Proposal for storing megolm keys serverside

[benparsons]

Documentation: #1538
Author: @ara4n, @uhoreg
Date: 23/11/2017

[uhoreg]

The API was designed for option 1, and mostly works for option 2, but it seems like the /room_keys/version API could work better with the PK encryption, and doesn't seem to support the "Verifying the device [new] with an existing device, so the device gets a copy of the recovery-key public key, and can start backing up into the same session" use case.

I think one way to support that is to have the client use the version API to upload the public key for the backup, signed with the device's signing key, along with the device ID. When a new device signs in and wants to back up to that version, then it prompts the user to verify one of the devices that signed the public key. Once the device has been verified, the new device can sign the public key and upload its signature, so that newer devices can check the public key by verifying that device. (Alternatively, we could integrate with the cross-signing data somehow, so that we don't need multiple signatures.)

I think most of this (other than uploading other signatures) can be done with the existing API by changing the contents of the auth_data.