@misc{cryptoeprint:2010:511,
author = {Koh-ichi Nagao},
title = {On the complexity of Decomposition Attack},
howpublished = {Cryptology ePrint Archive, Report 2010/511},
year = {2010},
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.