From: Eric Biggers <ebiggers@kernel.org>
To: Li Tian <litian@redhat.com>
Cc: linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-fscrypt@vger.kernel.org,
Herbert Xu <herbert@gondor.apana.org.au>,
"David S . Miller" <davem@davemloft.net>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Jaegeuk Kim <jaegeuk@kernel.org>
Subject: Re: [PATCH RFC] crypto/hkdf: Fix salt length short issue in FIPS mode
Date: Wed, 26 Nov 2025 17:51:41 -0800 [thread overview]
Message-ID: <20251127015141.GA29380@sol> (raw)
In-Reply-To: <CAHhBTWsTqP3LzJV+=_usvttJcMFoLYSY5Sqt2H-U-oki3Hu0Mw@mail.gmail.com>
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
next prev parent reply other threads:[~2025-11-27 1:53 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-11-26 13:42 [PATCH RFC] crypto/hkdf: Fix salt length short issue in FIPS mode Li Tian
2025-11-26 17:41 ` Eric Biggers
[not found] ` <CAHhBTWuOy1nC1rYqye8BzE+unoC+3M9Dsw+Mj54=3eeFwqyTXw@mail.gmail.com>
2025-11-27 1:14 ` Eric Biggers
[not found] ` <CAHhBTWsTqP3LzJV+=_usvttJcMFoLYSY5Sqt2H-U-oki3Hu0Mw@mail.gmail.com>
2025-11-27 1:51 ` Eric Biggers [this message]
[not found] ` <CAHhBTWs6rWq2huD8Ech79OVOxK3v3ijU3KFFOGLQ+pr7277Vew@mail.gmail.com>
2025-11-27 3:23 ` Eric Biggers
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