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 constructing the master 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 unseal.
The data stored by Vault is stored encrypted. Vault needs the encryption key in order to decrypt the data. The encryption key is also stored with the data, but encrypted with another encryption key known as the master key. The master key isn't stored anywhere.
Therefore, to decrypt the data, Vault must decrypt the encryption key which requires the master key. Unsealing is the process of reconstructing this master key.
Instead of distributing this master key as a single key to an operator, Vault uses an algorithm known as Shamir's Secret Sharing to split the key into shards. A certain threshold of shards is required to reconstruct the master key.
This is the unseal process: the shards are added one at a time (in any order) until enough shards are present to reconstruct the key and decrypt the data.
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
on multiple computers and it will work. This allows each shard of the master
key to be on a distinct machine for better security.
Once a Vault is unsealed, it remains unsealed until one of two things happens:
It is resealed via the API (see below).
The server is restarted.
Note: Unsealing makes the process of automating a Vault install difficult. Automated tools can easily install, configure, and start Vault, but unsealing it is a very manual process. We have plans in the future to make it easier. For the time being, the best method is to manually unseal multiple Vault servers in HA mode. Use a tool such as Consul to make sure you only query Vault servers that are unsealed.
There is also an API to seal the Vault. This will throw away the master key 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 master key shards.
» Auto Unseal
Auto Unseal was developed to aid in reducing the operational complexity of keeping the master key secure. This feature delegates the responsibility of securing the master key from users to a trusted device or service. Instead of only constructing the key in memory, the master key is encrypted with one of these services or devices and then stored in the storage backend allowing Vault to decrypt the master key at startup and unseal automatically.
When using a Auto Unseal, there are certain operations in Vault that still require a quorum of users to perform an operation such as generating a root token. During the initialization process, a set of Shamir keys are generated that are called Recovery Keys and are used for these operations.
For a list of examples and supported providers, please see the seal documentation.
» Seal Migration
The seal can be migrated between Shamir keys and automatic migration and vice versa.
To migrate from Shamir keys to Auto Unseal, take your server cluster offline and update
the seal configuration with the appropriate seal
configuration. When you bring up your server back up, run the unseal process with the
-migrate flag. All unseal commands must specify the
-migrate flag. Once the
required threshold of unseal keys are entered, the unseal keys will be migrated to
$ vault operator unseal -migrate
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 up your server back up, run the unseal process
-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.