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»Key Rotation

Vault has multiple encryption keys that are used for various purposes. These keys support rotation so that they can be periodically changed or in response to a potential leak or compromise. It is useful to first understand the high-level architecture before learning about key rotation.

As a review, Vault starts in a sealed state. Vault is unsealed by providing the unseal keys. By default, Vault uses a technique known as Shamir's secret sharing algorithm to split the master key into 5 shares, any 3 of which are required to reconstruct the master key. The master key is used to protect the encryption key, which is ultimately used to protect data written to the storage backend.

Vault Shamir Secret Sharing Algorithm

To support key rotation, we need to support changing the unseal keys, master key, and the backend encryption key. We split this into two separate operations, rekey and rotate.

The rekey operation is used to generate a new master key. When this is being done, it is possible to change the parameters of the key splitting, so that the number of shares and the threshold required to unseal can be changed. To perform a rekey a threshold of the current unseal keys must be provided. This is to prevent a single malicious operator from performing a rekey and invalidating the existing master key.

Performing a rekey is fairly straightforward. The rekey operation must be initialized with the new parameters for the split and threshold. Once initialized, the current unseal keys must be provided until the threshold is met. Once met, Vault will generate the new master key, perform the splitting, and re-encrypt the encryption key with the new master key. The new unseal keys are then provided to the operator, and the old unseal keys are no longer usable.

The rotate operation is used to change the encryption key used to protect data written to the storage backend. This key is never provided or visible to operators, who only have unseal keys. This simplifies the rotation, as it does not require the current key holders unlike the rekey operation. When rotate is triggered, a new encryption key is generated and added to a keyring. All new values written to the storage backend are encrypted with the new key. Old values written with previous encryption keys can still be decrypted since older keys are saved in the keyring. This allows key rotation to be done online, without an expensive re-encryption process.

Both the rekey and rotate operations can be done online and in a highly available configuration. Only the active Vault instance can perform either of the operations but standby instances can still assume an active role after either operation. This is done by providing an online upgrade path for standby instances. If the current encryption key is N and a rotation installs N+1, Vault creates a special "upgrade" key, which provides the N+1 encryption key protected by the N key. This upgrade key is only available for a few minutes enabling standby instances to do a periodic check for upgrades. This allows standby instances to update their keys and stay in-sync with the active Vault without requiring operators to perform another unseal.

»NIST Rotation Guidance

Periodic rotation of the encryption keys is recommended, even in the absence of compromise. Due to the nature of the AES-256-GCM encryption used, keys should be rotated before approximately 232 encryptions have been performed, following the guidelines of NIST publication 800-38D. Operators can estimate the number of encryptions by summing the following:

  • The vault.barrier.put telemetry metric.
  • The vault.token.creation metric where the token_type label is batch.
  • The merkle.flushDirty.num_pages metric.
  • The WAL index.

The simplest strategy may be to use those metrics to determine a frequency of rotation and make that part of the operational process. For example, if one determines that the estimated rate is 40 million operations per day, then rotating the key every three months is sufficient.