Alexandra Dirksen

Master's Thesis by Tilman Stehr

Certificate Transparency (CT) is an extension to the web’s PKI that allows insight into the issuance of
TLS certificates by introducing public append-only logs, in which all certificates must be included.
Currently, CT can be circumvented by an attacker controlling a CA and several CT logs. We present
an attacker model for this attacker and derive security goals from it. Additionally, we derive design
goals from a review of related work.
We introduce LogPicker, which improves CT’s security by involving multiple logs in the logging of a
certificate. The logs use a distributed randomness protocol to unpredictably chose the log that is to
include the certificate. They generate proof of LogPicker’s execution with an aggregate signature
scheme.
An analysis LogPicker and related protocols to determine the probability of correctness depending
on the number of logs and the trust in each log is presented. The analysis shows that LogPicker can
significantly improve trust in the web’s PKI. Tests with a prototype implementation indicate that
LogPicker has reasonable performance, scalability, and failure tolerance.
We conclude that LogPicker constitutes a useful addition to CT that can be realistically implemented.
Further research into LogPicker is recommended, we suggest formal verification of the protocol
and expansion of the prototype implementation.