Welcome to the resource topic for 2019/1416
Title:
The Signal Private Group System and Anonymous Credentials Supporting Efficient Verifiable Encryption
Authors: Melissa Chase, Trevor Perrin, Greg Zaverucha
Abstract:In this paper we present a system for maintaining a membership list of users in a group, designed for use in the Signal Messenger secure messaging app. The goal is to support (\mathit{private}) (\mathit{groups}) where membership information is readily available to all group members but hidden from the service provider or anyone outside the group. In the proposed solution, a central server stores the group membership in the form of encrypted entries. Members of the group authenticate to the server in a way that reveals only that they correspond to some encrypted entry, then read and write the encrypted entries. Authentication in our design uses a primitive called a keyed-verification anonymous credential (KVAC), and we construct a new KVAC scheme based on an algebraic MAC, instantiated in a group (\mathbb{G}) of prime order. The benefit of the new KVAC is that attributes may be elements in (\mathbb{G}), whereas previous schemes could only support attributes that were integers modulo the order of (\mathbb{G}). This enables us to encrypt group data using an efficient Elgamal-like encryption scheme, and to prove in zero-knowledge that the encrypted data is certified by a credential. Because encryption, authentication, and the associated proofs of knowledge are all instantiated in (\mathbb{G}) the system is efficient, even for large groups.
ePrint: https://eprint.iacr.org/2019/1416
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