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: 7+ 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
2025-11-28 23:15 ` kernel test robot
2025-11-29 1:01 ` kernel test robot
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20251127015141.GA29380@sol \
--to=ebiggers@kernel.org \
--cc=davem@davemloft.net \
--cc=herbert@gondor.apana.org.au \
--cc=jaegeuk@kernel.org \
--cc=linux-crypto@vger.kernel.org \
--cc=linux-fscrypt@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=litian@redhat.com \
--cc=tytso@mit.edu \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.