INSUBCONTINENT EXCLUSIVE:
The flow of adding new members to a WhatsApp group message is:A group member sends an unsigned message to the WhatsApp server that
designates which users are group members, for instance, Alice, Bob, and CharlieThe server informs all existing group members that Alice,
Bob, and Charlie have been addedThe existing members have the option of deciding whether to accept messages from Alice, Bob, and Charlie,
and whether messages exchanged with them should be encryptedWith no cryptographic signatures verifying an existing member wants to add a new
member, additions can be made by anyone with the ability to control the server or messages that flow into it
Using the common fictional scenario for illustrating end-to-end encryption, this lack of cryptographic assurance leaves open the possibility
cryptographic assurances for new group members
The Telegram messenger, meanwhile, offers no end-to-end encryption for group messages, making the app among the weakest for ensuring the
confidentiality of group messages.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
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,
(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.)
