[api]

Crypto? You can know who your peer is with a single packet if you've already exchanged keys, and other cleverness is also possible.

[ndesaulniers]

At the cost of perfect forward secrecy, since then you're no longer using ephemeral keys?

[_urga]

QUIC has a mechanism to upgrade to ephemeral keys once the connection has started.

[ndesaulniers]

As will TLS 1.3, starting from page 3: http://www.ietf.org/proceedings/92/slides/slides-92-tls-3.pd...