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Date:   Tue, 26 Jul 2022 19:45:38 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Cc:     Eric Biggers <ebiggers@kernel.org>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        linux-fscrypt@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-btrfs@vger.kernel.org, osandov@osandov.com,
        kernel-team@fb.com
Subject: Re: [PATCH RFC 4/4] fscrypt: Add new encryption policy for btrfs.
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On Mon, Jul 25, 2022 at 10:16:07PM -0400, Sweet Tea Dorminy wrote:
> On 7/25/22 19:32, Eric Biggers wrote:
> > On Sat, Jul 23, 2022 at 08:52:28PM -0400, Sweet Tea Dorminy wrote:

> > Given that this new proposal uses per-block metadata, has
> > support for authenticated encryption been considered? Has space been reserved
> > in the per-block metadata for authentication tags so that authenticated
> > encryption support could be added later even if it's not in the initial version?
> 
> I don't know sufficiently much about authenticated encryption to have 
> considered it. As currently drafted, btrfs encrypts before checksumming 
> if checksums are enabled, and checks against checksums before 
> decrypting. Although at present we haven't discussed authentication 
> tags, btrfs could store them in a separate itemtype which could be added 
> at any time, much as we currently store fsverity data. We do have 
> sufficient room saved for adding other encryption types, if necessary; 
> we could use some of that to indicate the existence of authentication 
> tags for the extents' data.

The AEAD tag can be used in place of checksum (also stored in the
checksum item).