@misc{cryptoeprint:2009:586,
author = {Rongquan Feng and Hongfeng Wu},
title = {A mean value formula for elliptic curves},
howpublished = {Cryptology ePrint Archive, Report 2009/586},
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.