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. This fixes the formatting of the itemized list too.
Signed-off-by: Daniel P. Berrange <berra...@redhat.com> --- qemu-img.texi | 19 ++++++++++++++++--- 1 file changed, 16 insertions(+), 3 deletions(-) diff --git a/qemu-img.texi b/qemu-img.texi index 174aae3..db4534b 100644 --- a/qemu-img.texi +++ b/qemu-img.texi @@ -544,16 +544,29 @@ 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 +@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 +@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 +@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. +@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
