Signal and Messaging Layer Security

Signal and Messaging Layer Security is a comprehensive guide to signal protocol. Used by widely deployed messengers such as Facebook Messenger, WhatsApp, Wire, Viber, and many others, Signal has become the standard for end-to-end encrypted (E2EE) communication. Despite its widespread adoption, the protocol remains largely unexplained in security literature. This book fills that gap by providing a clear, accessible explanation of the Signal protocol for security professionals with a basic understanding of cryptography.

Beyond the Signal protocol, the book also addresses the crucial challenge of scaling E2EE messaging to group communication. While Signal was designed for two-party messaging, extending its security guarantees to large groups is a complex problem. The second focus of this book is the Messaging Layer Security (MLS) protocol, currently being standardized by the Internet Engineering Task Force (IETF.) MLS is designed for large-scale deployments, enabling secure group messaging for thousands of participants.

By exploring both Signal and MLS, this book offers a deep dive into the protocols shaping secure communication today. It provides a clear and comprehensive guide to understanding the core principles behind modern E2EE messaging. Readers will gain an understanding of forward secrecy, post-compromise security, and the intricacies of encrypted messaging in both one-on-one and group settings making it an essential resource for anyone working in the field of secure communications.

Chapter 1 Introduction
1.1 Problem Statement
1.2 Historical Summary
1.3 Book Outline

Chapter 2 Cryptographic Toolbox
2.1 Unkeyed Cryptosystems
2.2 Secret Key Cryptosystems
2.3 Public Key Cryptosystems
2.4 Final Remarks

Chapter 3 Evolution
3.1 Phase 1: First Solutions
3.2 Phase 2: Synchronous Messaging
3.3 Phase 3: Asynchronous Messaging
3.4 Phase 4: Group Messaging
3.5 Final Remarks

Chapter 4 Signal Protocol
4.1 Introduction
4.2 X3DH
4.3 PQXDH
4.4 Double Ratchet Mechanism
4.5 User Authentication
4.6 Security Analysis
4.7 Final Remarks

Chapter 5 Group Communication
5.1 Introduction
5.2 Signal Protocol in Groups
5.3 Sender Keys
5.4 More Efficient Technologies and Protocols
5.5 Final Remarks

Chapter 6 MLS Protocol
6.1 Introduction
6.2 Architectural Setting
6.3 Protocol
6.4 Security Analysis
6.5 Final Remarks

Chapter 7 Further Developments and Improvements
7.1 Multi-Device Support
7.2 Cloud Storage
7.3 Self-Destructing Messages
7.4 Private Groups
7.5 Message Franking
7.6 Meta-Data Hiding

Chapter 8 Conclusions and Outlook

Chapter A ECC

Chapter B Quantum Computers and PQC
B.1 Quantum Computers
B.2 PQC

Chapter C Mathematical Notation

Chapter D Abbreviations and Acronyms

About the Author