What encryption protocol does WhatsApp use for end-to-end encryption?

WhatsApp uses the Signal Protocol for end-to-end encryption. The protocol protects messages so that they are encrypted on the sender’s device and decrypted only on the intended recipient’s device, preventing WhatsApp from reading the message contents in transit.

WhatsApp partnered with Open Whisper Systems, the organization behind Signal, in 2014 to incorporate the protocol. The rollout expanded over time, and on April 5, 2016, WhatsApp announced that end-to-end encryption covered every form of communication on the service, including messages, group chats, photos, videos, voice messages, and calls.

The Signal Protocol is a system rather than a single cipher. It combines mechanisms such as the Double Ratchet Algorithm, prekeys, and key-agreement procedures. AES-256 is one cryptographic primitive associated with implementations, but it is not the name of WhatsApp’s overall encryption protocol. TLS protects network connections, while OTR is a separate messaging protocol.