@misc{cryptoeprint:2010:496,
author = {Jan Camenisch and Thomas Gro{\ss}},
title = {Efficient Attributes for Anonymous Credentials (Extended Version)},
howpublished = {Cryptology ePrint Archive, Report 2010/496},
year = {2010},
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.