  This paper systematizes log based Transparency Enhancing Technologies. Based
on work on transparency from multiple disciplines we outline the purpose,
usefulness, and pitfalls of transparency.
  We describe the mechanisms that allow log based transparency enhancing
technologies to be implemented, in particular logging mechanisms, sanitisation
mechanisms and the trade-offs with privacy, data release and query mechanisms,
and how transparency relates to the external mechanisms that enable contesting
a system and holding system operators accountable.
  We illustrate this with two examples, Certificate Transparency and
cryptocurrencies, and show the role that transparency plays in their function
as well as the issues these systems face in delivering transparency.
