Realistically, 1) the model will be one of a small number of available models which can be tested against, 2) LLMs converge on similar predictions, so even an "unknown" model can hardly be considered cryptographic. The advantage of using an LLM this way is the ability to hide an information stream in an innocuous looking text, in the form of subtle deviations from the most likely next word. Hence, steganography.
Incidentally, this technique is actually an old one that has already seen use in magic shows and confidence tricks; two people who wish to communicate covertly, such as a magician and an "audience member", can exchange small amounts of information by varying the wording of simple responses: "yes", "that's right", "quite so". This can be used to, for instance, discreetly reveal the value of a card that the magician is "guessing" through "ESP".
Incidentally, this technique is actually an old one that has already seen use in magic shows and confidence tricks; two people who wish to communicate covertly, such as a magician and an "audience member", can exchange small amounts of information by varying the wording of simple responses: "yes", "that's right", "quite so". This can be used to, for instance, discreetly reveal the value of a card that the magician is "guessing" through "ESP".