@misc{cryptoeprint:2000:017,
author = {Rosario Gennaro and Luca Trevisan},
title = {Lower Bounds on the Efficiency of Generic Cryptographic Constructions},
howpublished = {Cryptology ePrint Archive, Report 2000/017},
year = {2000},
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.