@misc{cryptoeprint:2013:095,
author = {Antoine Joux},
title = {A new index calculus algorithm with complexity $L(1/4+o(1))$ in very small characteristic},
howpublished = {Cryptology ePrint Archive, Report 2013/095},
year = {2013},
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.