Support linking/pairing with other devices

[kazenshi]

Description

One valuable feature I would love to see out of a messaging app is the ability to link/pair with other devices. On Signal and WhatsApp the feature is called Linked Devices and on Google Messages it is called Device Pairing.

The use case for this feature is sending, viewing, and responding to messages from any device that I have linked/paired.

[e227474]

There's 2 ways I can see this being accomplished.

1. A local webserver running on the phone and that's used to relay the messages to a client on another device on the same WiFi network.

2. A GrapheneOS web service is used to forward the messages with end-to-end encryption to the linked device.

This part is applicable to both options: a web client accessible via a standard website is the most universal option but problematic as it requires trusting the website with the messages as there's not a way to automatically detect tampering of the code from a malicious server.

Isolated Web Apps would be the most ideal option but currently they're only supported on ChromeOS behind a toggle or for sysadmins to deploy to users. An alternative would be using an offline HTML file which has periodic manual update checks, but it still trusts the server whenever you "update" it by downloading a new HTML file as there's no signature verification. Instead desktops apps for macOS, Windows which are WebView wrappers with a self-update mechanism with signature verification could be made as well as an Android app. Apple doesn't allow WebView wrappers on iOS/iPadOS so an alternative approach would be needed for that.

[kazenshi]

I actually use Beeper which uses Matrix bridges under the hood. I connect signal via a local bridge bridge implementation that they maintain which keeps everything E2E encrypted and they don't see the traffic.

I'm open to either option so long as E2E encryption is maintained

[e227474]

@kazenshi You loose PFS by using a matrix bridge, not to mention that matrix has absolutely horrendous metadata privacy when compared to Signal. While technically E2EE is maintained, the security and privacy is greatly degraded.

[Anonymous941]

Another option could be to sync messages to a directory with .json files so you could use services like Syncthing or Rclone, which have encryption. It probably couldn't be used to send messages but it's easy to implement; it's already mostly implemented with the dump raw messages debug feature.