[Whonix-devel] [qubes-devel] How secure is Qubes dom0 backup tool encryption?

Mon May 8 00:52:42 CEST 2017

On 05/07/2017 06:43 PM, Peter Todd wrote:
> On Sun, May 07, 2017 at 12:49:06PM -0500, Andrew David Wong wrote:
>> They're not mutually exclusive. You can do both.
>>
>> I'm the one who reported the key derivation issue [1], but even I
>> think qvm-backup is plenty safe as long as you use a high-entropy
>> passphrase. (This will no longer be an issue when we switch to scrypt
>> in 4.0. [1]) I personally rely on it for my most confidential data,
>> and I'm confident that it's not the weakest link in my setup.
>
> FWIW, personally while I frequently use qvm-backup, I always use the password
> "a", and instead backup to LUKS-encrypted partitions formatted with BTRFS (for
> crappy authentication via BTRFS's checksums).
>
> I already rely on LUKS, so I don't see any reason to add another potential
> vulnerability to my setup. For my usage pattern, I'd actually prefer an option
> to completely remove both encryption and authentication to reduce CPU usage
> during backups. Based on CPU load, this appears to be the bottleneck on many of
> my machines (though this could be parallelized).
>

I believe the largest qvm-backup bottlenecks to be related to disk I/O. 
For one, qb writes all data to a temporary file before sending it to the 
destination. Second, it seems to inefficiently read all parts of a 
sparse image file (although it does pack the file efficiently on the 
destination) so a 40GB image with only 8GB used will take a long time to 
process in relation to the space used -- these are the parts where the 
CPU is busy.

Encryption should add very little to the backup overhead.

-- 

Chris Laprise, tasket at openmailbox.org
https://twitter.com/ttaskett
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