'Retrieves the client secret associated with the client key.
This method must allow the use of a dummy client_key value.
Fetching the secret using the dummy key must take the same amount of
time as fetching a secret for a valid client.
Note that the returned key must be in plaintext.'

def get_client_secret(self, client_key):

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