@misc{cryptoeprint:2005:119,
author = {Claus Diem},
title = {Index Calculus in Class Groups of Plane Curves of Small Degree},
howpublished = {Cryptology ePrint Archive, Report 2005/119},
year = {2005},
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.