Concurrent Signatures without a Conventional Keystone

摘要

The notion of concurrent signatures was introduced by Chen, Kudla, and Paterson in Eurocrypt 2004. A concurrent signature scheme allows two parties to concurrently exchange their signatures. A concurrent signature scheme has two phases. In the first phase, two parties exchange their ambiguous signatures, which are verifiable only by themselves. In the second phase, the ambiguity is removed by throwing out a "keystone", a secret known to one of involved parties. However, concurrent signatures are still one step short from fully solving the fair exchange problem and they require a normal public key infrastructure. Nevertheless, concurrent signatures have many useful applications. We present a variant of the notion of concurrently exchanging digital signatures and propose a generic construction and a concrete construction to show how to exchange signatures between two parties as fair as possible without using a conventional keystone.