Re: SHA-2 based HMAC algorithm...

["denis bider" <d3%denisbider.com@localhost>]

> I was asking for a motivation, since I thought you might
> have had some reason, besides the annoying incompatibility,
> to complain on the existing implementations using 16 byte
> or 20 byte hmac keys for hmac-sha256%ssh.com@localhost). To me it
> would make some sense to stick to a 20 byte (160 bit)
> key for all hmac-sha2 variants.

If we're moving towards a 256-bit hash for key exchange as well as data 
integrity, then the output of the key generation function will be 
256-bit, and we would have to truncate that output if we want the HMAC 
key to be 160 bits. I don't see a compelling reason to not use the whole 
key, given that we're generating it anyway.

That's my response to "why 32-byte key for hmac-sha2-256".

As for whether the key size for hmac-sha2-512 is 32 bytes or 64 bytes, I 
don't have a strong preference either way.

denis