@misc{cryptoeprint:2007:425,
author = {Koray Karabina and Edlyn Teske},
title = {On prime-order elliptic curves with embedding degrees k=3,4 and 6},
howpublished = {Cryptology ePrint Archive, Report 2007/425},
year = {2007},
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.