@misc{cryptoeprint:2009:154,
author = {Yasufumi Hashimoto},
title = {Algorithms to solve massively under-defined systems of multivariate quadratic equations},
howpublished = {Cryptology ePrint Archive, Report 2009/154},
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.