@misc{cryptoeprint:2008:189,
author = {Thomas Ristenpart and Thomas Shrimpton},
title = {How to Build a Hash Function from any Collision-Resistant Function},
howpublished = {Cryptology ePrint Archive, Report 2008/189},
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.