[Cryptography] How crypto killed Admiral Yamamoto

[Arnold Reinhold]

On Sun, 18 Apr 2021 08:19:22 +1000 (EST) Dave Horsfall wrote:

> On this day in 1943, Admiral Yamamoto (the architect of the Pearl Harbor 
> attack) was shot down by a squadron of P-38 Lightnings that just 
> "happened" to be loitering in the area at their extreme range where he was 
> known to be (the Japanese had a thing about punctuality).
> 
> I think that it was "Purple" that was broken?  In any case, there was some 
> debate about whether to shoot him down (thereby revealing that their 
> crypto had been broken) or let him go (thereby keeping the secret); well, 
> we know what happened next...
> 
> I believe that his "Betty" bomber slammed into a tree at about 100 mph 
> (along with his aircraft chair), and was identifiable by his teeth and his 
> Samurai sword.

The message detailing Admiral Isoroku Yamamoto’s itinerary was sent in the cipher system the U.S. called JN-25d, the primary code of the Imperial Japanese Navy. PURPLE, also an American code name, was a Roman alphabet machine cipher based on telephone stepping switches that was used for diplomatic traffic. Military versions JADE and CORAL, based on similar mechanisms, were also broken after the PURPLE breach, but were not widely used by the Japanese. The big intelligence coup from these breaks came from Japanese Vice Admiral Abe, based in Berlin, who radioed back to Tokyo detailed reports on German thinking and plans using the CORAL machine.  

The Japanese military mostly used code books that translated words and phrases into 5 digit code groups that were then enciphered with large tables of additives and increasingly disguised indicator systems. The code books and additive books were changed periodically and the massive effort in recovering the systems were facilitated by the large teams of women that the US Army and Navy, as well as the British, had recruited. JN-25 was one of those systems and the -d version had only recently been partially recovered. There is a detailed discussion in the Wikipedia article on “Operation Vengeance,” which also has a different account of Yamamoto’s death, his body being recovered largely intact with a likely fatal head wound.

American leadership reportedly concluded that the death of the highly successful Admiral would be equivalent to winning a major military battle and was therefore worth the risk of possibly revealing our knowledge of Japanese codes. I think Yamamoto would have appreciated the implied honor.

Arnold Reinhold