## Cryptology ePrint Archive: Report 2001/051

Black-Box Concurrent Zero-Knowledge Requires $\tilde\Omega(\log n)$ Rounds

Ran Canetti, Joe Kilian, Erez Petrank, Alon Rosen

Abstract: We show that any concurrent zero-knowledge protocol for a non-trivial language (i.e., for a language outside $\BPP$), whose security is proven via black-box simulation, must use at least $\tilde\Omega(\log n)$ rounds of interaction. This result achieves a substantial improvement over previous lower bounds, and is the first bound to rule out the possibility of constant-round concurrent zero-knowledge when proven via black-box simulation. Furthermore, the bound is polynomially related to the number of rounds in the best known concurrent zero-knowledge protocol for languages in $\NP$.

Category / Keywords: cryptographic protocols / zero-knowledge

Publication Info: An extended abstract to appear in STOC01