In contrast, the open source Signal messenger provides a cryptographic assurance that only an existing group member designated as the group admin can add new members. In an email, researcher Benjamin Dowling, also of King’s College, explained:
Signal implements “cryptographic group management.” Roughly this means that the administrator of a group, a user, signs a message along the lines of “Alice, Bob and Charley are in this group” to everyone else. Then, everybody else in the group makes their decision on who to encrypt to and who to accept messages from based on these cryptographically signed messages, [meaning] who to accept as a group member. The system used by Signal is a bit different [than WhatsApp], since [Signal] makes additional efforts to avoid revealing the group membership to the server, but the core principles remain the same.
On a high-level, in Signal, groups are associated with group membership lists that are stored on the Signal server. An administrator of the group generates a GroupMasterKey that is used to make changes to this group membership list. In particular, the GroupMasterKey is sent to other group members via Signal, and so is unknown to the server. Thus, whenever an administrator wants to make a change to the group (for instance, invite another user), they need to create an updated membership list (authenticated with the GroupMasterKey) telling other users of the group who to add. Existing users are notified of the change and update their group list, and perform the appropriate cryptographic operations with the new member so the existing member can begin sending messages to the new members as part of the group.
Most messaging apps, including Signal, don’t certify the identity of their users. That means there’s no way Signal can verify that the person using an account named Alice does, in fact, belong to Alice. It’s fully possible that Malory could create an account and name it Alice. (As an aside, and in sharp contrast to Signal, the account members that belong to a given WhatsApp group are visible to insiders, hackers, and to anyone with a valid subpoena.)
