@misc{cryptoeprint:2008:033,
author = {Boaz Barak, Mohammad Mahmoody-Ghidardy},
title = {Lower Bounds on Signatures From Symmetric Primitives},
howpublished = {Cryptology ePrint Archive, Report 2008/033},
year = {2008},
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.