Gilles Brassard Claude Crépeau Moti Yung Everything in NP can be Argued in Perfect Zero-Knowledge in a Bounded Number of Rounds. 123-136 1989 conf/icalp/1989 ICALP db/conf/icalp/icalp89.html#BrassardC89 http://dx.doi.org/10.1007/BFb0035756