數(shù)字信封是用來(lái)解決()。A、公鑰分發(fā)問(wèn)題 B、私鑰分發(fā)問(wèn)題 C、對(duì)稱(chēng)密鑰分發(fā)問(wèn)題 D、數(shù)據(jù)完整性問(wèn)題