Notices where this attachment appears
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More is Less: How group chats weaken the security of instant messengers #Signal, #WhatsApp and !threema https://eprint.iacr.org/2017/713.pdf
TL;DR: Attackers can often rewind members of group to previous state (replay attack), WhatsApp can add arbitrary users and thus circumvent #e2e crypto in all group chats easily.
BTW: #Threema fixed the issues, WhatsApp ignored, Signal wants to introduce a new protocol in the future.
/cc !verschluesselung