What is Signal SPQR / Triple Ratchet?

SPQR is a post-quantum ratchet that Signal layers onto its Double Ratchet, adding a third chain in which key material is refreshed using a lattice-based key encapsulation mechanism so forward secrecy and post-compromise security are intended to hold against a quantum adversary. Because encapsulation keys are far larger than elliptic-curve ones, this chain advances sparsely and splits key material across several messages instead of sending a fresh encapsulation with every one, which is why the combined construction is called the Triple Ratchet.