77d27cb968
* Add NIST guidance on rotating keys used for AES-GCM encryption * Capture more places barrier encryption is used * spacing issue * Probabilistically track an estimated encryption count by key term * Un-reorder imports * wip * get rid of sampling
75 lines
4.3 KiB
Plaintext
75 lines
4.3 KiB
Plaintext
---
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layout: docs
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page_title: Key Rotation
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sidebar_title: Key Rotation
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description: Learn about the details of key rotation within Vault.
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---
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# Key Rotation
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Vault has multiple encryption keys that are used for various purposes. These keys support
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rotation so that they can be periodically changed or in response to a potential leak or
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compromise. It is useful to first understand the
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[high-level architecture](/docs/internals/architecture) before learning about key rotation.
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As a review, Vault starts in a _sealed_ state. Vault is unsealed by providing the unseal keys.
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By default, Vault uses a technique known as [Shamir's secret sharing algorithm](https://en.wikipedia.org/wiki/Shamir's_Secret_Sharing)
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to split the master key into 5 shares, any 3 of which are required to reconstruct the master
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key. The master key is used to protect the encryption key, which is ultimately used to protect
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data written to the storage backend.
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[](/img/vault-shamir-secret-sharing.svg)
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To support key rotation, we need to support changing the unseal keys, master key, and the
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backend encryption key. We split this into two separate operations, `rekey` and `rotate`.
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The `rekey` operation is used to generate a new master key. When this is being done,
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it is possible to change the parameters of the key splitting, so that the number of shares
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and the threshold required to unseal can be changed. To perform a rekey a threshold of the
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current unseal keys must be provided. This is to prevent a single malicious operator from
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performing a rekey and invalidating the existing master key.
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Performing a rekey is fairly straightforward. The rekey operation must be initialized with
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the new parameters for the split and threshold. Once initialized, the current unseal keys
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must be provided until the threshold is met. Once met, Vault will generate the new master
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key, perform the splitting, and re-encrypt the encryption key with the new master key.
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The new unseal keys are then provided to the operator, and the old unseal keys are no
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longer usable.
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The `rotate` operation is used to change the encryption key used to protect data written
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to the storage backend. This key is never provided or visible to operators, who only
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have unseal keys. This simplifies the rotation, as it does not require the current key
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holders unlike the `rekey` operation. When `rotate` is triggered, a new encryption key
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is generated and added to a keyring. All new values written to the storage backend are
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encrypted with the new key. Old values written with previous encryption keys can still
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be decrypted since older keys are saved in the keyring. This allows key rotation to be
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done online, without an expensive re-encryption process.
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Both the `rekey` and `rotate` operations can be done online and in a highly available
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configuration. Only the active Vault instance can perform either of the operations
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but standby instances can still assume an active role after either operation. This is
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done by providing an online upgrade path for standby instances. If the current encryption
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key is `N` and a rotation installs `N+1`, Vault creates a special "upgrade" key, which
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provides the `N+1` encryption key protected by the `N` key. This upgrade key is only available
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for a few minutes enabling standby instances to do a periodic check for upgrades.
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This allows standby instances to update their keys and stay in-sync with the active Vault
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without requiring operators to perform another unseal.
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## NIST Rotation Guidance
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Period rotation of the encryption keys is recommended, even in the absence of
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compromise. Due to the nature of the AES-256-GCM encryption used, keys should be
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rotated before approximately 2<sup>32</sup> encryptions have been performed, following
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the guidelines of NIST publication 800-38D. Operators can estimate the number
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of encryptions by summing the following:
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* The `vault.barrier.put` telemetry metric.
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* The `vault.token.creation` metric where the `token_type` label is `batch`.
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* The `merkle.flushDirty.num_pages` metric.
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* The WAL index.
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The simplest strategy may be to use those metrics to determine a frequency of
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rotation and make that part of the operational process. For example, if one
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determines that the estimated rate is 40 million operations per day, then
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rotating the key every three months is sufficient.
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