  Transparency is crucial in security-critical applications that rely on
authoritative information, as it provides a robust mechanism for holding these
authorities accountable for their actions. A number of solutions have emerged
in recent years that provide transparency in the setting of certificate
issuance, and Bitcoin provides an example of how to enforce transparency in a
financial setting. In this work we shift to a new setting, the distribution of
software package binaries, and present a system for so-called "binary
transparency." Our solution, Contour, uses proactive methods for providing
transparency, privacy, and availability, even in the face of persistent
man-in-the-middle attacks. We also demonstrate, via benchmarks and a test
deployment for the Debian software repository, that Contour is the only system
for binary transparency that satisfies the efficiency and coordination
requirements that would make it possible to deploy today.
