path: root/qemu-img.texi
diff options
authorDaniel P. Berrange <berrange@redhat.com>2014-01-22 15:47:10 +0000
committerStefan Hajnoczi <stefanha@redhat.com>2014-01-31 22:05:03 +0100
commit136cd19d0522c03b6dccc3e344886feab6faee43 (patch)
treee18069cca7e4a0616a712478ed0c49059e16326a /qemu-img.texi
parent89e4a51ca9546a7bbe1998c4e3d4a3ac3a0c19be (diff)
Describe flaws in qcow/qcow2 encryption in the docs
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. [Changed "Qcow" to "qcow" for consistency. --Stefan] Signed-off-by: Daniel P. Berrange <berrange@redhat.com> Reviewed-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'qemu-img.texi')
1 files changed, 20 insertions, 3 deletions
diff --git a/qemu-img.texi b/qemu-img.texi
index 526d56a458..f84590ebf0 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -409,10 +409,27 @@ File name of a base image (see @option{create} subcommand)
@item backing_fmt
Image format of the base image
@item encryption
-If this option is set to @code{on}, the image is encrypted.
+If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC.
-Encryption uses the AES format which is very secure (128 bit keys). Use
-a long password (16 characters) to get maximum protection.
+The use of encryption in qcow and qcow2 images is considered to be flawed by
+modern cryptography standards, suffering from a number of design problems:
+@itemize @minus
+@item The AES-CBC cipher is used with predictable initialization vectors based
+on the sector number. This makes it vulnerable to chosen plaintext attacks
+which can reveal the existence of encrypted data.
+@item The user passphrase is directly used as the encryption key. A poorly
+chosen or short passphrase will compromise the security of the encryption.
+@item In the event of the passphrase being compromised there is no way to
+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.
+@end itemize
+Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
+recommended to use an alternative encryption technology such as the
+Linux dm-crypt / LUKS system.
@item cluster_size
Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster