@misc{cryptoeprint:2005:226,
author = {Liqun Chen and Zhaohui Cheng},
title = {Security Proof of Sakai-Kasahara's Identity-Based Encryption Scheme},
howpublished = {Cryptology ePrint Archive, Report 2005/226},
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.