What is Signal PQXDH (Post-Quantum Extended Diffie-Hellman)?

Private messaging relies on public-key cryptography, where a person publishes a public key that anyone can use to encrypt messages to them. Most public-key algorithms in use today rest on maths problems that a sufficiently powerful quantum computer could easily solve, so cryptographers are developing post-quantum algorithms intended to resist such machines. PQXDH is the key-agreement step of the Signal Protocol, the end-to-end encryption design used by Signal and other messaging apps, which sets up a shared secret between two parties. It combines the classic X25519 exchange with the post-quantum CRYSTALS-Kyber mechanism, so an attacker must break both. Signal announced it on 19 Sep 2023 as an upgrade to its earlier X3DH specification.

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