@misc{cryptoeprint:2011:306,
author = {Craig Costello and Kristin Lauter},
title = {Group Law Computations on Jacobians of Hyperelliptic Curves},
howpublished = {Cryptology ePrint Archive, Report 2011/306},
year = {2011},
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.