'Retrieves a previously stored client provided RSA key.
This method must allow the use of a dummy client_key value. Fetching
the rsa key using the dummy key must take the same aount of time
as fetching a key for a valid client.
Note that the key must be returned in plaintext.'

def get_rsa_key(self, client_key):

    raise NotImplementedError('Subclasses    must    implement    this    function.')
