German SKM Code
This extract from the pages 77 and 78 of the book "Dead Wake" by Erik Larson, briefly describes the German SKM code used by the German navy and the diplomatic corps during World War 1.
"In London, two blocks from the Thames and adjacent to the parade ground of the Horse Guards, stood a five-story building with a facade of pale stone and whiskey-colored brick. Familiar to everyone in the Admiralty, the structure was known, for short, as the Old Building, or, for shorter, O.B. Far less familiar was a secret operation located along one of its corridors in a group of offices centered on Room 40. The purpose of this room resided with nine senior officials, including First Lord Churchill and Adm. Jacky Fisher.
Every day, the watchkeepers in Room 40 received hundreds of coded and enciphered German messages that had been intercepted by an array of wireless stations erected on the British coast, and then sent to the Old Building by land telegraph. Germany had been forced to rely almost exclusively on wireless communication after Britain, in the first days of the war, had followed through on its 1912 plan to cut Germany's undersea cables. The intercepted messages arrived in the basement of the Admiralty building and were then relayed to Room 40.
It was the task of Room 40 to translate these messages into the King's English, a process made possible by a series of nearly miraculous events that occurred in the closing months of 1914 and put the Admiralty in possession of three code books governing Geman naval and diplomatic communications. By far the most important, and secret, was the German navy's SKM code, short for Signalbuch der Kaiserlichen Marine. In August 1914, a German destroyer, the Magdeburg, ran aground and was cornered by Russian ships. Exactly what happened next remains unclear, but one story holds that the Russians found a copy of the codebook still clutched in the arms of a dead German signalman whose body had washed ashore after the attack. If so, it was probably the codebook that killed him. It was large and heavy, 15 inches by 12 inches by 6 inches, and contained 34,304 three-letter groups used to encode messages. The letters MUD, for example, stood for Nantucket; Liverpool was FC]. The Russians in fact recovered three copies of the codebook, presumably not from the same body, and in October 1914 gave one to the Admiralty.
The code books were invaluable but did not by themselves reveal the contents of the intercepted messages. Their German authors used the volumes to obscure the original plain-text messages but then subjected the encoded versions to a further scrambling through the use of a cipher. Only holders of a cipher "key" could divine the underlying text, but possessing the code books made the whole process of solving the messages far simpler".
Contributors and Credits:
1) Dead Wake - The Last Crossing of the Lusitania by Erik Larson. 2016 Broadway Books. New York City. ISBN 979-0-307-40887-7