Re: [PATCH RFC] crypto/hkdf: Fix salt length short issue in FIPS mode

[Eric Biggers]

On Thu, Nov 27, 2025 at 09:24:43AM +0800, Li Tian wrote:
> > Why do you think the salt needs to be at least 32 bytes?
> 
> Forgive my inaccuracy. Under FIPS, salt needs to be at least the hash length
> (32bytes for sha256 and 64bytes for sha512) because NIST requires that the
> HMAC key used in Extract has *full security strength*. 32 is just the
> number I
> tested with.
> 
> Li Tian

It seems that you're confusing the salt with the input keying material.
The entropy for the key comes from the input keying material.  The salt
is a non-secret value that usually is just set to all-zeroes.  In fact,
both users of HKDF in the kernel just set it to all-zeroes.

- Eric