libvirt/docs/formatstorageencryption.htm...

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    <h1>Storage volume encryption XML format</h1>

    <ul id="toc"></ul>

    <h2><a id="StorageEncryption">Storage volume encryption XML</a></h2>

    <p>
      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.
    </p>
    <p>
      The top-level tag of volume encryption specification
      is <code>encryption</code>, with a mandatory
      attribute <code>format</code>.  Currently defined values
      of <code>format</code> are <code>default</code>, <code>qcow</code>,
      and <code>luks</code>.
      Each value of <code>format</code> implies some expectations about the
      content of the <code>encryption</code> tag.  Other format values may be
      defined in the future.
    </p>
    <p>
      The <code>encryption</code> tag can currently contain a sequence of
      <code>secret</code> tags, each with mandatory attributes <code>type</code>
      and either <code>uuid</code> or <code>usage</code>
      (<span class="since">since 2.1.0</span>). The only currently defined
      value of <code>type</code> is <code>volume</code>. The
      <code>uuid</code> is "uuid" of the <code>secret</code> while
      <code>usage</code> is the "usage" subelement field.
      A secret value can be set in libvirt by the
      <a href="html/libvirt-libvirt-secret.html#virSecretSetValue">
      <code>virSecretSetValue</code></a> 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 <code>uuid</code>.
    </p>
    <h3><a id="StorageEncryptionDefault">"default" format</a></h3>
    <h3><a id="StorageEncryptionQcow">"qcow" format</a></h3>
    <p>
      <span class="since">Since 4.5.0,</span> encryption formats
      <code>default</code> and <code>qcow</code> 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.
    </p>
    <h3><a id="StorageEncryptionLuks">"luks" format</a></h3>
    <p>
      The <code>luks</code> 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
      <code>&lt;secret type='passphrase'...&gt;</code> element is expected.
      <span class="since">Since 2.1.0</span>.
    </p>
    <p>
      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.
    </p>

    <dl>
      <dt><code>cipher</code></dt>
      <dd>This element describes the cipher algorithm to be used to either
          encrypt or decrypt the luks volume. This element has the following
          attributes:
          <dl>
            <dt><code>name</code></dt>
            <dd>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.</dd>
            <dt><code>size</code></dt>
            <dd>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.</dd>
            <dt><code>mode</code></dt>
            <dd>An optional cipher algorithm mode such as 'cbc', 'xts',
            'ecb', etc. Support of the specific cipher mode is
            hypervisor dependent.</dd>
            <dt><code>hash</code></dt>
            <dd>An optional master key hash algorithm such as 'md5', 'sha1',
            'sha256', etc. Support of the specific hash algorithm is
            hypervisor dependent.</dd>
          </dl>
      </dd>
      <dt><code>ivgen</code></dt>
      <dd>This optional element describes the initialization vector
          generation algorithm used in conjunction with the
          <code>cipher</code>. If the <code>cipher</code> is not provided,
          then an error will be generated by the parser.
          <dl>
            <dt><code>name</code></dt>
            <dd>The name of the algorithm, such as 'plain', 'plain64',
            'essiv', etc. Support of the specific algorithm is hypervisor
            dependent.</dd>
            <dt><code>hash</code></dt>
            <dd>An optional hash algorithm such as 'md5', 'sha1', 'sha256',
            etc. Support of the specific ivgen hash algorithm is hypervisor
            dependent.</dd>
          </dl>
      </dd>
    </dl>


    <h2><a id="example">Examples</a></h2>

    <p>
      Assuming a <a href="formatsecret.html#VolumeUsageType">
      <code>luks volume type secret</code></a> is already defined,
      a simple example specifying use of the <code>luks</code> format
      for either volume creation without a specific cipher being defined or
      as part of a domain volume definition:
    </p>
    <pre>
&lt;encryption format='luks'&gt;
  &lt;secret type='passphrase' uuid='f52a81b2-424e-490c-823d-6bd4235bc572'/&gt;
&lt;/encryption&gt;
    </pre>

    <p>
      Here is an example specifying use of the <code>luks</code> format for
      a specific cipher algorithm for volume creation:
    </p>
    <pre>
&lt;volume&gt;
  &lt;name&gt;twofish.luks&lt;/name&gt;
  &lt;capacity unit='G'&gt;5&lt;/capacity&gt;
  &lt;target&gt;
    &lt;path&gt;/var/lib/libvirt/images/demo.luks&lt;/path&gt;
    &lt;format type='raw'/&gt;
    &lt;encryption format='luks'&gt;
       &lt;secret type='passphrase' uuid='f52a81b2-424e-490c-823d-6bd4235bc572'/&gt;
       &lt;cipher name='twofish' size='256' mode='cbc' hash='sha256'/&gt;
       &lt;ivgen name='plain64' hash='sha256'/&gt;
    &lt;/encryption&gt;
  &lt;/target&gt;
&lt;/volume&gt;
    </pre>

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