@misc{cryptoeprint:2004:039,
author = {Victor K. Wei},
title = {A Bilinear Spontaneous Anonymous Threshold Signature for Ad Hoc Groups},
howpublished = {Cryptology ePrint Archive, Report 2004/039},
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.