@misc{cryptoeprint:2011:071,
author = {Gilles Barthe and Boris Köpf},
title = {Information-theoretic Bounds for Differentially Private Mechanisms},
howpublished = {Cryptology ePrint Archive, Report 2011/071},
year = {2011},
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.