WhatsApp’s multi-device support is already reaching more and more users: this is how it works on PC even without the mobile connected

In July 2021, WhatsApp announced the arrival of cross-device support. This new version allowed for example be able to use WhatsApp on the PC without having the mobile connected.

The feature was initially rolled out in beta, but the version already reaches all users. Those who have been testing it for a long time are very satisfied, and it’s a good time to analyze how this version works and its limits.

WhatsApp’s cross-device support comes out of beta

Although it’s been in preview for months, WhatsApp’s cross-device support started rolling out to all users late last year. When this is the case, the tab of the PC browser we are using displays a small message informing us of this option:

From there, all you have to do is click on this message to start the update process, which once again tells us that thanks to this version it will no longer be necessary for our mobile to remain connected to use WhatsApp on its web version, on your computer or on other devices.

When updating the version, the WhatsApp session that we had active on the PC will be disconnected, but we can reconnect our phone immediately, and from that moment this WhatsApp session will be will allow you to send and receive messages even if your mobile is not connected to a Wi-Fi network or a mobile data network.

What is multi-device WhatsApp and how does its beta version work?

The option This is very reminiscent of the native operation of other email clients like Telegram, which are not associated with a mobile number —session authentication is requested by sending an SMS to our smartphone— and which have a different operating scheme from that offered by WhatsApp from the start.

In Meta (formerly Facebook, the parent company of WhatsApp) they explained in July 2021 how the idea was to “remove smartphones from the equation”. Until then, the mobile was the true source of data “and the only device capable of end-to-end message encryption or initiating calls”. What they did with options like the native WhatsApp for Windows client or its web client was basically “reflect content” through these interfaces.

The problem is that this limited the options for auxiliary devices and clients, especially if the mobile was disconnected or had a low battery. Additionally, we could only have one client/session/device associated with the session on mobile: if you were using two computers and wanted to use WhatsApp Web on both, you could not: each showed you the message “Use here” so that this “mirror” functionality of WhatsApp on mobile is offered on this device/browser and only on that one.

How WhatsApp works and its cross-device support

So far every WhatsApp user has been identified with a single unique key by which all encrypted communication keys were derived. Unlike this solution, in multi-device support, each device has its own identity key.

This was the system used until now to send a message on WhatsApp. Mobile addiction was absolute.

It is the WhatsApp server that maintains a sort of card that records each user’s account and all of their device identities. When someone wants to send a message, their passwords are taken from the list of devices they have created on the server, and thanks to this it is possible use one of them independently: Mobile is actually out of the equation.

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This scheme also includes several security mechanisms that verify the devices to which messages are sent and establish a system of trust between the devices on our list.

With the new architecture, the mobile is independent of linked devices, which can send and receive messages even if this mobile loses connection or is switched off.

To maintain privacy, multi-device WhatsApp uses a system where the WhatsApp client sending the message encrypts it and stream it n times on n different devices (those in the sender’s and receiver’s device lists). Each message is individually encrypted using an encrypted session between device pairs, and messages are not stored on the server once delivered.

Regarding the video calls and callsTo maintain end-to-end encryption across multiple devices, WhatsApp does the following:

  • The sender generates a set of random 32-byte SRTP master secret keys for each of the receiver’s devices.
  • The sender sends an incoming call message (using the client-fanout approach we discussed earlier) to each of the receiver’s devices. Each receiving device receives this message, which contains the encrypted SRTP master secret key.
  • If the contact answers the call from one of the devices, an SRTP encrypted call is initiated, protected by the SRTP master secret key generated for that device.

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For group calls, the server randomly select a device that is in communication (either the sender or a device on which a user accepted the call) to generate the master key, which it then sends to other active participating devices via end-to-end peer-to-peer encryption. This process is repeated and the keys are reset each time someone joins or leaves the call.

Another question that may arise is whether the chat history is kept between all devices? The answer is yes. WhatsApp syncs message history and other app data, such as contact names or whether there are starred messages in a conversation across all devices. All this data is also encrypted.

The service works very well although yes, there are some limitations

There are users who have been using this version of WhatsApp for a long time thanks to the fact that it was available in beta. In the Xataka team we have very good impressions of two of our colleagues: Joseph Garcia (@josedextro) told us how he has been using this version for a few months and “the truth is that it is very good, since you do not need to have the phone connected at all times and messages sync seamlessly across multiple devices“.

There is something important in his impressions: for the user “the experience does not change too much to use it with the connected mobile. In fact, it’s virtually identical, just with fewer strings attached.“. This is one of the keys that makes this update practically “invisible”, since we will not notice anything particularly different except for the advantage of being able to use WhatsApp on several devices at the same time even if the mobile is disconnected.

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Of course, Jose reminds us that the function “does not work with two phones. In other words, you can use one mobile and up to four devices (laptops, PCs, etc.), but not two mobiles, what I miss the most, devoting myself to what I’m doing due to analytics frequent smartphones that we do at Xataka”.

Henri Perez (@lyzanor) explained to us how for him there is a clear leading role for WhatsApp Web because “I have a few WhatsApp groups where they talk all day” and working from a PC or laptop makes it more comfortable to use this option to chat from mobile.

“The advantage of this new option of not needing a mobile phone is that I don’t have to worry about the battery”, commented Enrique, adding that “I have a somewhat old mobile phone with a fairly weak battery and on top of that it does not charge me well, because some days I wake up and find that it has not been charged at night and I leave with 5% of the day before. These days having WhatsApp Web in this way helps me a lot“.

That the multi-device support of WhatsApp is therefore good news, but it must be said that this version has certain limitsto know:

  • You will not be able to see real-time location on paired devices.
  • You won’t be able to pin chats on WhatsApp Web or Desktop.
  • You won’t be able to view or reset group invites or join from WhatsApp Web or Desktop. To do this, you will need to use your phone.
  • You will not be able to message or call contacts using a very outdated version of WhatsApp on their phones from your linked device.
  • You will not be able to call from Portal or WhatsApp Desktop to linked devices that are not using the multi-device beta.
  • Other WhatsApp accounts will not work on your portal if they have not been added to the cross-device beta.
  • WhatsApp Business users will not be able to edit their business name or labels from WhatsApp Web or Desktop.

It is likely that many of these limitations will disappear in future versions of WhatsApp, but of course the cross-device support is attractive for many users who can start using it directly. thanks to this deployment which is finally consolidating and reach the general public.