@misc{cryptoeprint:2007:005,
author = {Lan Nguyen},
title = {Privacy-Protecting Coupon System Revisited},
howpublished = {Cryptology ePrint Archive, Report 2007/005},
year = {2007},
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.