aboutsummaryrefslogtreecommitdiff
path: root/qemu-img.texi
diff options
context:
space:
mode:
authorDaniel P. Berrange <berrange@redhat.com>2017-06-23 17:24:02 +0100
committerMax Reitz <mreitz@redhat.com>2017-07-11 17:44:55 +0200
commit0b4ee9090e278a46b00b21624ba610552d0106d8 (patch)
tree16f3b53bbd39ec919fed11a1066ce2975ca157bd /qemu-img.texi
parent4a47f85431aa6bed22157f683196affe8b06c69e (diff)
downloadqemu-0b4ee9090e278a46b00b21624ba610552d0106d8.zip
qemu-0b4ee9090e278a46b00b21624ba610552d0106d8.tar.gz
qemu-0b4ee9090e278a46b00b21624ba610552d0106d8.tar.bz2
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: Max Reitz <mreitz@redhat.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Signed-off-by: Daniel P. Berrange <berrange@redhat.com> Message-id: 20170623162419.26068-4-berrange@redhat.com Signed-off-by: Max Reitz <mreitz@redhat.com>
Diffstat (limited to 'qemu-img.texi')
-rw-r--r--qemu-img.texi19
1 files changed, 16 insertions, 3 deletions
diff --git a/qemu-img.texi b/qemu-img.texi
index 5b925ec..f335139 100644
--- a/qemu-img.texi
+++ b/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