Paper 2019/1416
The Signal Private Group System and Anonymous Credentials Supporting Efficient Verifiable Encryption
Melissa Chase and Trevor Perrin and 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.
Note: See the related blog post at https://signal.org/blog/signal-private-group-system/
Metadata
- Available format(s)
- Category
- Cryptographic protocols
- Publication info
- Preprint. MINOR revision.
- Keywords
- secure messagingsecure group messaginganonymous credentialsverifiable encryptionprivacy-preserving systems
- Contact author(s)
- melissac @ microsoft com,trevp @ signal org,gregz @ microsoft com
- History
- 2020-11-10: last of 5 revisions
- 2019-12-09: received
- See all versions
- Short URL
- https://ia.cr/2019/1416
- License
-
CC BY