Prof. Greg Jackson

Basically, he's running everything in the Pacific outside of Douglas MacArthur's command.

And Chester is busy crossing his fingers that his codebreakers, the Combat Intelligence Unit under the command of Joseph John Rochefort, has figured out what the Japanese Navy is up to.

It's pretty clear that the Kido Butai has been practicing maneuvers for a great battle.

The million-dollar question is, where do they plan to have it?

And I guess the second million-dollar question then becomes, when do they want to have it?

It's Thursday, May 14th, 1942.

We're in Captain Joe Rochefort's windowless basement, dubbed The Dungeon, an office in the old administration building of Pearl Harbor's Navy Yard, where the expert codebreaker has been working basically around the clock to crack the subsect of Japanese JN-25 code they've termed JN-25B.

And it seems like he's getting close.

He'd probably like to focus without us hanging over his shoulder though.

So as Joe hammers out the final details of what might be a huge breakthrough, let's take a second to understand this code.

The JN-25 consists of 40,000 to 45,000 five-digit number groups, some of which are just fillers designed specifically to fool codebreakers.

On top of those number groups, the coded messages are further enciphered.

Tell you what, I'll let historian Craig Simons explain it to you.

The encoder selected a five-digit number from this cipher tablet and added it to the first number group in the message.

The next cipher number was added to the second number group, and so on throughout the message.

An indicator buried in the message itself revealed the exact location—page number, column, and line—where the cipher number additives could be found in the secondary tablet.

Thus, the code group for East might be 10,236, but it would be encrypted again by adding another five-digit number from the cipher tablet.

If the encoder added the number 45,038, the word East became 55,264.

To decrypt the message, the recipient needed the initial code book, the secondary code tablet, and the indicator.

Close quote.