Verifies the signature nodes:
         - Checks that are Response or Assertion
         - Check that IDs and reference URI are unique and consistent.

        :returns: The signed elements tag names
        :rtype: list

def process_signed_elements(self):
        """
        Verifies the signature nodes:
         - Checks that are Response or Assertion
         - Check that IDs and reference URI are unique and consistent.

        :returns: The signed elements tag names
        :rtype: list
        """
        sign_nodes = self.__query('//ds:Signature')

        signed_elements = []
        verified_seis = []
        verified_ids = []
        response_tag = '{%s}Response' % OneLogin_Saml2_Constants.NS_SAMLP
        assertion_tag = '{%s}Assertion' % OneLogin_Saml2_Constants.NS_SAML

        for sign_node in sign_nodes:
            signed_element = sign_node.getparent().tag
            if signed_element != response_tag and signed_element != assertion_tag:
                raise OneLogin_Saml2_ValidationError(
                    'Invalid Signature Element %s SAML Response rejected' % signed_element,
                    OneLogin_Saml2_ValidationError.WRONG_SIGNED_ELEMENT
                )

            if not sign_node.getparent().get('ID'):
                raise OneLogin_Saml2_ValidationError(
                    'Signed Element must contain an ID. SAML Response rejected',
                    OneLogin_Saml2_ValidationError.ID_NOT_FOUND_IN_SIGNED_ELEMENT
                )

            id_value = sign_node.getparent().get('ID')
            if id_value in verified_ids:
                raise OneLogin_Saml2_ValidationError(
                    'Duplicated ID. SAML Response rejected',
                    OneLogin_Saml2_ValidationError.DUPLICATED_ID_IN_SIGNED_ELEMENTS
                )
            verified_ids.append(id_value)

            # Check that reference URI matches the parent ID and no duplicate References or IDs
            ref = OneLogin_Saml2_XML.query(sign_node, './/ds:Reference')
            if ref:
                ref = ref[0]
                if ref.get('URI'):
                    sei = ref.get('URI')[1:]

                    if sei != id_value:
                        raise OneLogin_Saml2_ValidationError(
                            'Found an invalid Signed Element. SAML Response rejected',
                            OneLogin_Saml2_ValidationError.INVALID_SIGNED_ELEMENT
                        )

                    if sei in verified_seis:
                        raise OneLogin_Saml2_ValidationError(
                            'Duplicated Reference URI. SAML Response rejected',
                            OneLogin_Saml2_ValidationError.DUPLICATED_REFERENCE_IN_SIGNED_ELEMENTS
                        )
                    verified_seis.append(sei)

            signed_elements.append(signed_element)

        if signed_elements:
            if not self.validate_signed_elements(signed_elements, raise_exceptions=True):
                raise OneLogin_Saml2_ValidationError(
                    'Found an unexpected Signature Element. SAML Response rejected',
                    OneLogin_Saml2_ValidationError.UNEXPECTED_SIGNED_ELEMENTS
                )
        return signed_elements