Authentication and Key Management Automation in Decentralized Secure Email and Messaging via Low-Entropy Secrets

TL;DR

This paper introduces a practical cryptographic method using password-authenticated key exchange (PAKE) for secure, automated, and user-friendly entity authentication in decentralized encrypted email and messaging systems, eliminating the need for public key infrastructure.

Contribution

It presents a novel PAKE-based solution for entity authentication that supports automation, enhances security, and is suitable for asynchronous communication without relying on trusted third parties.

Findings

Identified vulnerabilities in voice-based out-of-band authentication.

Proposed PAKE-based secure equality test for entity authentication.

Enables features like automated key renewal, multi-device sync, and post-quantum security.

Abstract

We revisit the problem of entity authentication in decentralized end-to-end encrypted email and secure messaging to propose a practical and self-sustaining cryptographic solution based on password-authenticated key exchange (PAKE). This not only allows users to authenticate each other via shared low-entropy secrets, e.g., memorable words, without a public key infrastructure or a trusted third party, but it also paves the way for automation and a series of cryptographic enhancements; improves security by minimizing the impact of human error and potentially improves usability. First, we study a few vulnerabilities in voice-based out-of-band authentication, in particular a combinatorial attack against lazy users, which we analyze in the context of a secure email solution. Next, we propose solving the problem of secure equality test using PAKE to achieve entity authentication and to establish a shared high-entropy secret key. Our solution lends itself to offline settings, compatible with the inherently asynchronous nature of email and modern messaging systems. The suggested approach enables enhancements in key management such as automated key renewal and future key pair authentications, multi-device synchronization, secure secret storage and retrieval, and the possibility of post-quantum security as well as facilitating forward secrecy and deniability in a primarily symmetric-key setting. We also discuss the use of auditable PAKEs for mitigating a class of online guess and abort attacks in authentication protocols.