Certificate Transparency is an ecosystem of logs, monitors, and auditors that
hold certificate authorities accountable while issuing certificates.  We show
how the amount of trust that TLS clients and domain owners need to place in
Certificate Transparency can be reduced, both in the context of existing gradual
deployments and the largely unexplored area of Tor.  Our contributions include
  improved third-party monitoring,
  a gossip protocol plugging into Certificate Transparency over DNS,
  an incrementally deployable gossip-audit model tailored for Tor Browser, and
  using certificates with onion addresses.
The methods used range from proof sketches to Internet measurements and
prototype evaluations.  An essential part of our evaluation in Tor is to assess
how the protocols used during website visits---such as requesting an inclusion
proof from a Certificate Transparency log---affect unlinkability between senders
and receivers.  We find that most false positives in website fingerprinting
attacks can be eliminated for all but the most frequently visited sites.  This
is because the destination anonymity set can be reduced due to how Internet
protocols work: communication is observable and often involves third-party
interactions.  Some of the used protocols can further be subject to side-channel
analysis.  For example, we show that remote (timeless) timing attacks against
Tor's DNS cache reliably reveal the timing of past exit traffic.  The severity
and practicality of our extension to website fingerprinting pose threats to the
anonymity provided by Tor.  We conclude that access to a so-called website
oracle should be an assumed attacker capability when evaluating website
fingerprinting~defenses.

\keywords
  Auditing,
  Certificate Transparency,
  DNS,
  Gossip,
  Side-Channels,
  Timing Attacks,
  Tor,
  Tor Browser,
  Website Fingerprinting,
  Website Oracles
