[tahoe-dev] Invitation protocol

[Michael Rogers]

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On 14/07/12 00:11, Brian Warner wrote:
> I'm most interested in using the invitation code to also
> *establish* a channel, since for things like Tahoe, there's nothing
> to bootstrap from. If the Tahoe client were also an IRC client, or
> an MUA, then it could quietly insert extra data into your IMs or
> emails to inform the agent on the other side of its IP address/etc.
> But when it's a standalone program, it's the human who gets the job
> of connecting agents together, which puts more of a burden on the
> protocol, since asking the human to transcribe an IP address and
> port number is boring and error-prone.

In Briar we use the six-digit invitation code to seed a PRNG, which
the transport plugin can use however it wants to arrange a rendezvous
between the endpoint devices.

The Bluetooth plugin uses the PRNG to generate a service UUID - the
responder's device creates a service with that UUID and the
initiator's device scans for it.

The LAN plugin uses the PRNG to generate the address and port of a
multicast group - both devices join the group, the initiator's device
sends a UDP packet to the group containing its TCP address and port,
and the responder connects to it.

Another possibility would be to use the PRNG to generate a BitTorrent
infohash; both devices would publish their IP addresses and ports in
the BitTorrent DHT under that infohash.

The invitation code is not assumed to be secret or unguessable - it
just serves to distinguish Alice and Bob from any other pairs of users
who may be trying to exchange keys in the same place at the same time.
For the DHT case, or any other case using a globally shared resource,
you'd probably want to use more than six digits for the invitation
code to reduce the probability of accidental collisions.

Cheers,
Michael

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