[bluejekyll]

> was guesswork on what the code word for Midway

More specifically, it was active counter intelligence where the US sent a false report of a water issue on Midway broadcast in the clear that they then picked up the Japanese report of the issue. They used that to discern which codeword Japan used for Midway.

[WalterBright]

You're right, but it was still scant information with which to bet the fleet on. The Japanese might have suspected that their code was broken, and so used disinformation to mislead the US Navy.

Hell, it's what I would have done whether I thought the code was broken or not.

The Germans had plenty of evidence that Enigma was broken. The High Command refused to believe it. I would have used the broken Enigma to send the Allies into a trap.

The way to play the code breaking game is to assume the enemy has broken it, and act accordingly to your own advantage.

[mannykannot]

Even if you know one of your widely-used codes or cyphers has been broken, I don't think it is that easy to make use of that fact, except perhaps briefly and in a limited way.

To conceal the fact that you know that it is broken, you would need to maintain use of that code at similar levels as before, without approximately doubling the signal traffic by sending the real communication under a new code. Furthermore, the fake traffic under the original code must be realistic to the degree the enemy can verify it, as they can read it, and if a major code has been broken for a period of a few weeks or so, the enemy presumably has plenty of information to use in verifying new messages, at least for a while (the verification need not be explicitly performed, at first; if new messages seem to be inconsistent with what is already known, questions are likely to be raised.)

Compromised minor cyphers and codes are another matter, and that is exactly how the Midway ruse worked.

[sidewndr46]

For Nazi Germany the "fake traffic" would not be needed for all the services. Key change happened at midnight Berlin time by all operators. The radio operators stayed up late into the night sending the personal correspondence of the various officers to their families. The codebreaking process used this huge volume of messages to feed into the "cribbing" process which aided in recovering the traffic. By the time they had extracted enough of the key to decrypt traffic, normal military communications had started

[mannykannot]

Thanks - I was unaware of that until now. It appears to be a major operational security lapse.

[mannykannot]

Correction: I wrote ‘without approximately doubling…’ where I meant ‘while approximately doubling…’ - and then one must take into account sidewndr46’s interesting point.

[yowzadave]

How do you keep your allies from believing your fake encoded messages and taking the same action that they would have taken, had you not suspected the code was broken?

[WalterBright]

I'm not sure what you mean.

[sidewndr46]

It sounds like they are suggesting there is some sort of unusual 3rd order deception going on, but I don't know really.

[sidewndr46]

It's like a known plaintext attack, but in the 1940s!

[wat10000]

There was a lot of this. The Enigma cracking team would use things like weather reports and convoy sightings as known plaintext for their work. If you pick up a submarine transmitting near a convoy, it’s probably saying that it saw a convoy at such and such coordinates. The same key was reused for the other messages from that day so cracking one let you read them all.

[WalterBright]

If I was running it and transmitting coordinates, I'd give the U-Boot commanders one-time pads to obfuscate them, and then encrypt the entire message.

> The same key was reused for the other messages from that day so cracking one let you read them all.

I know. The Germans were simply idiots in their hubris about Enigma. The evidence it was cracked was overwhelming, but Doenitz just dismissed it all. Rommel was also defeated by decoded Enigma messages, and he dismissed all evidence of its subversion.

[sidewndr46]

The weird thing is that it's possible the Enigma could have been used during WWII for communications. With the right number of rotors, the right choice of keys, and the right key rotation schedule it is possible.

Of course when you believe you are in the most advanced nation in the US, what is the incentive to improve?

[wat10000]

I wonder how hard it would have been to provision each submarine with enough one-time pad to cover all secure communication for their entire patrol. I don’t know how much radio traffic there was, but it seems like that would not have been a major burden.

[WalterBright]

Hand them a newspaper. Plenty of text there to use as a pad. The code breakers would need to know both the newspaper used, and the algorithm used to select letters from the paper in order to crack it. Or use a magazine to provide the algorithm.

Every U-Boot mission gets another newspaper. Every U-Boot has a different newspaper. There weren't that many U-Boots, so this would be manageable.

Even decoding one U-Boot's transmissions would not compromise the others.

[sidewndr46]

The U-Boats have plenty of other operational failures. Whenever U-Boats would be potentially damaged by the Allies, they'd put a notice in at one of the British listening stations. When the U-boat came into port, it'd radio in advance that it was damaged and might need special provisions (couldn't steer well, etc.) or even a tug to make it into the harbor. This all happened in German on the HF radio, which propagates really well. At least to Britain and possibly all the way to the mainland US.

The Allies basically got free reports of exact damage they inflicted on subs this way. On the other hand if a report didn't show up in a few days, they probably sunk it.