@misc{cryptoeprint:2009:479,
author = {Yu Sasaki and Lei Wang and Kazumaro Aoki},
title = {Preimage Attacks on 41-Step SHA-256 and 46-Step SHA-512},
howpublished = {Cryptology ePrint Archive, Report 2009/479},
year = {2009},
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.