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From:   Paul Crowley <paulcrowley@google.com>
Date:   Tue, 19 May 2020 09:33:11 -0700
Message-ID: <CA+_SqcB09GJJoTBm-U7ZwyTjuumyp4QwhLyxj8wbObd47qJOWw@mail.gmail.com>
Subject: Re: [PATCH] fscrypt: add support for IV_INO_LBLK_32 policies
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     linux-fscrypt@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, linux-ext4@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-crypto@vger.kernel.org,
        linux-mmc@vger.kernel.org, "Theodore Y . Ts'o" <tytso@mit.edu>,
        Satya Tangirala <satyat@google.com>,
        Jaegeuk Kim <jaegeuk@kernel.org>
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On Fri, 15 May 2020 at 13:50, Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> The eMMC inline crypto standard will only specify 32 DUN bits (a.k.a. IV
> bits), unlike UFS's 64.  IV_INO_LBLK_64 is therefore not applicable, but
> an encryption format which uses one key per policy and permits the
> moving of encrypted file contents (as f2fs's garbage collector requires)
> is still desirable.
>
> To support such hardware, add a new encryption format IV_INO_LBLK_32
> that makes the best use of the 32 bits: the IV is set to
> 'SipHash-2-4(inode_number) + file_logical_block_number mod 2^32', where
> the SipHash key is derived from the fscrypt master key.  We hash only
> the inode number and not also the block number, because we need to
> maintain contiguity of DUNs to merge bios.

Reviewed-by: Paul Crowley <paulcrowley@google.com>

This is the best that can be done cryptographically on such hardware.