@misc{cryptoeprint:2013:505,
author = {Zhengjun Cao and Lihua Liu},
title = {Improvement of One Anonymous Identity-Based Encryption},
howpublished = {Cryptology ePrint Archive, Report 2013/505},
year = {2013},
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.