A team of researchers confirmed that behavior in a recently released formal analysis of WhatsApp group messaging. They reverse-engineered the app, described the formal cryptographic protocols, and provided theorems establishing the security guarantees that WhatsApp provides. Overall, they gave the messenger a clean bill of health, finding that it works securely and as described by WhatsApp.
They did, however, confirm a behavior that should give some group messaging users pause: Like other messengers billed as secure—with the notable exception of Signal—WhatsApp doesn’t provide any sort of cryptographic means for group management.
“This means that it is possible for the WhatsApp server to add new members to a group,” Martin R. Albrecht, a researcher at King's College in London, wrote in an email. “A correct client—like the official clients—will display this change but will not prevent it. Thus, any group chat that does not verify who has been added to the chat can potentially have their messages read.” //
By 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. //
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.)
Signal does, however, offer a feature known as safety numbers. It makes it easy for a user to verify the security of messages or calls with specific contacts. When two users verify out-of-band—meaning using a known valid email address or cell phone number of the other—that Signal is displaying the same safety number on both their devices, they can be assured that the person claiming to be Alice is, in fact, Alice.
