code: func (srv *server) AuthSign(req, id []byte, provider auth.Provider) ([]byte, error) {
  	return srv.authReq(req, id, provider, "sign")
  }
docs: // AuthSign fills out an authenticated signing request to the server,
  // receiving a certificate or error in response.
  // It takes the serialized JSON request to send, remote address and
  // authentication provider.