@misc{cryptoeprint:2010:298,
author = {Elena Andreeva, Bart Mennink and Bart Preneel},
title = {On the Indifferentiability of the Gr{\o}stl Hash Function},
howpublished = {Cryptology ePrint Archive, Report 2010/298},
year = {2010},
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.