On the security of two sealed-bid auction schemes

In this paper, we cryptanalyze two sealed-bid auction schemes and show that both schemes are not secure against the known-bid attack. In the first scheme, we show that a dishonest sealer can swap the price of a sealed-bid and yet its validity remains intact; in the second scheme, we show that a dishonest auctioneer can forge a bidder's bid if the bidder was once a winning bidder. We propose a workaround for the former using a designated confirmer signature but not the latter, which is fundamentally flawed.