A system and method for achieving authorization in confidential group communications in terms of an ordered list of data blocks representing a tamper-resistant chronological account of group membership updates. This method permits ad-hoc and decentralized group definition, dynamic and decentralized membership updates, open sharing, tamper resistance, and tracking of membership history. There are many applications of these techniques. One such application is enabling end-to-end encryption of instant messaging, content sharing, and streamed media.