A more dramatic revelation of 2018—an outsider can secretly eavesdrop on your private end-to-end encrypted group chats on WhatsApp and Signal messaging apps.
Considering protection against three types of attackers—malicious user, network attacker, and malicious server—an end-to-end encryption protocol plays a vital role in securing instant messaging services.
The primary purpose of having end-to-end encryption is to stop trusting the intermediate servers in such a way that no one, not even the company or the server that transmits the data, can decrypt your messages or abuse its centralized position to manipulate the service.
In order words—assuming the worst-case scenario—a corrupt company employee should not be able to eavesdrop on the end-to-end encrypted communication by any mean.
However, so far even the popular end-to-end encrypted messaging services, like WhatsApp, Threema and Signal, have not entirely achieved zero-knowledge system.
Researchers from Ruhr-Universität Bochum (RUB) in Germany found that anyone who controls WhatsApp/Signal servers can covertly add new members to any private group, allowing them to spy on group conversations, even without the permission of the administrator.
As described by the researchers, in the pairwise communication (when only two users communicate with each other) server plays a limited role, but in case of multi-user chats (group chat where encrypted messages are broadcasted to many users), the role of servers increases to manage the entire process.
That’s where the issue resides, i.e. trusting the company’s servers to manage group members (who eventually have full access to the group conversation) and their actions.
As explained in the newly published RUB paper, titled « More is Less: On the End-to-End Security of Group Chats in Signal, WhatsApp, and Threema, » since both Signal and WhatsApp fail to properly authenticate that who is adding a new member to the group, it is possible for an unauthorized person—not a group administrator or even a member of the group—to add someone to the group chat.
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