@misc{cryptoeprint:2006:234,
author = {Ivan Damg{\aa}rd and Michael {\O}stergaard},
title = {RFID Security: Tradeoffs between Security and Efficiency},
howpublished = {Cryptology ePrint Archive, Report 2006/234},
year = {2006},
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.