The salt could be stored as a cookie and you can follow the session but never be able to reverse the hash yourself. Any match you get in the brute-force attempt might as well be a collision.

The entropy can be in the salt, you're all making it sound way too easy. The requirement is "non-reversible". Given infinite time everything can be brute-forced, but this is the mossad/not-mossad problem.

It's good enough for storing passwords, where the salt is plain-text.