[RichieAHB]

I’m not sure being German or from 1945 has much to do with this. I’d think if I had some candidate plaintext that looked German and where a letter didn’t map to itself it’s likely to be right.

Sure, you could argue that it’s not quite as unambiguous as JSON but, in the realm of all possible plaintext, I’d say JSON and 1945 German aren’t too far apart in terms of likelihood of getting a false positive on a string of non-trivial size.

If you want a bit more certainty you could just try it on another string (which is the next step in the process either way), and then another. At least to me the constraints of “German and a char doesn’t map to itself” seem like pretty limiting factors here.

[DSMan195276]

I don't think it's as simple as you're assuming. The "char doesn't map to itself" is a feature of the hardware design, there's no such thing as a key that allows that to happen so it doesn't reduce your search space. But additionally, the messages were extremely terse (~250 character limit), had no punctuation or spaces, and only had the 26 alphabet characters. Depending on the message in question it already doesn't look much like German, and the ones writing these messages also purposely vary their spelling/names/wording to make it harder to analyze. IMO the chance of finding "potentially plausible messages" seems higher than you'd want if you're starting with zero idea what the plaintext is supposed to be.

[RichieAHB]

Interesting. That probably does makes it a bit harder to verify but there are still plenty of “words” that are to be expected. My point being that if a German receiver can’t parse it, then what’s the point of send it. And if they can parse it we can encode the rules they’re using to parse it (i.e. at the very least a dictionary of “words”) to give us some decent confidence. And beyond that, using the key on multiple messages effectively increases the char limit to “max chars * message count”.

Also, I agree it doesn’t reduce the search space but I think the post I was replying to was talking more about difficulty of verifying the correctness of a decrypt that reducing the search space. That was definitely my point ant least. Either way, as you say, this definitely doesn’t reduce the search space.

[wkat4242]

Why was there a character limit? I always thought the only limit was the slow transmission speed of morse messages.

[DSMan195276]

From what I have read it was specifically to make it harder to decrypt the messages :D Longer messages are easier to analyze.

[dylan604]

It would be awesome if there's a message that when decoded incorrectly said "Drink your Ovaltine", but when decoded correctly was a different message

[RichieAHB]

Remind me of “Deniable Encryption”[1], where a single encrypted input decrypts to multiple plausible plain texts (including the real one, obviously) so that, if, for example, you were caught and asked to decrypt it, you can decrypt to the non-incriminating version.

[1] https://en.m.wikipedia.org/wiki/Deniable_encryption