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From: Markus Armbruster <armbru@redhat.com>
References: <1390393704-16067-1-git-send-email-berrange@redhat.com>
Date: Wed, 22 Jan 2014 16:32:11 +0100
In-Reply-To: <1390393704-16067-1-git-send-email-berrange@redhat.com> (Daniel
	P. Berrange's message of "Wed, 22 Jan 2014 12:28:24 +0000")
Message-ID: <8761pcxcas.fsf@blackfin.pond.sub.org>
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Subject: Re: [Qemu-devel] [PATCH v2] Describe flaws in qcow/qcow2 encryption
	in the docs
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, Peter Maydell <peter.maydell@linaro.org>

"Daniel P. Berrange" <berrange@redhat.com> writes:

> 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

Sector number zero-extended to 128 bits.

>                                   it is vulnerable to chosen
> plaintext attacks which can reveal the existance 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>

Amateur crypto.

Reviewed-by: Markus Armbruster <armbru@redhat.com>