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From: Eric Biggers <ebiggers@kernel.org>
To: linux-block@vger.kernel.org, linux-fscrypt@vger.kernel.org
Cc: linux-arm-msm@vger.kernel.org, kernel-team@android.com,
	Thara Gopinath <thara.gopinath@linaro.org>,
	Gaurav Kashyap <gaurkash@codeaurora.org>,
	Satya Tangirala <satyaprateek2357@gmail.com>
Subject: Re: [RFC PATCH v2 4/5] fscrypt: allow 256-bit master keys with AES-256-XTS
Date: Mon, 20 Sep 2021 20:16:44 -0700	[thread overview]
Message-ID: <YUlOnG4bfj7beDah@sol.localdomain> (raw)
In-Reply-To: <20210916174928.65529-5-ebiggers@kernel.org>

On Thu, Sep 16, 2021 at 10:49:27AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> fscrypt currently requires a 512-bit master key when AES-256-XTS is
> used, since AES-256-XTS keys are 512-bit and fscrypt requires that the
> master key be at least as long any key that will be derived from it.
> 
> However, this is overly strict because AES-256-XTS doesn't actually have
> a 512-bit security strength, but rather 256-bit.  The fact that XTS
> takes twice the expected key size is a quirk of the XTS mode.  It is
> sufficient to use 256 bits of entropy for AES-256-XTS, provided that it
> is first properly expanded into a 512-bit key, which HKDF-SHA512 does.
> 
> Therefore, relax the check of the master key size to use the security
> strength of the derived key rather than the size of the derived key
> (except for v1 encryption policies, which don't use HKDF).
> 
> Besides making things more flexible for userspace, this is needed in
> order for the use of a KDF which only takes a 256-bit key to be
> introduced into the fscrypt key hierarchy.  This will happen with
> hardware-wrapped keys support, as all known hardware which supports that
> feature uses an SP800-108 KDF using AES-256-CMAC, so the wrapped keys
> are wrapped 256-bit AES keys.  Moreover, there is interest in fscrypt
> supporting the same type of AES-256-CMAC based KDF in software as an
> alternative to HKDF-SHA512.  There is no security problem with such
> features, so fix the key length check to work properly with them.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  fs/crypto/fscrypt_private.h |  5 ++--
>  fs/crypto/hkdf.c            | 11 +++++--
>  fs/crypto/keysetup.c        | 57 +++++++++++++++++++++++++++++--------
>  3 files changed, 56 insertions(+), 17 deletions(-)

I've applied this patch to fscrypt.git#master for 5.16, as it's a useful cleanup
which isn't dependent on the hardware-wrapped keys feature.  I also fixed this
patch to update the documentation, which I had overlooked in this case.

- Eric

  parent reply	other threads:[~2021-09-21  3:17 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-16 17:49 [RFC PATCH v2 0/5] Support for hardware-wrapped inline encryption keys Eric Biggers
2021-09-16 17:49 ` [RFC PATCH v2 1/5] block: add basic hardware-wrapped key support Eric Biggers
2021-09-16 17:49 ` [RFC PATCH v2 2/5] block: add ioctls to create and prepare hardware-wrapped keys Eric Biggers
2021-09-16 17:49 ` [RFC PATCH v2 3/5] fscrypt: improve documentation for inline encryption Eric Biggers
2021-09-21  3:13   ` Eric Biggers
2021-09-16 17:49 ` [RFC PATCH v2 4/5] fscrypt: allow 256-bit master keys with AES-256-XTS Eric Biggers
2021-09-17 17:46   ` Paul Crowley
2021-09-21  3:16   ` Eric Biggers [this message]
2021-09-16 17:49 ` [RFC PATCH v2 5/5] fscrypt: add support for hardware-wrapped keys Eric Biggers
2021-09-27 18:12 ` [RFC PATCH v2 0/5] Support for hardware-wrapped inline encryption keys Eric Biggers

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