Tang is a server for binding data to network presence.

This sounds fancy, but the concept is simple. You have some data, but you only
want it to be available when the system containing the data is on a certain,
usually secure, network. This is where Tang comes in.

First, the client gets a list of the Tang server's advertised asymmetric keys.
This can happen online by a simple HTTP GET. Alternatively, since the keys are
asymmetric, the public key list can be distributed out of band.

Second, the client uses one of these public keys to generate a unique,
cryptographically strong encryption key. The data is then encrypted using this
key. Once the data is encrypted, the key is discarded. Some small metadata is
produced as part of this operation which the client should store in a convenient
location. This process of encrypting data is the provisioning step.

Third, when the client is ready to access its data, it simply loads the metadata
produced in the provisioning step and performs an HTTP POST in order to recover
the encryption key. This process is the recovery step.