[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.