The Signal Private Group System and Anonymous Credentials Supporting Efficient Verifiable Encryption

Speaker: Trevor Perrin and Melissa Chase , Signal and Microsoft Research

Date: Thursday, April 01, 2021

Time: 2:00 PM to 3:00 PM Note: all times are in the Eastern Time Zone

Public: Yes


Event Type: Seminar

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Host: Srini Devadas, CSAIL

Contact: Kyle L Hogan,

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Reminder Subject: TALK: The Signal Private Group System and Anonymous Credentials Supporting Efficient Verifiable Encryption


We present a system for maintaining a membership list of users in a group, which has been deployed in the Signal Messenger secure messaging app. The goal is to support *private groups* where membership information is readily available to all group members but hidden from the service provider or anyone outside the group. In our 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 G of prime order. The benefit of the new KVAC is that attributes may be elements in G whereas previous schemes could only support attributes that were integers modulo the order of 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 G the system is efficient, even for large groups.


Topic: CSAIL Security Seminar
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Security & Cryptography

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Created by Kyle L Hogan Email at Thursday, February 18, 2021 at 3:25 PM.