[noise] hkdf branch with arbitrary-length keyed hashing functions

[Jason A. Donenfeld]

Hi,

Here's a more general question about using blake2b with noise than my
previous email.

Functions like blake2b(key, data) can produce an arbitrary length
output: blake2b(key, data, size). So, I find it appealing to replace:

    HKDF(chaining_key, data):
        temp_key = HMAC-HASH(chaining_key, data)
        output1 = HMAC-HASH(temp_key, 0x01)
        output2 = HMAC-HASH(temp_key, output1 || 0x02)
        return (output1, output2)

With:

    Blake2b-KDF(chaining_key, data):
        key_material = Blake2b(chaining_key, data, 64 bytes)
        return (key_material[0:31], key_material[32:63])

Any objections or security considerations with regards to this?

Thanks,
Jason