If you spent the time, it could be a sizable subset of any. The only constraint is the number of characters. Nothing stops you from making a dozen different pos

If you spent the time, it could be a sizable subset of any. The only constraint is the number of characters. Nothing stops you from making a dozen different possible answers since the key is a pseudo random string with a 1-to-1 match with the crypt. You could send multiple messages with the same crypt string, so long as the recipients all have their different keys needed to read it. It's not a bad way to send instructions between say, 3 different people, who all have the same crypt, but also all have different keys from which to receive their specific instructions. They do have to be generated before disseminating the keys, though, since they are literally 1-to-1 transformations. The trade off is everyone has a gigantic key instead of a small set to remember. A one time pad that's the same length as the data it's used for, can do quite a bit.