Suggestion: Multiple devices

The optimal location for this suggestion is the Wiki. But I do not have access to that, so I decided to post this here. Feel free to move it and close this issue.

This is an alternative proposal for the one at http://wiki.tox.im/Multiple_Devices. A key feature is the fact that it does not require a change to the Tox network protocol and a client (such as Venom) can implement this.

_Summary:_ Instead of creating 2 identities and linking them, the same identity is shared over the 2 devices transparently. This reduces the technical complexity a lot, and reduces user confusion: Each user will have only one public key to share with his friends.

_How it works_:

Linking the devices (Sharing the identity): Alice already has an identity - which is a public/private key pair - on her PC, Alice decides to use her smartphone with Tox. Alice clicks a "Add an existing identity" button on the smartphone. Alice then inputs her public key. In the background, Alice's smartphone creates a temporarily identity and contacts Alice's PC via DHT. It then sends a "linking request", Alice accepts that request on her PC (We could make her type a secret phrase on both ends for security). Once the request is accepted, the Private/Public key pair is transferred - securely, either via DHT or p2p - from Alice's PC to Alice's smartphone. Alice's smartphone discards the temporarily identity and adopts the newly received one. Alice's smartphone then "goes online", see next section.

When a device goes online - Cooperation between devices sharing the same key: When a devices starts up / finishes the step above, it checks if another device is already online with the same public key (Scanning the DHT possibly requires a temporarily identity, correct me here). 2 Things can occur:

A: There is a device already online with that same public key: We'll call the device which is already online the "master" (the PC in Alice's case). We'll call the newly connecting device a "slave" (the smartphone in Alice's case).

The slave contacts the master via DHT (via a temporarily identity), and asks to be synced. The slave can easily prove it has possession of the same private key as the master by completing a challenge. Once this is proven, the master and the slave sync up: The slave(Smartphone) initiates a peer-to-peer connection to the master(PC), it then opts out of the DHT. Now, the phone is merely a "remote interface" to the PC, which is the actual Tox peer: Any action Alice performs on the slave(smartphone) is sent to the master (PC), only the master contacts the DHT and/or Alice's friends. Any action the PC receives from the DHT/from alice's friends is sent to the smartphone.

To sum it up, the slave is a "remote session" of the master.

B: There is no device already online with that same public key: In that case, the newly connecting devices connects normally using the normal Tox procedure, no science here. The devices then becomes the "master".

Some remarks:

1. This also works with multiple slaves. The master should randomly choose a "top slave" among all the connected slaves, in case the master disconnects, the "top slave" becomes the master and the remaining slaves reconnect to their new master.
2. The linking phase is really easy and already partially implemented: It is very similar to accepting a friend request.
3. The entire procedure can be implemented via a client and does not require changes to the Tox network protocol. (But it's best to implement in Tox Core)