Kichisaburō Nomura was ambassador to the United States 1939-42. His biography on Wikipedia is at [1].

After World War II, Nomura denied that he knew beforehand of the attack on Pearl Harbour. Reportedly Nomura and Kurusu had to personally decode the radioed message of Japan's breaking off the negotiations with the United States (which given the circumstances practically meant war), as it had been sent from Japan on Monday, December 8 and was received when the embassy's technical support staff was still on Sunday holiday. Nomura stated that this is why he had been unable to deliver the message until after the actual attack had taken place. In his memoirs, Cordell Hull (US Secretary of State) credited Nomura with having been sincere in trying to prevent war between Japan and the USA.

See also Purple (cipher machine), and, on Wikipedia, the Hull Note].