[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.

[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?

[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.