Integrate Castellan for Key Management

Castellan is a key manager interface library that is intended to be usable with multiple back ends, including Barbican. The Castellan code is based on the basic key manager interface that resides in Nova and Cinder. Now that the key manager interface lives in a separate library, the key manager code can be removed from Nova and Cinder, and Castellan can be used as the key manager interface instead.

Problem description

As encryption features in OpenStack projects are becoming more common, the projects typically need a way to interface with a key manager. Different deployers may have different requirements for key managers, so the key manager interface must also be configurable to have different back ends. The Castellan key manager interface was based off the key manager interfaces found in Cinder and Nova. Now that the shared key manager interface lives in a separate library, the original key manager interface embedded in Nova can be removed and Castellan used instead.

Use Cases

Castellan supports existing features such as ephemeral storage encryption and volume encryption.

Proposed change

Castellan by default pulls configuration options from a Castellan-specific configuration file in /etc/castellan, but can also take in configuration options if passed in directly. The configuration options for the key manager can still be specified in nova.conf, and passed along to Castellan.

The old key manager interface code and back end implementations in nova/keymgr and tests in nova/tests/unit/keymgr can be removed. Any place in the Nova code where the key manager interface was called will be replaced by calls to Castellan instead. Castellan does not include ConfKeyManager, an insecure fixed-key key manager that reads the key from the configuration file. The implementation for ConfKeyManager will remain in Nova as the Nova community agrees that it provides a valuable test fixture.


Castellan was integrated into Nova, but ConfKeyManager still remains in the Nova source code. There are a few options for improving the integration. The goals in determining a path forward are the following:

  • Keep Castellan a key manager interface for production-ready back ends

  • Deprecate class-based loading

  • Find a back end to serve as a test fixture for encryption features

However, class-based loading is a Castellan feature, and so the spec for deprecating class-based loading should live in the Castellan/Barbican specs. The followng are possible alternatives, which solve one or more of the goals:

  • Remove and replace ConfKeyManager

    One strategy for a path forward is to deprecate and remove ConfKeyManager and find an alternative back end suitable for testing. The ConfKeyManager back end reads a single, fixed key from a configuration file. It does not live in Castellan because ConfKeyManager is very insecure and is only suitable for testing. It is only useful for basic testing of encryption features using one key, such as Cinder volume encryption. If any administrators decided to use ConfKeyManager in their production deployment, they will be able to store the fixed key in the new back end as part of the migration necessary after deprecation. Other security features such as Glance image signing and verification use certificates and cannot be tested with ConfKeyManager. A back end closer to what is used in production would provide better testing. The following are options for replacing ConfKeyManager:

    • Option 1: KMIP Castellan back end

      The Key Manager Interoperability Protocol (KMIP) is a standardized protocol for interacting with a key manager. The PyKMIP library [6] includes not only client code necessary for interacting with a KMIP hardware device but also a KMIP software server with Keystone authentication that is useful for functional testing where a hardware device is not an option. Work on a KMIP Castellan back end has already started [7], but would need to be completed for this option. The PyKMIP software server is already used in the Barbican functional gate. New DevStack gate checks could be configured to use the PyKMIP server for the encryption Tempest tests, or the existing ones could be modified. This option satisfies all three of the goals listed above.

    • Option 2: Barbican Castellan back end

      A Barbican back end already exists for Castellan. This option entails editing DevStack gate jobs and/or DevStack itself to configure and launch Barbican. This option is beneficial because it would test encryption features as they should be used in production, as Barbican is the recommended back end. However, just 2% of production deployments use Barbican [4] so it may not make sense to include it in all of the gates. This option would satisfy all three of the goals listed above.

    • Option 3: New database back end

      This option is to create a new Castellan test fixture back end that can store multiple objects in a database. While this option will not provide a deployment-ready back end, it will be better than ConfKeyManager and will be able to support functional testing of features such as signed image verification that need to retrieve certificates. This is an improvement from using ConfKeyManager because this will allow the key manager testing code to be closer to what a deployment configuration would look like. However, this back end does not exist yet, and would require work to implement the database interactions. Option 1 or Option 2 would require less Castellan development work. Once completed, this option would satisfy two of the three goals.

  • Move ConfKeyManager elsewhere

    The community has expressed concern about ConfKeyManager living in the Nova code base, but moving ConfKeyManager into Castellan is not preferred. The following are options for if ConfKeyManager cannot be deprecated:

    • Option 4: Move ConfKeyManager to Tempest

      The Tempest tests are the only place where ConfKeyManager should be used, so the back end could be moved to Tempest. As long as Castellan provides an option to register back ends if class-based loading is deprecated, this option could satisfy all three of the goals above.

    • Option 5: Move ConfKeyManager to Castellan

      This is not a recommended option. The ConfKeyManager does not support testing of features such as signed image verification [8], which uses certificates, not keys. Moving ConfKeyManager to Castellan will push the problem of not having an adequate testing back end down the road.

  • Revert the Castellan integration patch

    • Option 6: Revert to nova/keymgr

      This is not a recommended option. The key manager interface will be left as it is in nova/keymgr, but this means that Nova’s key manager will not benefit from the updates, new features, and future additional back ends available in Castellan. The key manager interface will not be unified across Nova, as the volume encryption feature and encrypted ephemeral storage feature will use nova/keymgr, but the image signature verification feature already uses Castellan.

Data model impact


REST API impact


Security impact

Castellan behaves very similarly to the current Nova key manager. Castellan has added improvements and bug fixes beyond what is currently in the Nova and Cinder key managers, making it more secure. The fixed-key key manager found in Nova and Cinder is insecure for deployments, but it is useful for testing. Castellan doesn’t include the fixed-key key manager, so the ConfKeyManager will remain in Nova.

Notifications impact


Other end user impact


Performance Impact


Other deployer impact

The deployer should be made aware of a change in the default key manager back end. The current default back end in Nova is a fixed key, but Castellan uses Barbican as the default. This means the deployer should ensure Barbican is running and the fixed key added to Barbican so it can continue to be used.

The options in the Nova configuration file for disk encryption will change. The option group ‘keymgr’ will be spelled out to ‘key_manager’. The key manager option group will still have an option ‘api_class’ to specify the desired back end, but an option to specify the fixed key will no longer be available. In the ‘barbican’ option group, a few new options will be available to increase the robustness of the back end, such as the number of times to check if a key has been successfully created.

To maintain backwards compatibility, the old options will still be listed as deprecated options. Standard deprecation policy will be followed, and these old options should be removed in the next release cycle.

Developer impact

Nova developers should not be impacted by this change. If developers find more uses for a key manager, Castellan should be just as easy to use as the current Nova key manager interface.



Primary assignee:

Kaitlin Farr <> kfarr on IRC

Other contributors:


Work Items

  • Remove calls to Nova’s key manager with calls to Castellan.

  • Remove Nova key manager code.

  • Update documentation.


This change depends on Castellan, version >= 0.2.0. Castellan is already in OpenStack’s global requirements.


This change can be unit tested using a simple in-memory back end. As actual deployments should be using Barbican, this feature should be tested using a Barbican back end, too.

Documentation Impact

These changes will be documented. Nova documentation for disk encryption will be updated to reference Castellan [5].


[1] Castellan source code

[2] Castellan in OpenStack’s global requirements

[3] Current Nova key manager implementation

[4] April 2016 OpenStack User Survey

[5] Disk encryption configuration reference

[6] PyKMIP source code

[7] KMIP backend for Castellan

[8] Glance image signing and verification specification



Release Name