Re: [Qemu-block] [PATCH v1 03/15] qcow: document another weakness of qcow AES encryption

[Daniel P. Berrange]

On Mon, Jan 16, 2017 at 08:37:57PM +0100, Max Reitz wrote:
> On 03.01.2017 19:27, Daniel P. Berrange wrote:
> > Document that use of guest virtual sector numbers as the basis for
> > the initialization vectors is a potential weakness, when combined
> > with internal snapshots or multiple images using the same passphrase.
> > 
> > Signed-off-by: Daniel P. Berrange <address@hidden>
> > ---
> >  qemu-img.texi | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/qemu-img.texi b/qemu-img.texi
> > index 174aae3..8efcf89 100644
> > --- a/qemu-img.texi
> > +++ b/qemu-img.texi
> > @@ -554,6 +554,15 @@ change the passphrase to protect data in any qcow 
> > images. The files must
> >  be cloned, using a different encryption passphrase in the new file. The
> >  original file must then be securely erased using a program like shred,
> >  though even this is ineffective with many modern storage technologies.
> > address@hidden Initialization vectors used to encrypt sectors are based on 
> > the
> > +guest virtual sector number, instead of the host physical sector. When
> > +a disk image has multiple internal snapshots this means that data in
> > +multiple physical sectors is encrypted with the same initialization
> > +vector. With the CBC mode, this opens the possibility of watermarking
> > +attacks if the attack can collect multiple sectors encrypted with the
> > +same IV and some predictable data. Having multiple qcow2 images with
> > +the same passphrase also exposes this weakness since the passphrase
> > +is directly used as the key.
> >  @end itemize
> 
> In the output manpage, this itemize looks pretty broken to me:
> 
> @item foo
> bar baz
> 
> is formatted as:
> 
> -<foo>
>     bar baz
> 
> Which may be used intentionally, but it certainly isn't here.
> 
> It should probably be written as:
> 
> @item
> foo bar baz
> 
> which becomes
> 
> -    foo bar baz
> 
> (which is what the other itemize in qemu-img.texi does)
> 
> Do you want to fix that in this series?

Yes, will do.

Regards,
Daniel
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