@misc{cryptoeprint:2008:270,
author = {Somitra Kumar Sanadhya and Palash Sarkar},
title = {New Collision attacks Against Up To 24-step SHA-2},
howpublished = {Cryptology ePrint Archive, Report 2008/270},
year = {2008},
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.