Post-Quantum Extended Diffie–Hellman

In cryptography, Post-Quantum Extended Diffie–Hellman (PQXDH) is a Kyber-based post-quantumkey exchange method based on Diffie–Hellman key exchange. It has been a key part of the Signal Protocol, a popular end-to-end encryption protocol, since 2023.

PQXDH is a variant of the X3DH protocol that uses both the quantum-resistant Kyber protocol as well as the classical elliptic curveX25519 protocol. This ensures that an attacker must break both of the encryption protocols to gain access to sensitive data. The protocol is designed for asynchronous communication where the clients exchange public keys through a server to derive a secure shared key which they can use to encrypt sensitive data without needing to constantly sync new keys with each other.

In October 2023, the protocol underwent formal verification which managed to "prove all the desired security properties of the protocol" for its second revision. However, PQXDH relies entirely on elliptic curve cryptography for authentication.