@misc{cryptoeprint:2003:079,
author = {Stéphane Flon and Roger Oyono},
title = {Fast arithmetic on Jacobians of Picard curves},
howpublished = {Cryptology ePrint Archive, Report 2003/079},
year = {2003},
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.