Revisiting User Privacy for Certificate Transparency

Abstract

Public key infrastructure (PKI) based on certificate authorities is one of the cornerstones of secure communication over the internet. Certificates issued as part of this PKI provide authentication of web servers among others. Yet, the PKI ecosystem is susceptible to certificate misissuance and misuse attacks. To prevent those attacks, Certificate Transparency (CT) facilitates auditing of issued certificates and detecting certificates issued without authorization. Users that want to verify inclusion of certificates on CT log servers contact the CT server directly to retrieve inclusion proofs. This direct contact with the log server creates a privacy problem since the users' browsing activities could be recorded by the log server owner. Lueks and Goldberg (FC 2015) suggested the use of Private Information Retrieval (PIR) in order to protect the users' privacy in the CT ecosystem. With the immense amount of certificates included on CT log servers, their approach runs into performance issues, however. Nevertheless, we build on this approach and extend it using multi-tier Merkle trees, and render it practical using multi-server PIR protocols based on distributed point functions (DPFs). Our approach leads to a scalable design suitable to handle the increasing number of certificates and is, in addition, generic allowing instantiations using secure accumulators and PIRs. We implement and test this mechanism for privacy-preserving membership proof retrieval and show that it can be integrated without disrupting existing CT infrastructure. Most importantly, even for larger CT logs containing 2 31 certificates, our approach using sub-accumulators can provide privacy with a performance overhead of less than 9 milliseconds in total. 3 https://www.section.io/blog/chrome-ct-compliance/ 4 Precertificates are certificates provided to the log before the issuance of the actual certificate. They contain a special critical poison extension that renders the certificate unusable in TLS connections.

SoK: SCT Auditing in Certificate Transparency

“…Lueks and Goldberg were the first to propose using PIR for CT [48], and a more performant solution was later proposed by Kales, Omolola, and Ramacher [35]. Recently, Kogan and Corrigan-Gibbs proposed a PIR solution, Checklist, for the related problem of Safe Browsing [36].…”

Privacy-Preserving Epidemiological Modeling on Mobile Graphs

“…As Android does not allow third-party developers to implement applications for Android's TEE Trusty [7], we use hardware-backed crypto operations already implemented by Android instead. We use the code of [70] to instantiate FSS-PIR. We implement the AGCT in C++ and follow previous work on cuckoo hashing [108] by using tabulation hashing for the hash functions.…”

“…We omit the overhead of remote attestation for the sake of simplicity. For RIPPLE PIR , we use the FSS-PIR scheme of [17,70] as the baseline and the addresses are hashed with SHA-256 and trimmed to 40−1+log 2 (p•𝐸 avg ), where p represents the number of participants and 𝐸 avg represents the average number of encounters per participant per simulation step. We set 𝐸 avg = 100 while benchmarking based on numbers provided by research on epidemiological modeling [40,95].…”

Postcertificates for Revocation Transparency

“…Tomescu et al [49] propose a transparency log system that minimizes proof sizes and bandwidth usage at the cost of the increased append times. Kales et al [22] study effects of CT on user privacy and implement privacy-preserving and efficient membership testing for CT logs, which can potentially enable the use of the logs for direct and secure postcertificate lookup by clients.…”