@misc{cryptoeprint:2010:645,
author = {Xinfeng Lei and Rui Xue and Ting Yu},
title = {A Timed Logic for Modeling and Reasoning about Security Protocols},
howpublished = {Cryptology ePrint Archive, Report 2010/645},
year = {2010},
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.