From: David Sterba <dsterba@suse.cz>
To: Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
Cc: "dsterba@suse.cz" <dsterba@suse.cz>,
Johannes Thumshirn <jth@kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
Eric Biggers <ebiggers@google.com>,
Richard Weinberger <richard@nod.at>
Subject: Re: [PATCH v3 0/3] Add file-system authentication to BTRFS
Date: Tue, 26 May 2020 13:53:00 +0200 [thread overview]
Message-ID: <20200526115300.GY18421@twin.jikos.cz> (raw)
In-Reply-To: <SN4PR0401MB35986E7B3F88F1EB288051A79BB00@SN4PR0401MB3598.namprd04.prod.outlook.com>
On Tue, May 26, 2020 at 07:50:53AM +0000, Johannes Thumshirn wrote:
> On 25/05/2020 15:11, David Sterba wrote:
> > On Thu, May 14, 2020 at 11:24:12AM +0200, Johannes Thumshirn wrote:
> > As mentioned in the discussion under LWN article, https://lwn.net/Articles/818842/
> > ZFS implements split hash where one half is (partial) authenticated hash
> > and the other half is a checksum. This allows to have at least some sort
> > of verification when the auth key is not available. This applies to the
> > fixed size checksum area of metadata blocks, for data we can afford to
> > store both hashes in full.
> >
> > I like this idea, however it brings interesting design decisions, "what
> > if" and corner cases:
> >
> > - what hashes to use for the plain checksum, and thus what's the split
> > - what if one hash matches and the other not
> > - increased checksum calculation time due to doubled block read
> > - whether to store the same parital hash+checksum for data too
> >
> > As the authenticated hash is the main usecase, I'd reserve most of the
> > 32 byte buffer to it and use a weak hash for checksum: 24 bytes for HMAC
> > and 8 bytes for checksum. As an example: sha256+xxhash or
> > blake2b+xxhash.
> >
> > I'd outright skip crc32c for the checksum so we have only small number
> > of authenticated checksums and avoid too many options, eg.
> > hmac-sha256-crc32c etc. The result will be still 2 authenticated hashes
> > with the added checksum hardcoded to xxhash.
>
> Hmm I'm really not a fan of this. We would have to use something like
> sha2-224 to get the room for the 2nd checksum. So we're using a weaker
> hash just so we can add a second checksum.
The idea is to calculate full hash (32 bytes) and store only the part
(24 bytes). Yes this means there's some information loss and weakening,
but enables a usecase.
> On the other hand you've asked
> me to add the known pieces of information into the hashes as a salt to
> "make attacks harder at a small cost".
Yes and this makes it harder to attack the hash, it should be there
regardless of the additional checksums.
next prev parent reply other threads:[~2020-05-26 11:54 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-14 9:24 [PATCH v3 0/3] Add file-system authentication to BTRFS Johannes Thumshirn
2020-05-14 9:24 ` [PATCH v3 1/3] btrfs: rename btrfs_parse_device_options back to btrfs_parse_early_options Johannes Thumshirn
2020-05-14 9:24 ` [PATCH v3 2/3] btrfs: add authentication support Johannes Thumshirn
2020-05-27 13:24 ` David Sterba
2020-05-27 13:54 ` Johannes Thumshirn
2020-05-27 14:01 ` Johannes Thumshirn
2020-05-27 18:04 ` Johannes Thumshirn
2020-06-01 14:30 ` David Sterba
2020-06-01 14:35 ` David Sterba
2020-05-14 9:24 ` [PATCH v3 3/3] btrfs: document btrfs authentication Johannes Thumshirn
2020-05-14 12:26 ` Jonathan Corbet
2020-05-14 14:54 ` Johannes Thumshirn
2020-05-14 15:14 ` Richard Weinberger
2020-05-14 16:00 ` Jonathan Corbet
2020-05-14 16:05 ` Richard Weinberger
2020-05-24 19:55 ` David Sterba
2020-05-25 10:57 ` Johannes Thumshirn
2020-05-25 11:26 ` David Sterba
2020-05-25 11:44 ` Johannes Thumshirn
2020-05-25 13:10 ` [PATCH v3 0/3] Add file-system authentication to BTRFS David Sterba
2020-05-26 7:50 ` Johannes Thumshirn
2020-05-26 11:53 ` David Sterba [this message]
2020-05-26 12:44 ` Johannes Thumshirn
2020-06-01 14:59 ` David Sterba
2020-05-27 2:08 ` Qu Wenruo
2020-05-27 11:27 ` David Sterba
2020-05-27 11:58 ` Qu Wenruo
2020-05-27 13:11 ` David Sterba
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