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PQXDH Protocol Overview

The Post-Quantum Extended Diffie-Hellman (PQXDH) protocol is a key agreement protocol that provides post-quantum security for establishing secure communication channels.

What is PQXDH?​

PQXDH extends the traditional Extended Diffie-Hellman (X3DH) protocol with post-quantum cryptographic primitives to ensure security against quantum computer attacks.

Key Components​

Classical ECDH​

  • Uses Curve25519 for backwards compatibility
  • Provides security against classical computers

Post-Quantum KEM​

  • Uses Kyber for quantum-resistant key encapsulation
  • Protects against future quantum attacks

Hybrid Approach​

The protocol combines both classical and post-quantum methods for maximum security.

Security Properties​

  • Forward Secrecy: Past communications remain secure even if long-term keys are compromised
  • Post-Quantum Security: Resistant to attacks by quantum computers
  • Deniability: Participants can deny having participated in conversations
  • Asynchronous: Participants don't need to be online simultaneously

Implementation​

The PQXDH implementation in Zoe Relay provides:

  • Secure key generation and exchange
  • Session establishment for encrypted messaging
  • Ephemeral key management for forward secrecy

For detailed API documentation, see the Rust API docs.