@misc{cryptoeprint:2004:178,
author = {S. G. Barwick and W.-A. Jackson and K. M. Martin and C. M. O'Keefe},
title = {Optimal Updating of Ideal Threshold Schemes},
howpublished = {Cryptology ePrint Archive, Report 2004/178},
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.