Re: [PATCH v3 0/3] Add file-system authentication to BTRFS

[David Sterba <dsterba@suse.cz>]

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.