Room 40
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Room 40, also known as 40 O.B., was the cryptanalysis section of the British Admiralty during the First World War.
Room 40 was on the first floor of the main wing of The Admiralty's Old Building, now known as the Ripley Building (built 1726) in Whitehall. It was on the same corridor as the old Board Room.
SMS Magdeburg aground off Odensholm
Zimmermann telegram as decoded by Room 40
Alastair Denniston
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Commander Alexander Guthrie Denniston was a Scottish codebreaker, deputy head of the Government Code and Cypher School and hockey player. Denniston was appointed operational head of GC&CS in 1919 and
Alastair Denniston in 1926
