@misc{cryptoeprint:2015:530,
author = {Pierre Karpman and Thomas Peyrin and Marc Stevens},
title = {Practical Free-Start Collision Attacks on 76-step SHA-1},
howpublished = {Cryptology ePrint Archive, Report 2015/530},
year = {2015},
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.