Paper 2024/1938

A Formal Treatment of Key Transparency Systems with Scalability Improvements

Abstract

Key Transparency (KT) systems have emerged as a critical technology for securely distributing and verifying the correctness of public keys used in end-to-end encrypted messaging services. Despite substantial academic interest, increased industry adoption, and IETF standardization efforts, KT systems lack a holistic and formalized security model, limiting their resilience to practical threats and constraining future development. In this paper, we introduce the first cryptographically sound formalization of KT as an ideal functionality, clarifying the assumptions, security properties, and potential vulnerabilities of deployed KT systems. We identify a significant security concern — a possible impersonation attack by a malicious service provider — and propose a backward-compatible solution. Additionally, we address a core scalability bottleneck by designing and implementing a novel, privacy-preserving verifiable Bloom filter (VBF) that significantly improves KT efficiency without compromising security. Experimental results demonstrate the effectiveness of our approach, marking a step forward in both the theoretical and practical deployment of scalable KT solutions.