[messaging] Identity keys and fingerprints

[zaki at manian.org]

What happens in David's formulation if one were to need to revoke one
device?

One downside of Trevor's prefered formulation is that loss of control of
one device means restart one's identity from scratch....

On Mon, Jan 5, 2015 at 10:03 PM, David Leon Gil <coruus at gmail.com> wrote:

> On Mon, Jan 5, 2015 at 4:18 PM, Trevor Perrin <trevp at trevp.net> wrote:
> > The most practical approaches are probably either synchronizing the
> > identity key between devices, or using it to sign device keys.  Either
> > way, adding a new device might increase communication in (f), since
> > Alice might have to retrieve additional device-specific prekeys,
> > and/or signed device keys.
>
> There is no need for an "identity key" to sign anything except an
> initial device key. Just chase cross-signatures back to a
> distinguished (by some flag) identity key that is stored offline, and
> use a hash of that as the fingerprint:
>
> xsign(device0t0, identity)
> xsign(device0t0, device1t0)
> xsign(device0t1, device2t0)
>
> chase_fingerprint(device2) == hash(identity)
>
> (Many thank to Yan for suggesting this approach to stable
> fingerprints, though I don't know that she endorses it.)
>
> This neatly avoids exposing any long-term keys to additional risk of
> compromise.
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