'Retrieves a previously stored client provided RSA key.
:param client_key: The client/consumer key.
:param request: An oauthlib.common.Request object.
:returns: The rsa public key as a string.
This method must allow the use of a dummy client_key value. Fetching
the rsa key using the dummy key must take the same amount of time
as fetching a key for a valid client. The dummy key must also be of
the same bit length as client keys.
Note that the key must be returned in plaintext.
This method is used by
* AccessTokenEndpoint
* RequestTokenEndpoint
* ResourceEndpoint
* SignatureOnlyEndpoint'

def get_rsa_key(self, client_key, request):

    raise self._subclass_must_implement(u'get_rsa_key')
