libvirt/docs/formatstorageencryption.rst

.. role:: since

====================================
Storage volume encryption XML format
====================================

.. contents::

Storage volume encryption XML
-----------------------------

Storage volumes may be encrypted, the XML snippet described below is used to
represent the details of the encryption. It can be used as a part of a domain or
storage configuration.

The top-level tag of volume encryption specification is ``encryption``, with a
mandatory attribute ``format``. Currently defined values of ``format`` are
``default``, ``qcow``, ``luks``, and ``luks2``. Each value of ``format`` implies
some expectations about the content of the ``encryption`` tag. Other format
values may be defined in the future.

The ``encryption`` tag supports an optional ``engine`` tag, which allows
selecting which component actually handles the encryption. Currently defined
values of ``engine`` are ``qemu`` and ``librbd``. Both ``qemu`` and ``librbd``
require using the qemu driver. The ``librbd`` engine requires qemu version >=
6.1.0, both ceph cluster and librbd1 >= 16.1.0, and is only applicable for RBD
network disks. If the engine tag is not specified, the ``qemu`` engine will be
used by default (assuming the qemu driver is used). Note that ``librbd`` engine
is currently only supported by the qemu VM driver, and is not supported by the
storage driver. Furthermore, the storage driver currently ignores the ``engine``
tag. :since:`since 9.3.0` RBD layered encryption is supported. Layered
encryption requires a secret per each encrypted layer. The first secret
corresponds to the (child) image itself, the second secret to the parent image,
and so forth.

The ``encryption`` tag can currently contain a sequence of ``secret`` tags, each
with mandatory attributes ``type`` and either ``uuid`` or ``usage`` (
:since:`since 2.1.0` ). The only currently defined value of ``type`` is
``volume``. The ``uuid`` is "uuid" of the ``secret`` while ``usage`` is the
"usage" subelement field. A secret value can be set in libvirt by the
`virSecretSetValue <html/libvirt-libvirt-secret.html#virSecretSetValue>`__ API.
Alternatively, if supported by the particular volume format and driver,
automatically generate a secret value at the time of volume creation, and store
it using the specified ``uuid``.

``qcow`` format
~~~~~~~~~~~~~~~

:since:`Since 4.5.0`, encryption formats ``default`` and ``qcow`` may no longer
be used to create an encrypted volume. Usage of qcow encrypted volumes in QEMU
began phasing out in QEMU 2.3 and by QEMU 2.9 creation of a qcow encrypted
volume via qemu-img required usage of secret objects, but that support was not
added to libvirt.

``luks`` format
~~~~~~~~~~~~~~~

The ``luks`` format is specific to a luks encrypted volume and the secret is
used in order to either encrypt during volume creation or decrypt the volume for
usage by the domain. A single ``<secret type='passphrase'...>`` element is
expected (except for the case of RBD layered encryption mentioned above).
:since:`Since 2.1.0`.

For volume creation, it is possible to specify the encryption algorithm used to
encrypt the luks volume. The following two optional elements may be provided for
that purpose. It is hypervisor dependent as to which algorithms are supported.
The default algorithm used by the storage driver backend when using qemu-img to
create the volume is 'aes-256-cbc' using 'essiv' for initialization vector
generation and 'sha256' hash algorithm for both the cipher and the
initialization vector generation.

``cipher``
   This element describes the cipher algorithm to be used to either encrypt or
   decrypt the luks volume. This element has the following attributes:

   ``name``
      The name of the cipher algorithm used for data encryption, such as 'aes',
      'des', 'cast5', 'serpent', 'twofish', etc. Support of the specific
      algorithm is storage driver implementation dependent.
   ``size``
      The size of the cipher in bits, such as '256', '192', '128', etc. Support
      of the specific size for a specific cipher is hypervisor dependent.
   ``mode``
      An optional cipher algorithm mode such as 'cbc', 'xts', 'ecb', etc.
      Support of the specific cipher mode is hypervisor dependent.
   ``hash``
      An optional master key hash algorithm such as 'md5', 'sha1', 'sha256',
      etc. Support of the specific hash algorithm is hypervisor dependent.
``ivgen``
   This optional element describes the initialization vector generation
   algorithm used in conjunction with the ``cipher``. If the ``cipher`` is not
   provided, then an error will be generated by the parser.

   ``name``
      The name of the algorithm, such as 'plain', 'plain64', 'essiv', etc.
      Support of the specific algorithm is hypervisor dependent.
   ``hash``
      An optional hash algorithm such as 'md5', 'sha1', 'sha256', etc. Support
      of the specific ivgen hash algorithm is hypervisor dependent.

``luks2`` format
~~~~~~~~~~~~~~~~

The ``luks2`` format is currently supported only by the ``librbd`` engine, and
can only be applied to RBD network disks (RBD images). Since the ``librbd``
engine is currently not supported by the libvirt storage driver, you cannot use
it to control such disks. However, pre-formatted RBD luks2 disks can be loaded
to a qemu VM using the qemu VM driver. A single
``<secret type='passphrase'...>`` element is expected (except for the case of
RBD layered encryption mentioned above).

``luks-any`` format
~~~~~~~~~~~~~~~~~~~

The ``luks-any`` format is currently supported only by the ``librbd`` engine,
and can only be applied to RBD network disks (RBD images). This format will try
to parse the disk as either LUKS or LUKS2, depending on the actual on-disk
format. A single ``<secret type='passphrase'...>`` element is expected (except
for the case of RBD layered encryption mentioned above) :since:`Since 9.3.0`.

Examples
--------

Assuming a `luks volume type secret <formatsecret.html#usage-type-volume>`__ is
already defined, a simple example specifying use of the ``luks`` format for
either volume creation without a specific cipher being defined or as part of a
domain volume definition:

::

   <encryption format='luks'>
     <secret type='passphrase' uuid='f52a81b2-424e-490c-823d-6bd4235bc572'/>
   </encryption>

Here is an example specifying use of the ``luks`` format for a specific cipher
algorithm for volume creation. :since:`Since 6.10.0`, the ``target`` format can
also support ``qcow2`` type with ``luks`` encryption.

::

   <volume>
     <name>twofish.luks</name>
     <capacity unit='G'>5</capacity>
     <target>
       <path>/var/lib/libvirt/images/demo.luks</path>
       <format type='raw'/>
       <encryption format='luks'>
          <secret type='passphrase' uuid='f52a81b2-424e-490c-823d-6bd4235bc572'/>
          <cipher name='twofish' size='256' mode='cbc' hash='sha256'/>
          <ivgen name='plain64' hash='sha256'/>
       </encryption>
     </target>
   </volume>