Identifiers:
   http://www.w3.org/2001/04/xmldsig-more#KeyName
   http://www.w3.org/2001/04/xmldsig-more#KeyValue
   http://www.w3.org/2001/04/xmldsig-more#PKCS7signedData
   http://www.w3.org/2001/04/xmldsig-more#rawPGPKeyPacket
   http://www.w3.org/2001/04/xmldsig-more#rawPKCS7signedData
   http://www.w3.org/2001/04/xmldsig-more#rawSPKISexp
   http://www.w3.org/2001/04/xmldsig-more#rawX509CRL
   http://www.w3.org/2001/04/xmldsig-more#RetrievalMethod