Off-the-Record (OTR) Messaging for Pidgin[1] provides this and hides the complexity from the user. Sadly, all of my Google Talk, AIM, QQ, and MSN contacts have moved to proprietary platforms like whatsapp, skype, and facebook.
I miss the days of 1 messaging platform for all my work and personal chatting. For a solid decade Gaim/Pidgin handled all of this for me.