[messaging] Multiple devices and key synchronization: some thoughts

Tony Arcieri bascule at gmail.com
Fri Jan 2 10:12:27 PST 2015

On Fri, Jan 2, 2015 at 1:47 AM, carlo von lynX <
lynX at i.know.you.are.psyced.org> wrote:

> Tony, I would like to challenge the idea of necessity of a "bootstrap
> message" - that is to write to a person by either using her master
> key or encrypting to all currently known keys. If you are in a social
> relationship with that person you must have absolved a communication
> bootstrap procedure (using QR codes, shared secrets, social graph
> adoption or bluetooth handshake.. whatever) and thus you should be
> having an ongoing ephemeral key for each person you talk to. Both
> Briar and Pond use ephemerals once the communication is started. The
> challenge in this case is rather to synchronize the ephemerals among
> the devices, and that can be done with a pubsub channel link between
> the devices. Doesn't that make sense?

How do you handle network partitions? As a more specific example, how do
you handle two devices which aren't ever necessarily on at the same time?
Do you synchronize this ephemeral key via an always-on server of some sort?

My larger question was about revocation though. If you've lost one of the
devices containing your long-lived key, now what? Do you revoke it or not
and if so, how do you synchronize the new key to all devices?

Having a separate key per device makes revocation easier, as you can just
revoke a device-specific key then go on your way.

Tony Arcieri
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