Measurement techniques to understand how diversity in TLS implementations & deployments influences protocol security

Abstract

TLS is a fundamental and widely-used network security protocol. On one hand, the protocol has undergone rigorous development over the past 25 years and offers sophisticated theoretical guarantees. At the same time, its adoption has grown from traditional computers to handheld devices and IoT ones, with these settings presenting varying constraints and caveats. As a consequence, a large number of TLS implementations and deployments exist and cater to different needs. Unfortunately, this results in a gap between what the protocol offers in theory vs how it works in practice; the diversity in the ecosystem not only increases the probability of a mistake during protocol development and use, but also leads to customizations with unexpected side effects.The thesis of this dissertation is that the rich diversity in TLS implementations & deployments introduces opportunities to harm protocol security, and that the harms can be identified (and mitigated) using rigorous measurement techniques.My work sheds light on previously unexplored aspects of TLS deployment in three different settings; web, mobile and IoT devices. More specifically, I (a) study web content availability and consistency over HTTP/S to better understand the obstacles to a TLS-by-default web, (b) conduct longitudinal experiments on a large number of consumer IoT devices to evaluate TLS effectiveness in that setting, and (c) revisit certificate pinning policies in mobile applications to examine implementations with advanced network security techniques that go beyond what the protocol offers.In addition to exploring diversity in deployments, my work leverages the diversity in TLS implementations alongside recent advances in generative language models to automate bug discovery. More specifically, I present a novel approach of generating synthetic TLS certificates using language models that reveal a wide range of previously unobserved and interesting implementation differences with security implications.My work has led to vulnerability disclosures, a security feature at a major CDN provider, a presentation at an IRTF body to inform protocol engineering, and novel auditing techniques that enable greater transparency about real-world protocol effectiveness. I believe the insights from my work can assist in better modeling of software security beyond TLS, the techniques proposed push state-of-the-art for network measurement, and the use of language models to generate synthetic test cases can prove valuable in domains where software inputs can be expressed in natural language.