International Association
for Cryptologic Research

In this paper we consider TLS MITM attacks in the context of web applications, where the attacker\'s goal is to impersonate the user to the legitimate server, and thus gain access to the user\'s online account. We describe in detail why the recently proposed TLS Channel ID-based client authentication, as well as client web authentication in general, cannot fully prevent such attacks.

We then leverage TLS Channel ID-based authentication and combine it with the concept of sender invariance to create a novel mechanism that we call SISCA: Server Invariance with Strong Client Authentication. SISCA resists user impersonation via TLS MITM attacks even if the attacker has obtained the private key of the legitimate server. We analyze our proposal and show how it can be integrated in today\'s web infrastructure.