@misc{cryptoeprint:2011:532,
author = {Bart Mennink and Bart Preneel},
title = {Hash Functions Based on Three Permutations: A Generic Security Analysis},
howpublished = {Cryptology ePrint Archive, Report 2011/532},
year = {2011},
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.