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

[Max Reitz]

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?

Max

>  
>  Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
>

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