[noise] New branch: psk (for NoisePSK)

[Trevor Perrin]

https://github.com/trevp/noise/tree/psk

This adds the "NoisePSK" idea we discussed.  Setting a PSK triggers an
extra MixKey() and MixHash() during initialization, then adds 32-byte
explicit randoms to the first 2 messages which also trigger MixKey().

One could ask whether we should (a) add explicit randoms to all
messages, or (b) add a NoiseER option so you could use optionally use
explicit randoms without PSK, which would let you reuse your
ephemerals for a time window, instead of having to make up new ones
every handshake so they serve as randomizers.

On (a), I'd rather not burden most Noise protocols with unneeded
explicit randoms and MixKey() steps.

On (b), I think the benefits of reusing ephemerals for a time window
are small.  This would complicate the spec with more options, and
possibly new patterns which would require extended randoms to be safe,
and new ways to shoot yourself in foot.  Seems not worth pursuing at
this point, maybe a v2 feature.


Trevor