[messaging] Identity keys and fingerprints

David Leon Gil coruus at gmail.com
Mon Jan 5 22:13:08 PST 2015

So, supposing that no other mechanism is available:

Take out the identity key from its Faraday cage, sign a statement of
which device keys are good and which are compromised, and publish that
statement. (And then hide the long-term key away again.)

Same identity afterwards.

On Mon, Jan 5, 2015 at 10:07 PM, zaki at manian.org <zaki at manian.org> wrote:
> 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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