Credential Encryption

As of the Newton release, keystone encrypts all credentials stored in the default sql backend. Credentials are encrypted with the same mechanism used to encrypt Fernet tokens, fernet. Keystone provides only one type of credential encryption but the encryption provider is pluggable in the event you wish to supply a custom implementation.

This document details how credential encryption works, how to migrate existing credentials in a deployment, and how to manage encryption keys for credentials.

Configuring credential encryption

The configuration for credential encryption is straightforward. There are only two configuration options needed:

[credential]
provider = fernet
key_repository = /etc/keystone/credential-keys/

[credential] provider defaults to the only option supplied by keystone, fernet. There is no reason to change this option unless you wish to provide a custom credential encryption implementation. The [credential] key_repository location is a requirement of using fernet but will default to the /etc/keystone/credential-keys/ directory. Both [credential] key_repository and [fernet_tokens] key_repository define locations for keys used to encrypt things. One holds the keys to encrypt and decrypt credentials and the other holds keys to encrypt and decrypt tokens. It is imperative that these repositories are managed separately and they must not share keys. Meaning they cannot share the same directory path. The [credential] key_repository is only allowed to have three keys. This is not configurable and allows for credentials to be re-encrypted periodically with a new encryption key for the sake of security.

How credential encryption works

The implementation of this feature did not change any existing credential API contracts. All changes are transparent to the user unless you’re inspecting the credential backend directly.

When creating a credential, keystone will encrypt the blob attribute before persisting it to the backend. Keystone will also store a hash of the key that was used to encrypt the information in that credential. Since Fernet is used to encrypt credentials, a key repository consists of multiple keys. Keeping track of which key was used to encrypt each credential is an important part of encryption key management. Why this is important is detailed later in the Encryption key management section.

When updating an existing credential’s blob attribute, keystone will encrypt the new blob and update the key hash.

When listing or showing credentials, all blob attributes are decrypted in the response. Neither the cipher text, nor the hash of the key used to encrypt the blob are exposed through the API. Furthermore, the key is only used internally to keystone.

Encryption key management

Key management of [credential] key_repository is handled with three keystone-manage commands:

1. keystone-manage credential_setup

2. keystone-manage credential_rotate

3. keystone-manage credential_migrate

keystone-manage credential_setup will populate [credential] key_repository with new encryption keys. This must be done in order for proper credential encryption to work, with the exception of the null key. This step should only be done once.

keystone-manage credential_rotate will create and rotate a new encryption key in the [credential] key_repository. This will only be done if all credential key hashes match the hash of the current primary key. If any credential has been encrypted with an older key, or secondary key, the rotation will fail. Failing the rotation is necessary to prevent overrotation, which would leave some credentials indecipherable since the key used to encrypt it no longer exists. If this step fails, it is possible to forcibly re-key all credentials using the same primary key with keystone-manage credential_migrate.

keystone-manage credential_migrate will check the backend for credentials whose key hash doesn’t match the hash of the current primary key. Any credentials with a key hash mismatching the current primary key will be re-encrypted with the current primary key. The new cipher text and key hash will be updated in the backend.