| --- |
| layout: docs |
| page_title: Key Rotation |
| description: Learn about the details of key rotation within Vault. |
| --- |
| |
| # 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](/vault/docs/internals/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](https://en.wikipedia.org/wiki/Shamir's_Secret_Sharing) |
| to split the root key into 5 shares, any 3 of which are required to reconstruct the master |
| key. The root key is used to protect the encryption key, which is ultimately used to protect |
| data written to the storage backend. |
| |
|  |
| |
| To support key rotation, we need to support changing the unseal keys, root 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 root 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 root 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 root 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. |
| |
| The `rotate/config` endpoint is used to configure the number of operations or time interval |
| between automatic rotations of the backend encryption key. |
| |
| ## 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 2<sup>32</sup> encryptions have been performed, following |
| the guidelines of NIST publication 800-38D. |
| |
| As of Vault 1.7, Vault will automatically rotate the backend encryption key |
| prior to reaching 2<sup>32</sup> encryption operations by default. |
| |
| 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. |
