[messaging] Multiple devices and key synchronization: some thoughts

[Joseph Bonneau]

On Mon, Dec 29, 2014 at 6:52 PM, Maxwell Krohn <themax at gmail.com> wrote:

> I mentioned to David off-list that we considered but didn't pursue another
> multi-device option for signatures.  It would be to use a protocol such as
> 2-Schnorr [1].
>

This is a nice protocol but it's solving a different problem being
discussed initially in this thread. I think it's worth starting from the
high-level user experience we want here before diving into the crypto,
because people are already discussing crypto protocols which provide a
pretty different UX. Ignoring setup/pairing, which is a pain in almost any
protocol, there are three possible versions of the "multi device UI" which
have already been proposed in this thread:

*A user has multiple devices, any one of which can read messages if it is
online (Trevor's #2/3/4 all fit here as do all of David's proposals)
*A user has multiple devices, one "master" (or "home server") of which must
be online for the user to be able to read messages at any other device
(this was Trevor's #1)
*A user has multiple devices, two of which must be online to sign something
and set up a channel (2-Schnorr?)

There are many other combos when you get in to issuing/revoking/changing
keys. For example, you might also use the 2-Schnorr protoocl only to
protect some meta-key to sign other device keys, and not for routine
messages.

In any case, I would advocate that any system needs to be flexible for
different users to choose multiple options based on their security
preferences. I suspect most users will want a simple baseline UI along the
lines of iMessage (or almost any other chat app) today, which is that you
can enroll any new device instantaneously with a username/password only and
no pairing protocol. I think if you want to design a mass-market system,
anything involving an explicit device pairing-protocol needs to be an
opt-in feature.
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