signals intelligence

By 1936, SIS had succeeded in cracking the main Japanese diplomatic cipher, to which it gave the code name Red. Red decrypts were probably the first SIGINT to be seen by Roosevelt as president. Early in 1937, for the first time in American history, SIGINT began to be regularly delivered to the White House, initially at the rate of about one decrypt a day. (…) Roosevelt, however, was more interested in the less reliable but superficially more exciting intelligence supplied by Astor’s Room and Donovan. The cryptanalysts, unlike the amateur agents, received no sign of the president’s interest in their work.

During 1938 Red decrypts revealed that a new Japanese cipher machine was under construction. On March 20, 1939, the first diplomatic messages were intercepted in a new machine cipher, code-named Purple.