@misc{cryptoeprint:2011:135,
author = {Omran Ahmadi and Robert Granger},
title = {On isogeny classes of Edwards curves over finite fields},
howpublished = {Cryptology ePrint Archive, Report 2011/135},
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.