Re: [Qemu-devel] [PATCH v3] Describe flaws in qcow/qcow2 encryption in the docs

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Jan 22, 2014 at 03:47:10PM +0000, Daniel P. Berrange wrote:
> The qemu-img.texi / qemu-doc.texi files currently describe the
> qcow2/qcow2 encryption thus
> 
>   "Encryption uses the AES format which is very secure (128 bit
>    keys). Use a long password (16 characters) to get maximum
>    protection."
> 
> While AES is indeed a strong encryption system, the way that
> QCow/QCow2 use it results in a poor/weak encryption system.
> Due to the use of predictable IVs, based on the sector number
> extended to 128 bits, it is vulnerable to chosen plaintext
> attacks which can reveal the existence of encrypted data.
> 
> The direct use of the user passphrase as the encryption key
> also leads to an inability to change the passphrase of an
> image. If passphrase is ever compromised the image data will
> all be vulnerable, since it cannot be re-encrypted. The admin
> has to clone the image files with a new passphrase and then
> use a program like shred to secure erase all the old files.
> 
> Recommend against any use of QCow/QCow2 encryption, directing
> users to dm-crypt / LUKS which can meet modern cryptography
> best practices.
> 
> Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
> ---
>  qemu-doc.texi | 23 ++++++++++++++++++++---
>  qemu-img.texi | 23 ++++++++++++++++++++---
>  2 files changed, 40 insertions(+), 6 deletions(-)
> 
> Changed in v3:
> 
>   - Addressed feedback/typos from Eric & Markus
> 
> In v2:
> 
>   - Addressed typos reported by Peter
> 
> Still welcome info about any other flaws qcow2 has in this
> area that should be documented.

Changed "Qcow" to "qcow" for consistency.

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan