The PQXDH (Post-Quantum Extended Diffie-Hellman) key agreement protocol is described in this document. PQXDH is designed to establish a shared secret key between two parties who authenticate each other based on public keys. It provides post-quantum forward secrecy and cryptographic deniability. The protocol is suitable for asynchronous settings where one user is offline and has published information to a server. Another user can use this information to send encrypted data to the offline user and establish a shared secret key for future communication. The document also outlines the parameters, roles, and steps involved in the PQXDH protocol.
https://signal.org/docs/specifications/pqxdh/