How Far is User Privacy Leakage: A Revisit of Client Certificate Usage

X.509 Certificates are an important medium for identity authentication when the client establishes a TLS connection with the server. As the security issues on TLS are more and more widely discussed, the TLS1.3 [1] protocol was officially released in 2018 for the sake of enhancing security. The TLS1.3 protocol shares the secret key in the ClientHello phase, so that all information except the hello message is encrypted. Although TLS1.3 is developing rapidly, in our observations, TLS1.2 [2] traffic is still the majority of TLS/SSL traffic, accounting for more than 60%. In this paper, we focus on the measurement of client certificates in the existing internet, by extracting useful information from a passive measurement framework deployed on an enterprise gateway. We perform a comprehensive analysis of the information and disclose some noteworthy flaws in the usage of client certificates that may possibly compromise user privacy.