[noise] post-handshake authentication

[David Wong]

Hello hello,

I've been thinking about doing a post-handshake authentication pattern
via Noise_NN, but the only API I have is GetHandshakeHash() which
gives out something based on `h` and not on `ck`. Ideally I would want
both endpoints to have an exportFingerprint() function that would
generate a different fingerprint based on the initiator or the
responder. Then the other peer would "activate" the CipherStates
created by Split() by entering that fingerprint (obtained out-of-band)
in authenticateSession() or something.

I want different fingerprints per endpoint because I do not want a
peer to export the secret himself and then active the session by using
the fingerprint HE generated.

Additionally, has anyone thought about integrating a
short-authentication-string protocol with Noise? Sometimes it's not
practical to share a "long" fingerprint out of band (e.g. embedded
device flashes a number of leds as the SAS)

Cheers,
David