'Find key based on header.
.. todo:: Supports only "jwk" header parameter lookup.
:returns: (Public) key found in the header.
:rtype: .JWK
:raises acme.jose.errors.Error: if key could not be found'

def find_key(self):

    if (self.jwk is None):
        raise errors.Error('No    key    found')
    return self.jwk
