On Constant-Round Concurrent Zero-Knowledge From a Knowledge Assumption

In this paper, the authors consider the long-standing open question of constructing constant-round concurrent zero-knowledge protocols in the plain model. Resolving this question is known to require non-black-box techniques. They consider non-black-box techniques for zero-knowledge based on knowledge assumptions, a line of thinking initiated by the work of Hada and Tanaka (CRYPTO 1998). Prior to their work, it was not known whether knowledge assumptions could be used for achieving security in the concurrent setting, due to a number of significant limitations that they discuss here.