The Signal app never did the "primary device serves message contents to companion devices" like WhatsApp did.

Ever since Signal introduced the desktop client, its multidevice functionality has involved fully fledged signal-protocol sessions between each device.

It sounds like WhatsApp is now adding something like this to their client, though it sounds like some of the details of the scheme differ.

In principal the signal protocol works fine for a multidevice scenario.

> The WhatsApp server maintains a mapping between each person’s account and all their device identities. When someone wants to send a message, they get their device list keys from the server.

Oh, no...

What's your concern?

> they get their device list keys from the server

Which means the server can just substitute keys in.

Right, but that doesn't compromise the security of the service necessarily.

Users can catch a malicious server injecting incorrect keys by looking at security notifications and comparing security codes. This is part of the Signal protocol.

This may be tedious but only needs to be done in the event of phone keys getting reset (a once in a year event?), as all companion device keys are automatically verified with signatures provided from an account owner's primary (phone) device

Source: https://www.whatsapp.com/security/WhatsApp_Security_Whitepap...

Maybe read the article?

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> Please don't comment on whether someone read an article. "Did you even read the article? It mentions that" can be shortened to "The article mentions that."