Seal/Unseal

When a Vault server is started, it starts in a sealed state. In this state, Vault is configured to know where and how to access the physical storage, but doesn't know how to decrypt any of it.

Unsealing is the process of obtaining the plaintext root key necessary to read the decryption key to decrypt the data, allowing access to the Vault.

Prior to unsealing, almost no operations are possible with Vault. For example authentication, managing the mount tables, etc. are all not possible. The only possible operations are to unseal the Vault and check the status of the seal.

Why?

The data stored by Vault is encrypted. Vault needs the encryption key in order to decrypt the data. The encryption key is also stored with the data (in the keyring), but encrypted with another encryption key known as the root key.

Therefore, to decrypt the data, Vault must decrypt the encryption key which requires the root key. Unsealing is the process of getting access to this root key. The root key is stored alongside all other Vault data, but is encrypted by yet another mechanism: the unseal key.

To recap: most Vault data is encrypted using the encryption key in the keyring; the keyring is encrypted by the root key; and the root key is encrypted by the unseal key.

Shamir seals

The default Vault config uses a Shamir seal. Instead of distributing the unseal key as a single key to an operator, Vault uses an algorithm known as Shamir's Secret Sharing to split the key into shares. A certain threshold of shares is required to reconstruct the unseal key, which is then used to decrypt the root key.

This is the unseal process: the shares are added one at a time (in any order) until enough shares are present to reconstruct the key and decrypt the root key.

Unsealing

The unseal process is done by running vault operator unseal or via the API. This process is stateful: each key can be entered via multiple mechanisms from multiple client machines and it will work. This allows each shares of the root key to be on a distinct client machine for better security.

Note that when using the Shamir seal with multiple nodes, each node must be unsealed with the required threshold of shares. Partial unsealing of each node is not distributed across the cluster.

Once a Vault node is unsealed, it remains unsealed until one of these things happens:

1. It is resealed via the API (see below).

2. The server is restarted.

3. Vault's storage layer encounters an unrecoverable error.

Sealing

There is also an API to seal the Vault. This will throw away the root key in memory and require another unseal process to restore it. Sealing only requires a single operator with root privileges.

This way, if there is a detected intrusion, the Vault data can be locked quickly to try to minimize damages. It can't be accessed again without access to the root key shares.

Auto unseal

Auto Unseal was developed to aid in reducing the operational complexity of keeping the unseal key secure. This feature delegates the responsibility of securing the unseal key from users to a trusted device or service. At startup Vault will connect to the device or service implementing the seal and ask it to decrypt the root key Vault read from storage.

There are certain operations in Vault besides unsealing that require a quorum of users to perform, e.g. generating a root token. When using a Shamir seal the unseal keys must be provided to authorize these operations. When using Auto Unseal these operations require recovery keys instead.

Just as the initialization process with a Shamir seal yields unseal keys, initializing with an Auto Unseal yields recovery keys.

It is still possible to seal a Vault node using the API. In this case Vault will remain sealed until restarted, or the unseal API is used, which with Auto Unseal requires the recovery key fragments instead of the unseal key fragments that would be provided with Shamir. The process remains the same.

For a list of examples and supported providers, please see the seal documentation.

Recovery key

When Vault is initialized while using an HSM or KMS, rather than unseal keys being returned to the operator, recovery keys are returned. These are generated from an internal recovery key that is split via Shamir's Secret Sharing, similar to Vault's treatment of unseal keys when running without an HSM or KMS.

Details about initialization and rekeying follow. When performing an operation that uses recovery keys, such as generate-root, selection of the recovery keys for this purpose, rather than the barrier unseal keys, is automatic.

Initialization

When initializing, the split is performed according to the following CLI flags and their API equivalents in the /sys/init endpoint:

• recovery-shares: The number of shares into which to split the recovery key. This value is equivalent to the recovery_shares value in the API endpoint.
• recovery-threshold: The threshold of shares required to reconstruct the recovery key. This value is equivalent to the recovery_threshold value in the API endpoint.
• recovery-pgp-keys: The PGP keys to use to encrypt the returned recovery key shares. This value is equivalent to the recovery_pgp_keys value in the API endpoint, although as with pgp_keys the object in the API endpoint is an array, not a string.

Additionally, Vault will refuse to initialize if the option has not been set to generate a key, and no key is found. See Configuration for more details.

Rekeying

Unseal key

Vault's unseal key can be rekeyed using a normal vault operator rekey operation from the CLI or the matching API calls. The rekey operation is authorized by meeting the threshold of recovery keys. After rekeying, the new barrier key is wrapped by the HSM or KMS and stored like the previous key; it is not returned to the users that submitted their recovery keys.

Recovery key

The recovery key can be rekeyed to change the number of shares/threshold or to target different key holders via different PGP keys. When using the Vault CLI, this is performed by using the -target=recovery flag to vault operator rekey.

Via the API, the rekey operation is performed with the same parameters as the normal /sys/rekey endpoint; however, the API prefix for this operation is at /sys/rekey-recovery-key rather than /sys/rekey.

Seal migration

The Seal migration process cannot be performed without downtime, and due to the technical underpinnings of the seal implementations, the process requires that you briefly take the whole cluster down. While experiencing some downtime may be unavoidable, we believe that switching seals is a rare event and that the inconvenience of the downtime is an acceptable trade-off.

Migration post Vault 1.16.0 via Seal HA for Auto Seals (Enterprise)

With Seal HA, migration between auto-unseal types (not including any Shamir seals) can be done fully online using Seal High Availability (Seal HA) without any downtime.

1. Edit the Vault configuration, and add the new, target seal configuration.
2. Send the Vault process the SIGHUP signal, triggering a configuration reload.
3. Monitor the sys/sealwrap/rewrap endpoints, to see that rewrap is running, and/or sys/seal-backend-status, endpoints, waiting for fully_wrapped to be true, indicating all seal wrapped values are now wrapped by the new seal. The logs also contain information about the rewrap progress.
4. Edit the Vault configuration, removing the old seal configuration.
5. Send the Vault process the SIGHUP signal, again allowing re-wrapping to complete.

Migration post Vault 1.5.1

These steps are common for seal migrations between any supported kinds and for any storage backend.

1. Take a standby node down and update the seal configuration.

   • If the migration is from Shamir seal to Auto seal, add the desired new Auto seal block to the configuration.
   • If the migration is from Auto seal to Shamir seal, add disabled = "true" to the old seal block.
   • If the migration is from Auto seal to another Auto seal, add disabled = "true" to the old seal block and add the desired new Auto seal block.

   Now, bring the standby node back up and run the unseal command on each key, by supplying the -migrate flag.

   • Supply Shamir unseal keys if the old seal was Shamir, which will be migrated as the recovery keys for the Auto seal.
   • Supply recovery keys if the old seal is one of Auto seals, which will be migrated as the recovery keys of the new Auto seal, or as Shamir unseal keys if the new seal is Shamir.

2. Perform step 1 for all the standby nodes, one at a time. It is necessary to bring back the downed standby node before moving on to the other standby nodes, specifically when Integrated Storage is in use for it helps to retain the quorum.

3. Step down the active node. One of the standby nodes will become the new active node. When using Integrated Storage, ensure that quorum is reached and a leader is elected.

4. The new active node will perform the migration. Monitor the server log in the active node to witness the completion of the seal migration process. Wait for a little while for the migration information to replicate to all the nodes in case of Integrated Storage. In enterprise Vault, switching an Auto seal implies that the seal wrapped storage entries get re-wrapped. Monitor the log and wait until this process is complete (look for seal re-wrap completed).

5. Seal migration is now completed. Take down the old active node, update its configuration to use the new seal blocks (completely unaware of the old seal type) ,and bring it back up. It will be auto-unsealed if the new seal is one of the Auto seals, or will require unseal keys if the new seal is Shamir.

6. At this point, configuration files of all the nodes can be updated to only have the new seal information. Standby nodes can be restarted right away and the active node can be restarted upon a leadership change.

Migration pre 1.5.1

Migration from shamir to auto unseal

To migrate from Shamir keys to Auto Unseal, take your server cluster offline and update the seal configuration with the appropriate seal configuration. Bring your server back up and leave the rest of the nodes offline if using multi-server mode, then run the unseal process with the -migrate flag and bring the rest of the cluster online.

All unseal commands must specify the -migrate flag. Once the required threshold of unseal keys are entered, unseal keys will be migrated to recovery keys.

$ vault operator unseal -migrate

Migration from auto unseal to shamir

To migrate from Auto Unseal to Shamir keys, take your server cluster offline and update the seal configuration and add disabled = "true" to the seal block. This allows the migration to use this information to decrypt the key but will not unseal Vault. When you bring your server back up, run the unseal process with the -migrate flag and use the Recovery Keys to perform the migration. All unseal commands must specify the -migrate flag. Once the required threshold of recovery keys are entered, the recovery keys will be migrated to be used as unseal keys.

Migration from auto unseal to auto unseal

To migrate from Auto Unseal to a different Auto Unseal configuration, take your server cluster offline and update the existing seal configuration and add disabled = "true" to the seal block. Then add another seal block to describe the new seal.

When you bring your server back up, run the unseal process with the -migrate flag and use the Recovery Keys to perform the migration. All unseal commands must specify the -migrate flag. Once the required threshold of recovery keys are entered, the recovery keys will be kept and used as recovery keys in the new seal.

Migration with integrated storage

Integrated Storage uses the Raft protocol underneath, which requires a quorum of servers to be online before the cluster is functional. Therefore, bringing the cluster back up one node at a time with the seal configuration updated, will not work in this case. Follow the same steps for each kind of migration described above with the exception that after the cluster is taken offline, update the seal configurations of all the nodes appropriately and bring them all back up. When the quorum of nodes are back up, Raft will elect a leader and the leader node that will perform the migration. The migrated information will be replicated to all other cluster peers and when the peers eventually become the leader, migration will not happen again on the peer nodes.

Seal High Availability (Enterprise)

Seal High Availability (Seal HA) allows the configuration of more than one auto seal mechanism such that Vault can tolerate the temporary loss of a seal service or device for a time. With Seal HA configured with at least two and no more than three auto seals, Vault can also start up and unseal if one of the configured seals is still available (though Vault will remain in a degraded mode in this case). While seals are unavailable, seal wrapping and entropy augmentation can still occur using the remaining seals, and values produced while a seal is down will be re-wrapped with all the seals when all seals become healthy again.

An operator should choose two seals that are unlikely to become unavailable at the same time. For example, they may choose KMS keys in two cloud regions, from two different providers; or a mix of HSM, KMS, or Transit seals.

When an operator configures an additional seal or removes a seal (one at a time) and restarts or triggers a configuration reload on receipt of a SIGHUP signal, Vault will automatically detect that it needs to re-wrap CSPs and seal wrapped values, and will start the process. Seal re-wrapping can be monitored via the logs or via the sys/seal-status endpoint. While a re-wrap is in progress (or could not complete successfully), changes to the seal configuration are not allowed.

In additional to high availability, Seal HA can be used to migrate between two auto seals in a simplified manner.

Use and Configuration

Refer to the configuration section for details on configuring Seal HA.

Seal Re-Wrapping

Whenever seal configuration changes, Vault must re-wrap all CSPs and seal wrapped values, to ensure each value has an entry encrypted by all configured seals. Vault detects these configuration changes automatically, and triggers a re-wrap. Re-wraps can take some time, depending on the number of seal wrapped values. While re-wrapping is in progress, no configuration changes to the seals can be made.

Progress of the re-wrap can be monitored using the sys/sealwrap/rewrap endpoint.

Limitations and Known Issues

In order to limit complexity and increase safety, there are some limitations to the use and configuration of Seal HA:

• Vault must be configured for a single seal at the time of initialization. Extra seals can then be added.
• Seals must be added or removed one at a time.
• Only auto seals can be used in HA configurations. Shamir and auto cannot be mixed.
• A maximum of three seals can be configured.
• As seal wrapped values must be wrapped by all configured seals, it is possible that large values may fail to persist as the size of the entry is multiplied by the number of seals causing it to exceed the storage entry size limit. An example would be storing a large document in KVv2 with seal wrapping enabled.
• It is not possible to rotate the data encryption key nor the recovery keys while unless all seals are healthy.