Tiny WireGuard Tweak

We show that a future adversary with access to a quantum computer, historic network traffic protected by WireGuard, and knowledge of a WireGuard user's long-term static public key can likely decrypt many of the WireGuard user's historic messages. We propose a simple, efficient alteration to the WireGuard protocol that mitigates this vulnerability, with negligible additional computational and memory costs. Our changes add zero additional bytes of data to the wire format of the WireGuard protocol. Our alteration provides transitional post-quantum security for any WireGuard user who does not publish their long-term static public key -- it should be exchanged out-of-band.