@misc{cryptoeprint:2005:052,
author = {Ziv Kfir and Avishai Wool},
title = {Picking Virtual Pockets using Relay Attacks on Contactless Smartcard Systems},
howpublished = {Cryptology ePrint Archive, Report 2005/052},
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.