  Trust in publicly verifiable Certificate Transparency (CT) logs is reduced
through cryptography, gossip, auditing, and monitoring. The role of a monitor
is to observe each and every log entry, looking for suspicious certificates
that interest the entity running the monitor. While anyone can run a monitor,
it requires continuous operation and copies of the logs to be inspected. This
has lead to the emergence of monitoring-as-a-service: a trusted party runs the
monitor and provides registered subjects with selective certificate
notifications, e.g., "notify me of all foo.com certificates". We present a
CT/bis extension for verifiable light-weight monitoring that enables subjects
to verify the correctness of such notifications, reducing the trust that is
placed in these monitors. Our extension supports verifiable monitoring of
wild-card domains and piggybacks on CT's existing gossip-audit security model.
