Storage volume encryption XML format

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, and luks. 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 can currently contain a sequence of secret tags, each with mandatory attributes type and either uuid or usage (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 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.

"default" format

"qcow" format

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. 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.

Examples

Assuming a luks volume type secret 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: 

<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>