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

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 26/05/2020 13:54, David Sterba wrote:
> 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.

I'm not enough a security expert to be able to judge this. Eric can I hear 
your opinion on this?

Thanks,
	Johannes