@misc{cryptoeprint:2004:282,
author = {Ventzislav Nikov and Svetla Nikova},
title = {New Monotone Span Programs from Old},
howpublished = {Cryptology ePrint Archive, Report 2004/282},
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.