@misc{cryptoeprint:2005:201,
author = {Zhaohui Cheng and Liqun Chen},
title = {On Security Proof of McCullagh-Barreto's Key Agreement Protocol and its Variants},
howpublished = {Cryptology ePrint Archive, Report 2005/201},
year = {2005},
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.