[Qemu-block] [PATCH v1 03/15] qcow: document another weakness of qcow AES encryption

[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 <address@hidden>
---
 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.
address@hidden 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