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.