  The modern Internet is highly dependent on trust communicated via
certificates. However, in some cases, certificates become untrusted, and it is
necessary to revoke them. In practice, the problem of secure revocation is
still open. Furthermore, the existing procedures do not leave a transparent and
immutable revocation history. We propose and evaluate a new revocation
transparency protocol that introduces postcertificates and utilizes the
existing Certificate Transparency (CT) logs. The protocol is practical, has a
low deployment cost, provides an immutable history of revocations, enables
delegation, and helps to detect revocation-related misbehavior by certificate
authorities (CAs). With this protocol, a holder of a postcertificate can bypass
the issuing CA and autonomously initiate the revocation process via submission
of the postcertificate to a CT log. The CAs are required to monitor CT logs and
proceed with the revocation upon detection of a postcertificate. Revocation
status delivery is performed independently and with an arbitrary status
protocol. Postcertificates can increase the accountability of the CAs and
empower the certificate owners by giving them additional control over the
status of the certificates. We evaluate the protocol, measure log and monitor
performance, and conclude that it is possible to provide revocation
transparency using existing CT logs.
