@misc{cryptoeprint:2008:199,
author = {David G. Harris},
title = {Simultaneous field divisions: an extension of Montgomery's trick},
howpublished = {Cryptology ePrint Archive, Report 2008/199},
year = {2008},
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.