If there was cryptographic verification, the same people doubting that video would suspect that his cryptographic keys had been compromised.

(Which, come to think of it, is a lot more likely than actors as disparate as Sean Hannity, John Pilger, the Swedish prosecutor, the Ecuadorean embassy and Pamela Anderson all being very happy to pretend to interact with a Fake Assange who keeps making very similar arguments to the inexplicably-disappeared Real Assange.)