One of the first acts by the Royal Navy during WW1was to have the German underwater telegraphic cables cut in the North Sea, thus denying them telegram communications with most of the world. It meant that they had to rely on high frequency radio transmissions to communicate with their embassies and overseas ships. Knowing this, the Admiralty engaged the help of Marconi and the Post Office to set up listening stations on the east coast of England to intercept the German signals. I cover all this in my WW1 naval intelligence novel, Now the Darkness Gathers.
Clearly, the German signals were encoded. However, the Admiralty had a break when they managed to obtain three of the Germans’ principal code books. Even so, they still could not decipher the German messages.
However, serving on the staff of the Naval Intelligence Division was a Fleet Paymaster, Charles Rotter, a character I feature in my book and with whom I identify in three ways; we both served in what became the Navy’s Supply and Secretariat specialisation; we both worked in intelligence and, of most relevance here, we were both interpreters (I in Chinese and he in German). Rotter was the head of the German section and Admiral Oliver, the Director of Naval Intelligence, accordingly passed the signal intercepts to him and asked what he could make of them.
Rotter was an expert in the German Navy. In common with several members of the naval intelligence community, he spent his leaves before the war in Germany. He could pass for a German national and spent much of his time mixing with German naval officers. As a result, he knew how they thought and expressed themselves. He, also, discovered that they never changed the form of words in routine signals sent at stated times, such as weather reports.
Rotter soon recognised that more than one encryption method had been employed. After encoding, the Germans had used a substitution key for the letters. Using his knowledge of German and the way the German Navy wrote its signals, he identified the most commonly used sets of words and letters. He was greatly aided by the routine weather reports as he could check the actual weather in the area on the day of the report. This, by the way, was a technique later used by Bletchley Park during WW2. The RAF even dropped mines in certain areas so that the decoders could work out how letters had been transposed from the latitude and longitudes of the mines reported in the German signals. Returning to WW1, within a week, Rotter was able to produce a table to show how the letters were substituted and thereafter the staff of Room 40 were able to read the enemy’s signal traffic with ease.
Unlike me, Rotter had a distinguished career and was to become a Companion of the Bath and a Rear Admiral. His contribution to winning the war has, sadly, been forgotten and I can find no photographs of him.