The Signal Protocol is a set of cryptographic specifications that provides end-to-end encryption for private communications. Signal is upgrading the X3DH specification to PQXDH to add a layer of protection against the threat of a future quantum computer that could break current encryption standards.
The new protocol, PQXDH, combines the elliptic curve key agreement protocol X25519 and the post-quantum key encapsulation mechanism CRYSTALS-Kyber to compute a shared secret, ensuring that an attacker must break both systems to access the same shared secret.