End-to-End Encryption in Modern Messaging

The Signal Protocol is the cryptographic foundation behind Signal, WhatsApp, and other secure messengers. It combines two algorithms: X3DH for establishing an initial shared secret when the recipient is offline, and the Double Ratchet for deriving a new encryption key for every single message. The result is strong confidentiality and forward secrecy that is transparent to the user.

X3DH allows two parties to establish a shared secret even when one is offline. It uses four Diffie-Hellman computations combining long-term identity keys, medium-term signed prekeys, and one-time prekeys. The server stores prekeys uploaded by the recipient in advance. The sender performs X3DH using those keys, derives a shared secret, and sends an encrypted initial message without waiting for the recipient to come online.