[noise] Wrong arguments for KDF in PSK mode

[Jason A. Donenfeld]

Hi Trevor,

In PSK mode, for NoiseIK (and similar), the first two calculations
wind up being:

ck, temp = KDF("NoisePSK_IK_blablabla", psk)
h = HASH(HASH(HASH("NoisePSK_IK_blablabla" || prologue) || temp) || static_pub)

The first line is somewhat problematic, because psk -- a key value --
is used as the input parameter, and not as the key parameter. These
arguments should be reversed:

ck, temp = KDF(psk, "NoisePSK_IK_blablabla")

That way, the psk is the key value. Some KDFs have issues when the key
parameter is made public.


Thanks,
Jason