@misc{cryptoeprint:2014:305,
author = {Florian Mendel and Vincent Rijmen and Martin Schläffer},
title = {Collision Attack on 5 Rounds of Grøstl},
howpublished = {Cryptology ePrint Archive, Report 2014/305},
year = {2014},
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.