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

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Message-ID: <20251127015141.GA29380@sol>
Date: Wed, 26 Nov 2025 17:51:41 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: Li Tian <litian@...hat.com>
Cc: linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fscrypt@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>,
	"Theodore Y . Ts'o" <tytso@....edu>,
	Jaegeuk Kim <jaegeuk@...nel.org>
Subject: Re: [PATCH RFC] crypto/hkdf: Fix salt length short issue in FIPS mode

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