WhatsApp is rolling out a limited public beta test, which will let certain users experience the multi-device capability that they have been working on. This will let users access the messaging service on their phone and up to four other non-phone devices simultaneously.
We’ve been working on this for a long time. Until now, @WhatsApp has only been available on one device at a time. And desktop and web support only worked by mirroring off your phone – which meant your phone had to be on and have an active internet connection.
— Will Cathcart (@wcathcart) July 14, 2021
WhatsApp says users will be able to access their messages on these other devices, even if their main cell phone’s battery has died. This will mark a change from the current setup where the phone remains the primary device for connectivity and the other devices only mirror off of it, which means that if the phone is not connected to the internet or is switched off, WhatsApp cannot be used.
The feature was a challenging one due to the nature of the encrypting that the messaging app prides itself on. Although airtight, it was difficult to translate it to a multi-device model.
Under the new system, the phone will no longer be the primary device and the user data will remain “seamlessly and securely synchronised and private,” according to a blog by Facebook.
A client-fanout approach would be applied for this, 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.” The message is “individually encrypted using the established pairwise encryption session with each device.”
With multi-device support, each will now have its own identity key. This is a shift from the existing system where a user is identified by a “single identity key from which all encrypted communication keys were derived,” notes the blog. Additionally, the WhatsApp server will maintain a mapping between each person’s account and all their device identities.
In the new multi-device support, claims the blog post on the Facebook engineering site, “each companion device will connect to your WhatsApp independently while maintaining the same level of privacy and security through end-to-end encryption…”
Currently, WhatsApp has made the feature available for only beta testers in certain countries. However, WhatsApp does plan on providing opt-in beta access for users on a stable version that will be accessible via the Linked devices screen in near future.
There is, however, a list of features that would not be supported by the current version. Viewing live location on companion devices, pinning chats on WhatsApp Web or Desktop, joining, viewing, and resetting group invites from WhatsApp Web and Desktop and others are going to be some features one would be void of on companion devices. To check the full list, you can visit the WhatsApp website here.