Holger Vogt Henning Pagnia Felix C. Gärtner Modular Fair Exchange Protocols for Electronic Commerce. 3-11 1999 conf/acsac/1999 ACSAC http://csdl.computer.org/comp/proceedings/acsac/1999/0346/00/03460003abs.htm db/conf/acsac/acsac1999.html#VogtPG99