@misc{cryptoeprint:2010:575,
author = {Otto Johnston},
title = {A Discrete Logarithm Attack on Elliptic Curves},
howpublished = {Cryptology ePrint Archive, Report 2010/575},
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.