@misc{cryptoeprint:2011:621,
author = {Zhenfu Cao, Zongyang Zhang , Yunlei Zhao},
title = {Adaptive Security of Concurrent Non-Malleable Zero-Knowledge},
howpublished = {Cryptology ePrint Archive, Report 2011/621},
year = {2011},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.