CryptoKVStorage offers a CryptoStorage implementation by extending a key
value store (KVStorage) to create and manage cryptographic keys. This is
useful for providing a simple CryptoStorage implementation based upon an
existing KVStorage engine (e.g. for test purposes).
A secure key/value storage engine. Create takes a policy that is enforced
internally by the actual backend. The policy contains public identities that
the backend can translate into a unique and private token for another
service. Hence get and set internally will pass the current service private
token to the backend to gain its permissions.