@misc{cryptoeprint:2006:077,
author = {Jaume Marti-Farre and Carles Padro},
title = {On Secret Sharing Schemes, Matroids and Polymatroids},
howpublished = {Cryptology ePrint Archive, Report 2006/077},
year = {2006},
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.