@misc{cryptoeprint:2009:310,
author = {Chong Hee Kim and Gildas Avoine},
title = {RFID distance bounding protocol with mixed challenges to prevent relay attacks},
howpublished = {Cryptology ePrint Archive, Report 2009/310},
year = {2009},
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.