[noise] KDF, part 9281274

[Jason A. Donenfeld]

Hey Trevor,

I just wanted to have a final pass at this issue, in light of the
other thread on KDF argument order and your requirements for a
dual-PRF.

What precisely would be wrong about greatly simplifying and
speeding-up the whole thing by defining KDF either as:

output = BLAKE2b-PRF(key, input)
return (output[0:31], output[32:63])

or

output = HMAC-SHA2-512(key, input)
return (output[0:31], output[32:63])

With the choice of which one of these two to use being specified in
the cipher suites. These are both solid in the ROM.

What precisely prevents you from using these?

Regards,
Jason