SecureJoin: Protecting chat messaging against network adversaries

Important

This document is severely outdated.

Delta Chat still uses the basic protocol described here, but no longer to verify the binding between an e-mail address and a key. The Delta Chat V2 release of July 2025 introduced key contacts, where a contact is a key, so there is no binding left to verify.

SecureJoin protocols provide a usable model for message end-to-end encryption which is secure against attackers trying to break authenticity, confidentiality or integrity of messages as can occur with compromised servers and networks. They are implemented, user-tested and continuously refined in production-releases of the cross-platform Delta Chat messenger. Other messenger implementors as well as researchers are welcome to submit remarks, questions or critique either through github or by contacting Delta Chat teams.

Today’s SecureJoin protocols are derived from respective key parts of CounterMitm protocols which were produced by researchers of the NEXTLEAP EU project (2016-2018) to explore decentralized messaging and privacy topics.

• 1. Introduction
  • 1.1. Attack model and terminology
  • 1.2. Disadvantages of other key-verification techniques
  • 1.3. Integrating key verification with general workflows
  • 1.4. Known Limitations and Issues
• 2. Securing communications against network adversaries
  • 2.1. Setup Contact protocol
  • 2.2. Verified Group protocol