A Symbolic Analysis of Privacy for TLS 1.3 with Encrypted Client Hello

TLS 1.3, the newest version of the Transport Layer Security (TLS) protocol, provides strong authentication and confidentiality guarantees that have been comprehensively analyzed in a variety of formal models. However, despite its controversial use of handshake meta-data encryption, the privacy guarantees of TLS 1.3 remain weak and poorly understood. For example, the protocol reveals the identity of the target server to network attackers, allowing the passive surveillance and active censorship of TLS connections. To close this gap, the IETF TLS working group is standardizing a new privacy extension called Encrypted Client Hello (ECH, previously called ESNI), but the absence of a formal privacy model makes it hard to verify that this extension works. Indeed, several early drafts of ECH were found to be vulnerable to active network attacks.