@misc{cryptoeprint:2006:021,
author = {Denis Charles and Eyal Goren and Kristin Lauter},
title = {Cryptographic hash functions from expander graphs},
howpublished = {Cryptology ePrint Archive, Report 2006/021},
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.