Ulrich Lang Dieter Gollmann Rudolf Schreiner Verifiable Identifiers in Middleware Security. 450-462 2001 conf/acsac/2001 ACSAC http://csdl.computer.org/comp/proceedings/acsac/2001/1405/00/14050450abs.htm db/conf/acsac/acsac2001.html#LangGS01