Message in old cipher led to Adm. Yamamoto's death: U.S. documents+

[Copper]

The successful U.S. decrypting of secret Japanese communication messages that led to Imperial Japanese Navy Adm. Isoroku Yamamoto's death during World War II was because one of those messages was written in an old cipher that should have earlier been destroyed, according to declassified U.S. intelligence documents.

Although the U.S. military said after the war that it ambushed him after deciphering related Japanese Navy messages, it has long been unknown how specifically the United States deciphered what kind of messages.

Yamamoto, then commander-in-chief of the Combined Fleet, was killed in an aerial ambush by U.S. Army Air Force planes on April 18, 1943, on the way to Ballalae in the Northern Solomon Islands from Rabaul, the main base of Japanese military and naval activity in the South Pacific at the time.

He was in charge of planning Japan's surprise attack on Pearl Harbor in December 1941. He was also responsible for an operation that led to the June 1942 Battle of Midway, in which Japan lost naval superiority in the Pacific.

The documents, found at the U.S. National Archives by Japanese historian Katsuhiro Hara, include a report by a wartime U.S. cryptanalytic activity unit and two decrypted Japanese Navy communication messages.

The report says the U.S. unit intercepted and deciphered a Japanese Navy message of April 13, 1943, conveying Yamamoto's plan of an inspection tour of forward positions in the South Pacific.

The message was written in a high-security cipher, which the unit called JN25E14, but was valid only from Jan. 3 through Feb. 14 that year, according to a related U.S. military document.

A decryption of the message, sent out from the Japanese Navy's Southeast Area Fleet, shows "CINC COMBINED FLEET," or the commander-in-chief of the Combined Fleet, will travel to Ballalae from Rabaul, now in Papua New Guinea.

The message contained specific details associated with Yamamoto's journey, including arrival and departure times and locations, as well as the number and types of planes that would carry and accompany him on the trip.

Another decrypted message dated April 14 from the Base Force No. 8, written in the less secure JN20H cipher, says of "the special visit of Commander Yamamoto," with an order that a garrison force at a base in Ballalae should "act as heretofore."

With these two messages, the U.S. military grasped Yamamoto's itinerary, with the cryptanalytic report saying, "Together, the two systems gave a complete account, and it was Admiral Yamamoto's death warrant."

After Yamamoto's death, the Japanese Navy said in a report that the U.S. military could not have decrypted the messages as it had introduced an updated cipher as of April 1 that year in place of the previous one.

Regarding the reason for the Japanese Navy's use of an old cipher for the April 13 communication, Hara, a veteran researcher of wartime history, said the new version may have not reached its Ballalae base.