@misc{cryptoeprint:2006:348,
author = {Krzysztof Pietrzak},
title = {Non-Trivial Black-Box Combiners for Collision-Resistant Hash-Functions don't Exist},
howpublished = {Cryptology ePrint Archive, Report 2006/348},
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.