[messaging] Multiple devices and key synchronization: some thoughts

Trevor Perrin trevp at trevp.net
Sat Jan 3 12:13:34 PST 2015

On Sat, Jan 3, 2015 at 12:06 AM, Michael Rogers
<michael at briarproject.org> wrote:
> On 02/01/15 20:44, Trevor Perrin wrote:
>> Without a single "master" or "identity" key though, I'm not sure
>> how TOFU or out-of-band verification (like "fingerprints") would
>> work.
>> For example, suppose I wanted to print something on my business
>> card that was sufficient for someone to send a message that all my
>> devices can decrypt.
> Let me say first of all that I don't think we should get hung up on
> business cards. Billions of people use mobile phones as their main or
> sole messaging devices; very few people print their own business
> cards. We should focus on the tech that people actually use.

OK, but that was just one example of the general case "Alice sends a
message to Bob asynchronously (i.e. Bob is not online), authenticated
by Bob's fingerprint".

Alice might have found Bob's fingerprint on his business card, or
social media profile, or through some directory lookup, or by
corroborating the service-provided fingerprint with her friends.

This case is possible if Bob has a single master public key that
either signs device-specific public keys, or whose private key is
shared between devices.  It's not possible if Alice must interact with
one of Bob's devices to learn about the others.

In asynchronous scenarios (like email or text messaging), this seems a
disadvantage of your approach compared to the (1)-(4) proposals.  What
do you see as the advantages?


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