This commit introduces Brontide: an authenticated key agreement
protocol in three acts. Brontide is the successor to lndc within lnd,
and ultimately within the greater Lighting Network. Brontide uses the
Noise_XK handshake for initial key agreement, then implements an AEAD
scheme which encrypts+authenticates both packets, and the lengths of
the packets on the wire. The initial authentication handshake preserves
the responder’s identity by never transmitting it to the initiator and
performing mutual authentication via an incremental Triple-DH based on
ECDH of secp256k1 and an HKDF which uses SHA-256.
Bronzed isn’t yet integrated within the wider daemon yet. Full
integration will land in a future pull request.
