he should have copies of the codebook which his cruiser had brought back to Britain, so that he could make use of it intercepting German signals.

Captain Round, working for Marconi, had been carrying out experiments for the army in France and Hall instructed him to build a direction-finding system for the navy.

Other stations were built at Lerwick, Aberdeen, York, Flamborough Head and Birchington and by May 1915 the Admiralty was able to track German submarines crossing the North Sea.

From June 1915, the regular intelligence reports of ship positions were no longer passed to all flag officers, only to Jellicoe, who was the only person to receive accurate charts of German minefields prepared from Room 40 information.

Some information was passed to Beatty (commanding the battlecruisers), Tyrwhitt (Harwich destroyers) and Keyes (submarines) but Jellicoe was unhappy with the arrangement.

No attempts were made by the German fleet to restrict its use of wireless until 1917 and then only in response to perceived British use of direction finding, not because it believed messages were being decoded.

The telegram was passed to the U.S. by Captain Hall, and a scheme was devised (involving a still unknown agent in Mexico and a burglary) to conceal how its plaintext had become available and also how the U.S. had gained possession of a copy.

In 1919, Room 40 was deactivated and its function merged with the British Army's intelligence unit MI1b to form the Government Code and Cypher School (GC&CS).

[38] This unit was housed at Bletchley Park during the Second World War and subsequently renamed Government Communications Headquarters (GCHQ) and relocated to Cheltenham.