A Characterization of Non-interactive Instance-Dependent Commitment-Schemes (NIC)

We provide a new characterization of certain zero-knowledge protocols as non-interactive instance-dependent commitment-schemes (NIC). To obtain this result we consider the notion of V-bit protocols, which are very common, and found many applications in zero-knowledge. Our characterization result states that a protocol has a V-bit zero-knowledge protocol if and only if it has a NIC. The NIC inherits its hiding property from the zero-knowledge property of the protocol, and vice versa. Our characterization result yields a framework that strengthens and simplifies many zero-knowledge protocols in various settings. For example, applying this framework to the result of Micciancio et al. [19] (who showed that some problems, including GRAPH-NONISOMORPHISM and QUADRATIC-RESIDUOUSITY, unconditionally have a concurrent zero-knowledge proof) we easily get that arbitrary, monotone boolean formulae over a large class of problems (which contains, e.g., the complement of any random self-reducible problem) unconditionally have a concurrent zero-knowledge proof.