Can quantum cryptography imply quantum mechanics? Reply to Smolin

Clifton, Bub, and Halvorson (CBH) have argued that quantum mechanics can be
derived from three cryptographic, or broadly information-theoretic, axioms. But
Smolin disagrees, and he has given a toy theory that he claims is a
counterexample. Here we show that Smolin's toy theory violates an independence
condition for spacelike separated systems that was assumed in the CBH argument.
We then argue that any acceptable physical theory should satisfy this
independence condition.