[messaging] Unlinkable rendezvous via human-sized keys (was: Re: Human sized keys)

Trevor Perrin trevp at trevp.net
Thu Mar 20 18:19:19 PDT 2014

On Thu, Mar 20, 2014 at 8:44 AM, Daniel Kahn Gillmor
<dkg at fifthhorseman.net>wrote:

> On 03/20/2014 03:11 AM, Trevor Perrin wrote:
> > You'd have users exchange ~160-bit ECDH keys directly.  I'd have users
> > exchange (introduction server name, ~128-bit fingerprint) and use these
> to
> > lookup an "introduction cert" where the fingerprinted long-term key
> signs a
> > short-term ECDH key.
> >
> > Your approach eliminates the need to mask the intro-cert lookup via PIR
> or
> > dummy traffic.  But it lowers the security of your long-term key from
> ~128
> > bits to ~80 bits, and reduces "forward-secrecy of linkages", since
> > compromise of the long-term ECDH key (which you've printed on your
> business
> > card, so you're not going to rotate it frequently) allows going through
> > published rendezvous messages and linking correspondents for the key's
> > lifetime.
> Watson's scheme is also doable with ephemeral keys, you just wouldn't
> have them on your business card -- each user could have their machinery
> generate a stash of ephemeral keys and print out one card per key.  each
> card would have the public key of a single ephemeral key written on one
> half of the card (the "peer" half), and a short tag on the other half
> that identifies the private key in your client's ephemeral stash (the
> "self" half)
> you meet someone who also plays this game, and the two of you take the
> top card from each of your stacks, tear it in half, give the peer half
> to the other person, and staple or tape or otherwise pair up the two
> pieces for use later when you're online.

Yeah, I think that loses the main benefit of a DH rendezvous though, which
is that each party has a single static value which they can print on biz
cards, publish widely for corroboration, or exchange via 3rd parties.

Your process requires:
 - printing and carrying around a stack of perforated tickets
 - tearing and exchanging them
 - attaching them with staples or tape

I think a lot fewer people would be willing / able to do that.

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