WhatsApp’s end-to-end encryption relies on the generation of unique security keys, using the acclaimed Signal protocol, developed by Open Whisper Systems, that are traded and verified between users to guarantee communications are secure and cannot be intercepted by a middleman.

However, WhatsApp has the ability to force the generation of new encryption keys for offline users, unbeknown to the sender and recipient of the messages, and to make the sender re-encrypt messages with new keys and send them again for any messages that have not been marked as delivered.