How WhatsApp enables multi-device capability

With this new capability, you can now use WhatsApp on your phone and up to four other nonphone devices simultaneously - even if your phone battery is dead. Each companion device will connect to your WhatsApp independently while maintaining the same level of privacy and security through end-to-end encryption that people who use WhatsApp have come to expect. The current WhatsApp experience for companion devices on web, macOS, Windows, and Portal uses a smartphone app as the primary device, making the phone the source of truth for all user data and the only device capable of end-to-end encrypting messages for another user, initiating calls, etc. The WhatsApp server maintains a mapping between each person's account and all their device identities. We use a combination of technologies to solve this: First, we have extended security codes to now represent the combination of all of someone's device identities so that anyone and their contact can always verify all the devices they are sending messages to. Where the WhatsApp client sending the message encrypts and transmits it N number of times to N number of different devices - those in the sender and receiver's device lists. We want to ensure that people have a consistent experience with WhatsApp no matter the device they are using. For message history: When a companion device is linked, the primary device encrypts a bundle of the messages from recent chats and transfers them to the newly linked device.