Before revision:  In contrast, when buyers' valuations are MHR, we show that the mild additional assumption of a cryptographically secure commitment scheme suffices for a simple two-round auction which is optimal, credible, and strategyproof .
After revision:  In contrast, when buyers' valuations are MHR, we show that the mild additional assumption of a cryptographically secure commitment scheme suffices for a simple two-round auction which is optimal, strategyproof, and credible (even when the number of bidders is only known by the auctioneer) .