[enhancement] allow MTP access for authenticated computers

[Jamie Strandboge]

In accordance with https://wiki.ubuntu.com/SecurityAndPrivacySettings/ProtectingUserData, MTP currently refuses to show any folders when connecting the device to a system and the device's screen is locked (good). A nice future enhancement might be to do something similar to what is planned with adb-- using a certificate or token for each system the device connects to. Eg, it might look something like this:

1. Janet plugs in a locked device to her new laptop
2. MTP checks to see if this system (the new laptop) is known to the device
3. Since it is not, MTP checks the state of the screensaver and sees that it is locked, so refuses to export any directories
4. Janet unlocks the screen on the device and the files are exported to her laptop
5. Meanwhile on the device, MTP prompts Janet with:
    Detected new system. Unconditionally export MTP files to this system in the future?
    [ ] yes [ ] no
6. Janet answers 'yes' and MTP adds Janet's new laptop to its database of known devices
7. Janet performs file transfers via MTP, then unplugs the device and goes out to dinner, taking a lot of fun pictures
8. Janet returns home and plugs her locked device into her laptop
9. MTP checks to see if this system (the new laptop) is known to the device
10. Since Janet answered 'yes' in step '6', the files are exported to her laptop (without having to unlock the screen) and she can copy her fun pictures to her laptop

If Janet answered 'no' in step '6', after performing step '9', MTP would proceed to step '3' instead of '10'.

This probably requires design for the user interactions. Eg, perhaps it would be good to remember if the user answered 'no' in step '5'. Also, it would be good to be able to revoke systems from the database of known devices.

This is not for RTM. This is not a security requirement. This is for UX.

[Mathieu Trudel-Lapierre (cyphermox)]

What's the API like for doing this? Anything I can speak to over DBus, or library calls that can be made to check certificates?

I'm guessing this would also only be available if the adb transport is also active.

[Mathieu Trudel-Lapierre (cyphermox)]

Assigned to ogra; only so I can get a bit more information on the APIs for doing that kind of thing.