[Qemu-block] [PATCH v5 03/18] 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.
This fixes the formatting of the itemized list too.

Reviewed-by: Max Reitz <address@hidden>
Reviewed-by: Alberto Garcia <address@hidden>
Signed-off-by: Daniel P. Berrange <address@hidden>
---
 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
address@hidden The AES-CBC cipher is used with predictable initialization 
vectors based
address@hidden
+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.
address@hidden The user passphrase is directly used as the encryption key. A 
poorly
address@hidden
+The user passphrase is directly used as the encryption key. A poorly
 chosen or short passphrase will compromise the security of the encryption.
address@hidden In the event of the passphrase being compromised there is no way 
to
address@hidden
+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.
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