@misc{cryptoeprint:2004:305,
author = {Paulo S. L. M. Barreto},
title = {A note on efficient computation of cube roots in characteristic 3},
howpublished = {Cryptology ePrint Archive, Report 2004/305},
year = {2004},
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.