@@ -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