[noise] NoisePSK

[Trevor Perrin]

On Thu, Nov 12, 2015 at 12:12 AM, Trevor Perrin <trevp at trevp.net> wrote:
>
> One idea is to have "NoisePSK" handshakes which differ from "Noise"
> handshakes in two ways:
>  (1) A "preshared_key" value is mixed into SymmetricState, e.g.
>
> Initialize():
>     # handle handshake_name = NoisePSK_[???], prologue
>     psk1, psk2 = HKDF(preshared_key, empty)
>     MixKey(psk1)
>     MixHash(psk2)

To optimize, we can take advantage of the fact that MixKey() calls
without intervening encryption means k isn't used:
  MixKey(psk1)
  ...
  MixKey(explicit_random)

So we can repurpose k, eliminating the extra HKDF():

Initialize():
  # handle handshake_name = NoisePSK_[???], prologue
  MixKey(preshared_key)
  MixHash(k)

Seems worth trying out in a branch.  Note nothing here effects
existing Noise protocols, this is just adding a variation for PSK.

Trevor