Builds the Signature
        :param data: The Request data
        :type data: dict

        :param saml_type: The target URL the user should be redirected to
        :type saml_type: string  SAMLRequest | SAMLResponse

        :param sign_algorithm: Signature algorithm method
        :type sign_algorithm: string

def __build_signature(self, data, saml_type, sign_algorithm=OneLogin_Saml2_Constants.RSA_SHA1):
        """
        Builds the Signature
        :param data: The Request data
        :type data: dict

        :param saml_type: The target URL the user should be redirected to
        :type saml_type: string  SAMLRequest | SAMLResponse

        :param sign_algorithm: Signature algorithm method
        :type sign_algorithm: string
        """
        assert saml_type in ('SAMLRequest', 'SAMLResponse')
        key = self.get_settings().get_sp_key()

        if not key:
            raise OneLogin_Saml2_Error(
                "Trying to sign the %s but can't load the SP private key." % saml_type,
                OneLogin_Saml2_Error.PRIVATE_KEY_NOT_FOUND
            )

        msg = self.__build_sign_query(data[saml_type],
                                      data.get('RelayState', None),
                                      sign_algorithm,
                                      saml_type)

        sign_algorithm_transform_map = {
            OneLogin_Saml2_Constants.DSA_SHA1: xmlsec.Transform.DSA_SHA1,
            OneLogin_Saml2_Constants.RSA_SHA1: xmlsec.Transform.RSA_SHA1,
            OneLogin_Saml2_Constants.RSA_SHA256: xmlsec.Transform.RSA_SHA256,
            OneLogin_Saml2_Constants.RSA_SHA384: xmlsec.Transform.RSA_SHA384,
            OneLogin_Saml2_Constants.RSA_SHA512: xmlsec.Transform.RSA_SHA512
        }
        sign_algorithm_transform = sign_algorithm_transform_map.get(sign_algorithm, xmlsec.Transform.RSA_SHA1)

        signature = OneLogin_Saml2_Utils.sign_binary(msg, key, sign_algorithm_transform, self.__settings.is_debug_active())
        data['Signature'] = OneLogin_Saml2_Utils.b64encode(signature)
        data['SigAlg'] = sign_algorithm