From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48057) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cOTov-0003xX-Mv for qemu-devel@nongnu.org; Tue, 03 Jan 2017 13:28:27 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cOTou-00016W-PK for qemu-devel@nongnu.org; Tue, 03 Jan 2017 13:28:25 -0500 From: "Daniel P. Berrange" Date: Tue, 3 Jan 2017 18:27:49 +0000 Message-Id: <20170103182801.9638-4-berrange@redhat.com> In-Reply-To: <20170103182801.9638-1-berrange@redhat.com> References: <20170103182801.9638-1-berrange@redhat.com> Subject: [Qemu-devel] [PATCH v1 03/15] qcow: document another weakness of qcow AES encryption List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Kevin Wolf , Max Reitz , qemu-block@nongnu.org, "Daniel P. Berrange" 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 --- 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. +@item 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 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are -- 2.9.3