The Signal Protocol is the most-deployed piece of cryptographic engineering in human history, sitting under Signal, WhatsApp, RCS, and Messenger. We walk what end-to-end encryption actually buys you, the X3DH initial key agreement, the symmetric and Diffie-Hellman ratchets, message ordering and out-of-order delivery, and what an attacker who compromises a device can and cannot recover.
End-to-End-Encryption
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The Signal Protocol and the Double Ratchet