@misc{cryptoeprint:2012:191,
author = {Liangliang Xiao and I-Ling Yen},
title = {Security Analysis and Enhancement for Prefix-Preserving Encryption Schemes},
howpublished = {Cryptology ePrint Archive, Report 2012/191},
year = {2012},
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.