Identifiers:
   http://www.w3.org/2001/04/xmldsig-more#hmac-sha224
   http://www.w3.org/2001/04/xmldsig-more#hmac-sha256
   http://www.w3.org/2001/04/xmldsig-more#hmac-sha384
   http://www.w3.org/2001/04/xmldsig-more#hmac-sha512