Christian Decker [ARCHIVE] on Nostr: 📅 Original date posted:2020-02-24 📝 Original message: Hi Bastien, seems you ...

📅 Original date posted:2020-02-24
📝 Original message:
Hi Bastien,

seems you were a bit quicker than I was with my writeup of my
proposal. I came up with a scheme that allows us to drop a large part of
the partial onion, so that it can indeed fit into an outer onion, and
the rendez-vous node RV can re-construct the original packet from the
included data [1].

The construction comes down to initializing the part of the routing info
string that is not going to be used, in such a way that the incremental
unwrappings at the nodes in the partial onion cancels out. Like you
mentioned in your mail it comes down extending the filler generation to
also cover the unused part and then applying all the encryption streams
xored to the unused space. By doing this we get the middle part of the
onion consisting of only 0x00 bytes.

I then decided to apply an additional ChaCha20 stream to this prefill,
such that the onion will not consist of mostly 0x00 bytes which would be
a dead giveaway to `RV+1` that `RV` was a rendez-vous node.

The process for the partial onion creator boils down to:

- Compute a path from `RV` of its choice to recipient `R`.
- Compute a shared secret using a random ephemeral private key and
`RV`s public key, and then generate a prefill-key
- Compute the prefill by combining the correct substrings of the
encryption streams for the nodes along the path, then add the
ChaCha20 stream keyed with the prefill-key.
- Wrap the onion, including payloads for each of the nodes along path
`RV` to `R`
- Trim out the unused space, which now will match the obfuscation
stream generated with the prefill-key

As an example such an onion, with 5 legacy hops (65 byte each) results
in a 325 + 66 bytes onion, and we save 975 bytes. See [2] for an example
of how this looks like.

The sender `S` then just does the following:

- Compute a route from `S` to `RV`
- Build an onion with the route, specifying the trimmed partial onion
as payload, along with the usual parameters, for `RV`
- Initiate payment with the constructed onion

Upon receiving an incoming HTLC with a partial onion the rendez-vous
node `RV` then just does the following:

- Verify all parameters as usual
- Extract the partial onion
- Use the ephemeral key from the partial onion to generate the shared
secret and the prefill key
- Generate the prefill stream and insert it in the correct place,
before the HMAC. This reconstitutes the original routing packet
- Swap out the original onion with the reconstituted onion and forward.

My writeup [1] is an early draft, but I wanted to get it out early to
give the discussion a basis to work off. I'll revisit it a couple of
times before opening a PR, but feel free to shout at me if I have
forgotten to consider something :-)

Cheers,
Christian

[1] https://github.com/lightningnetwork/lightning-rfc/blob/rendez-vous/proposals/0001-rendez-vous.md
[2] https://gist.github.com/cdecker/ec06452bc470749d9f6d2de73651c5fd

Bastien TEINTURIER via Lightning-dev
<lightning-dev at lists.linuxfoundation.org> writes:
> Good morning list,
>
> After exploring decoys [1], which is a cheap way of doing route blinding,
> I'm turning back to exploring rendezvous.
> The previous mails on the mailing list mentioned that there was a
> technicality
> to make the HMACs check out, but didn't provide a lot of details.
> The issue is that the filler generation needs to take into account some hops
> that will be added *later*, by the payer.
>
> However it is quite easy to work-around, with a few space trade-offs.
> Let's consider a typical rendezvous setup, where Alice wants to be paid via
> rendezvous Bob, and Carol wants to pay that invoice:
>
> Carol -> ... -> Bob -> ... -> Alice
>
> If Alice knows how many bytes Carol is going to use for her part of the
> onion
> payloads, Alice can easily take them into account when generating her
> filler by
> pre-pending the same amount of `0` bytes. It seems reasonable to impose a
> fixed
> number of onion bytes for each side of the rendezvous (650 each?) so Alice
> would
> know that amount.
>
> When Carol completes the onion with her part of the route, she simply needs
> to
> generate filler data for her part of the route following the normal Sphinx
> protocol
> and apply it to the onion she found in the invoice.
>
> But the tricky part is that she needs to give Bob a way of generating the
> same
> filler data to unapply it. Then all HMACs correctly check out.
>
> I see two ways of doing that:
>
> * Carol simply sends that filler (650 bytes), probably via a TLV in
> `update_add_htlc`.
> This means every intermediate hop needs to forward that, which is painful
> and
> potentially leaking too much data.
> * Carol provides Bob with the rho keys used to generate her filler, and the
> length
> used by each hop. This leaks to Bob an upper bound on the number of hops
> and the
> number of bytes sent to each hop.
>
> Since shift-and-xor kind of crypto is hard to read as equations, but very
> easy to
> read as diagrams, I spent a bit of time doing beautiful ASCII art [2].
> Don't hesitate
> to have a look at it to find more details about how that works. You can
> also print
> that on t-shirts to look fancy at conferences. I also have some sample code
> working
> in eclair [3] for those who can read Scala without getting headaches.
>
> Are there other tricks we can use to reconcile both sides of the onion at
> Bob's?
> Maybe cdecker (or someone else) has an ace up his sleeve for me there? :)
>
> One important thing to note is that rendezvous on normal onions will be
> costly to
> integrate into invoices: it takes 1366 bytes to include one onion, and if
> we want
> to handle route failures or let the sender use multi-part, we will need to
> have a
> handful of pre-encrypted onions in the invoice (hence a few kB, which may
> not be
> practical for QR codes).
>
> But I did mention before that doing rendezvous on the trampoline onion
> could have
> better properties [4]. When doing that, having Carol transmit her filler
> data only
> to Bob, via the outer onion payload becomes practical and doesn't leak
> information.
> Multi-part would work with a single trampoline onion in the invoice (~500
> bytes),
> because nodes can do MPP between trampoline nodes thanks to the
> onion-in-onion
> construction. We simply need to decide the size of the trampoline onion to
> allow
> each side of the rendezvous to be able to insert a number of hops we're
> comfortable
> with. You can find more details in the "Rendezvous on a trampoline" section
> of [2].
>
> I'm really interested in other approaches to making rendezvous work with
> the HMACs
> correctly checking out. If people on this list have drafts, intuitions or
> random
> thoughts about possible constructions, please share them, I'd be happy to
> dive into
> them to explore alternatives to the one I found, hoping we can make this
> work and
> provide this feature to our users in the near future.
>
> A small side-note on Hornet. Hornet does offer many features that I believe
> we will
> want in Lightning in the future. It may seem that doing a custom rendezvous
> scheme
> is a waste of time since we'll ditch it once/if we implement Hornet. While
> that is
> true in the long run, I believe that if we're able to find a rendezvous
> scheme that
> isn't too much work to implement, it makes sense to have something
> available soon-ish.
> Hornet will likely be a longer-term effort that we won't get as soon as
> we'd like
> (especially since it will probably require a network-wide update). But who
> knows, maybe
> we may see that we are trying to create many features that are already
> built into Hornet
> (rendezvous, directed message support, etc) and will decide to implement
> Hornet sooner
> than expected?
>
> Cheers,
> Bastien
>
> [1]
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-January/002435.html
> [2] https://gist.github.com/t-bast/ab42a7f52eb2e73105557957c8359601
> [3] https://github.com/ACINQ/eclair/tree/sphinx-rendezvous
> [4]
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-October/002237.html
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