In this paper, we initiate a formal study of transparency, which in recent years has become an increasingly
critical requirement for the systems in which people place trust. We present the abstract concept of
a transparency overlay, which can be used in conjunction with any system to give it provable transparency
guarantees, and then apply the overlay to two settings: Certificate Transparency and Bitcoin. In the latter
setting, we show that the usage of our transparency overlay eliminates the need to engage in mining and
allows users to store a single small value rather than the entire blockchain. Our transparency overlay is
generically constructed from a signature scheme and a new primitive we call a dynamic list commitment,
which in practice can be instantiated using a collision-resistant hash function
