@misc{cryptoeprint:2010:352,
author = {Masahiro Yagisawa},
title = {A Digital Signature Using Multivariate Functions on Quaternion Ring},
howpublished = {Cryptology ePrint Archive, Report 2010/352},
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.