Signal Protocol

• Signal Protocol - Open protocol providing end-to-end encryption with forward secrecy and post-compromise security
• libsignal - Official Signal Protocol library (Java, C, Rust)
• Double Ratchet Specification - Signal’s core key derivation and message encryption algorithm
• X3DH Key Agreement - Extended Triple Diffie-Hellman for establishing shared secrets
• x3dh-ratchet - Crypto primitives based on the Signal Protocol for secure async messaging
• 2key-ratchet - Double Ratchet and X3DH implementation in JavaScript
• Signal SPQR Post-Quantum Ratchet - Sparse Post-Quantum Ratchet (SPQR) integrating ML-KEM (2025)

Matrix Protocol

• vodozemac - Pure Rust implementation of Matrix Olm and Megolm cryptographic ratchets
• Matrix.org - Open protocol for decentralized, federated real-time communication
• Olm - Double Ratchet implementation for 1:1 Matrix encryption
• Megolm - Sender-key ratchet for efficient group encryption in Matrix
• Matrix E2EE Implementation Guide - Official guide to Matrix’s end-to-end encryption architecture
• Element - Flagship Matrix client for web, desktop, and mobile

Messaging Layer Security (MLS)

• OpenMLS - Rust implementation of the RFC 9420 Messaging Layer Security protocol
• Wire Core Crypto - MLS-based end-to-end encryption library used by Wire clients
• RFC 9420: MLS Protocol - IETF standard for efficient group end-to-end encryption
• MLS Architecture - Architecture document for MLS deployment
• TreeKEM - External-Operations TreeKEM and the security of MLS in RFC 9420
• MLS RFC Overview - Detailed walkthrough of RFC 9420 design and security properties

Metadata-Resistant Messaging

• SimpleX Chat - First messaging network operating without user identifiers of any kind
• SimpleX Messaging Protocol - Transport-agnostic protocol for distributed secure unidirectional messaging
• SimpleX v6.4.1 - July 2025 release with contact management and group protection features
• SimpleX Security Audit - Trail of Bits cryptographic design review for v6.1
• Cwtch - Decentralized, metadata-resistant messaging over Tor v3 onion services
• Cwtch Implementation - Official peer implementation for asynchronous, metadata-resistant Cwtch applications
• Briar - Peer-to-peer encrypted messaging via Tor, Bluetooth, and WiFi
• Briar Desktop 0.6.0-beta - Beta desktop client adding blogs support to secure P2P messaging
• Briar 1.5.17 - March 2026 Android release with improved connectivity
• Quiet - Private peer-to-peer team chat with no central servers, built on Tor and IPFS-style syncing
• Session - Decentralized private messenger using onion-routed service nodes and account IDs instead of phone numbers
• Session Desktop - Decentralized onion-routed messenger using service nodes for offline storage and IP protection

Modern Encryption Tools

• age - Simple, modern file encryption tool (Go) — a GPG replacement
• minisign - Minimalistic tool for signing files and verifying signatures
• OpenPGP.js - OpenPGP implementation in JavaScript (maintained by Proton Mail)
• Sequoia PGP - Modern OpenPGP implementation in Rust
• Gopenpgp - Go wrapper around GPG for high-performance OpenPGP operations

Privacy Infrastructure

• Oblivious HTTP (OHTTP) - IETF protocol for anonymous HTTP transactions via relay
• Oblivious DoH (ODoH) - DNS resolution via oblivious relay to hide client identity from resolver
• RFC 9458: OHTTP - IETF standard for Oblivious HTTP