What I’m saying is that if the key is truly random and full length, it will produce random ciphertext. This ciphertext is equally likely (cipherwise) to have come from any clear text passage of the same length, of which there are infinitely many. So there are an infinite number of solutions, and if the key is random, there’s no clue which are more likely than others. You could make an educated guess based on subject matter and what you know about the sender, but remember, ANY 1000 character clear text can produce the cipher text, so it’s pretty much like knowing nothing at all. Now, one could reason in reverse and calculate what the key would have had to have been for any conceivable clear text, and if you think the sender is cryptographically sophisticated, you could throw out the candidates that would need crappy keys to generate the ciphertext, but I don’t see what else you can do.
“What I’m saying is that if the key is truly random and full length, it will produce random ciphertext”
Whats full length? Keys come in increments of 8 with the normal key length being 256. Some software tools will use keys of over 2000.
There is not an infinite number of solutions. It is definately a finite although very high number. With todays computers it is not nearly as difficult as it once was to break a key.
“but I don’t see what else you can do.”
There is a science used to break these codes.