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Diffstat (limited to 'qemu-doc.texi')
| -rw-r--r-- | qemu-doc.texi | 23 |
1 files changed, 20 insertions, 3 deletions
diff --git a/qemu-doc.texi b/qemu-doc.texi index ce61f30d6e..ad31f2d2d0 100644 --- a/qemu-doc.texi +++ b/qemu-doc.texi @@ -547,10 +547,27 @@ File name of a base image (see @option{create} subcommand) @item backing_fmt Image format of the base image @item encryption -If this option is set to @code{on}, the image is encrypted. +If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC. + +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 +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 +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 +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. +@end itemize -Encryption uses the AES format which is very secure (128 bit keys). Use -a long password (16 characters) to get maximum protection. +Use of qcow / qcow2 encryption is thus strongly discouraged. Users are +recommended to use an alternative encryption technology such as the +Linux dm-crypt / LUKS system. @item cluster_size Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster |