[Cryptography] Why is a short HMAC key zero-padded instead of hashed?

[Ron Garret]

On Feb 2, 2017, at 2:10 PM, Francisco Corella <fcorella at pomcor.com> wrote:

> Please keep in mind that HMAC is also standardized in NIST FIPS 198-1.
> Let's not introduce unnecessary confusion by putting standards out of
> sync.  I don't see any reason for changing HMAC.  Regarding the
> message that started this thread, there is a clear reason for padding
> the key with zeros when shorter than the block size rather than
> hashing it: hashing has a substantial computational cost.  Supposedly
> "simplifying the code" is not an argument for introducing an
> unnecessary hash.

If that’s your quality metric, why hash a long key then instead of just (say) truncating it?

rg

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