From 136cd19d0522c03b6dccc3e344886feab6faee43 Mon Sep 17 00:00:00 2001 From: "Daniel P. Berrange" Date: Wed, 22 Jan 2014 15:47:10 +0000 Subject: 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 Reviewed-by: Markus Armbruster Reviewed-by: Eric Blake Signed-off-by: Stefan Hajnoczi --- qemu-img.texi | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) (limited to 'qemu-img.texi') 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 -- cgit v1.2.3