Alvin D. Kramer was a Lieutenant Commander of the US Navy and a cryptographer from OP-20-G, the signals intelligence wing of the Navy, during World War II. In 1941, Kramer and Rufus S. Bratton monitored Japanese communications from "Purple" and discovered an imminent attack on the naval base at Pearl Harbor, Hawaii, believing that it would occur in November. They later believed that it would occur on 6 December, but it would occur the next day, taking the Americans by surprise.