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

<encryption format="default"/> can be specified only when creating a qcow volume. If the volume is successfully created, the encryption formats, parameters and secrets will be auto-generated by libvirt and the attached encryption tag will be updated. The unmodified contents of the encryption tag can be used in later operations with the volume, or when setting up a domain that uses the volume.

"qcow" format

The qcow format specifies that the built-in encryption support in qcow- or qcow2-formatted volume images should be used. A single <secret type='passphrase'> element is expected. Note that this encryption is inherently broken and should not be used any more.

"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

Here is a simple example, specifying use of the qcow format:

<encryption format='qcow'>
   <secret type='passphrase' uuid='c1f11a6d-8c5d-4a3e-ac7a-4e171c5e0d4a' />
</encryption>

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>