qcow: document another weakness of qcow AES encryption

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: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-4-berrange@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

qemu-img.texi

@@ -567,16 +567,29 @@ 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