In 1939, she made important inroads into JN-25, the Japanese fleet's operational code used for the most important of messages. She successfully solved the cipher component of the "5-num" system which used number groups as substitutes for words and numbers which was further encrypted with a digital cipher. After that, the Navy could read some standard format messages, such as weather reports, but the bulk of the messages remained to be discovered. This work was later developed and exploited after the attack on Pearl Harbor for the rest of the Pacific War and provided advance warning of the Japanese attack on Midway Atoll. She was unable to take part in this work because, in October 1940, she was transferred to a team working to break the German naval Enigma cipher.