[Lightning-dev] A Payment Point Feature Family (MultiSig, DLC, Escrow, ...)

Lloyd Fournier lloyd.fourn at gmail.com
Fri Oct 11 00:41:34 UTC 2019

Hi Nadav,

I've thought about similar problems before. Essentially you are trying to
create an "access structure" on discrete logarithm (the completion of the
adaptor signature in "pay-to-point"). I think the term for arbitrary
combinations of AND and ORs and even N-of-M is called a *monotone
access structure*.

> Essentially the idea is to use the point S + ECDH(B, E) where S is the
seller's point, B is the buyer's point and E is the escrow's point

I can't see how you can create an access structure like this with ECDH.
Neither B nor E know the discrete log of ECDH(B,E). I can see that you can
hash it and use it as a scalar but then you have to make a heavy zkp to
prove the validity (or interact with the escrow which violates the premise).

Fortunately, I think it is possible to create any monotone access structure
on a discrete logarithm using *verifiable encryption*.  Applying this to
the Escrow payment problem, the buyer decides on the payment point B and
verifiably encrypts the discrete logarithm of B under the Escrow's public
key E and sends the encryption to the seller. If there's a dispute, the
seller sends the encryption to the Escrow. If the Escrow resolve the
dispute in favour of the seller they just decrypt the ciphertext and send
the discrete log of B to the seller. The seller can now redeem the payment.

If we want to create more complex access structures then we use verifiable
secret sharing where the discrete log of B is split up into shares and
distributed according the the desired structure.

So how do we do this verifiable encryption/secret sharing? Well it's not
really straight forward. In the case of Escrow, Camenisch-Shoup [1]
verifiable encryption might be attractive since the Escrow can be trusted
to produce the Paillier modulus properly. Otherwise there's
Camenisch-Damgaard [2] which is much less efficient but only relies on CDH
assumption. As far as I know there are no usable implementations of either
of these schemes but Camenisch-Damgaard is relatively straightforward and I
think it's practical.

[1] https://www.shoup.net/papers/verenc.pdf
[2] https://eprint.iacr.org/1999/008


On Thu, Oct 10, 2019 at 10:43 AM Nadav Kohen <nadav at suredbits.com> wrote:

>  Hi list,
> I'm back again with another idea about Payment Points and fun things to do
> with them! Hopefully this time I'm not entirely just hashing out old ideas
> in public like an out-of-the-loop person :)
> *TLDR: Adding and ECDH-ing points gives us AND and OR functionality which
> we can compose to make cool lightning contracts like Multisig, Escrow, and
> DLCs*
> So when looking at the following (likely incomplete) list of things you
> can do with payment points:
> 1) Payment De-correlation
> 2) "Stuckless" Payments
> 3) High AMP
> 4) Selling Signatures
> 5) Selling Pedersen De-commitment
> 6) Escrow Contracts
> I started of trying to classify what kind of thing these new features are
> in hopes of coming across new ones. The first three I clumped into a group
> I called "Payment point addition allows us to do cool things while
> maintaining the Proof of Payment (PoP) property". The next two (4 and 5) I
> called "Commitment applications where point is public". But ZmnSCPxj's
> proposal for lightning escrow contracts (
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-June/002028.html)
> struck me as something different that might somehow be made more general.
> Essentially the idea is to use the point S + ECDH(B, E) where S is the
> seller's point, B is the buyer's point and E is the escrow's point. This
> means that the scalar can be discovered by the seller in collaboration with
> the buyer or the escrow, that is, S AND (B OR E). I propose that under
> certain circumstances (such as the parties involved being able to
> interact), this can be generalized to have payments conditioned on
> arbitrary AND/OR circuits.
>  I believe that AND is very straightforward as you simply take two
> conditions A and B and add them together to get a point that requires both
> of their scalars are discoverable (except maybe under certain bad
> circumstances that can be avoided where like B = C - A, this must be
> guarded against).
> OR is harder but I think that it can be achieved in the two party case by
> ECDH and in the n-party case by multi-party key exchanges (which I know
> pretty much nothing about other than that they exist). Given some key
> exchange protocol (preferably non-interactive), KE, KE(A_1, ..., A_n)
> should result in a number known only to those who know any scalar a_1, ...,
> a_n and no one else. Assuming this exists and we can manage to trustlessly
> (in some possibly stretched sense of the word) compute shared keys
> (including such things as KE(A+B, C)), then KE(A, B) acts as A OR B in our
> payment condition contract.
> To restate the escrow contract idea in this setting, the payment is
> conditioned on S + KE(B, E). Important to note is that not all parties must
> know about the details of the payment itself: the Escrow in this example
> knows nothing about this payment (other than that some payment exists)
> unless there is a dispute.
> Lastly, note that contracts following this scheme look indistinguishable
> to normal payments on the network, and are fully compatible with High AMPs
> since we can simply take the payment point specified by our contract and
> add that point to each partial payment point.
> Well this is all nice in theory, but is there anything that will actually
> come out of this thinking? I'll detail the two things I've thought of so
> far for which I'd love critique! I'd also love to hear if anyone else
> things of any cool application using this line of thought (or really
> anything cool with payment points :P).
> Idea 1: "MultiSignature" Lightning Contracts
> I mean "MultiSignature" here only in the sense that m-of-n parties must
> agree to the payment in order for it to happen, no actual signatures are
> used. A "MultiSignature" contract is simply a bunch of ANDs and ORs! For
> example a 2-of-3 multisig between parties A, B, and C can be represented as
> (A AND B) OR (B AND C) OR (C AND A). As such, if some seller has a point S
> and three parties have points A, B, and C where a certain payment must go
> through if any two of them think it should, then the payment point used for
> the payment should be S + KE(A + B, B + C, C + A). We add S to this point
> so that the scalar, s, can act as proof of payment. And that's it! I
> haven't thought long enough to come up with any situation where this might
> be useful but hoping someone who reads this will!
> Idea 2: DLCs Routed over Lightning
> Say that some DLC oracle will either broadcast s_A or s_B whose public
> points are S_A and S_B (which can be computed from public information as
> per the DLC scheme). Then say that Alice and Bob enter into a contract
> under which Alice wins some amount if s_A is broadcasted and Bob if s_B is
> broadcasted. Say Alice has a point A and Bob has a point B. They each send
> the other a payment with the amount that party must receive if they win
> with the payment point A + S_A for Bob's payment to Alice and B + S_B for
> Alice's payment to Bob. And this is it! If s_A is broadcasted then Alice
> gets paid (and Bob gets proof of payment a, which is the scalar to A),
> otherwise s_B is broadcasted and Bob gets paid (with Alice receiving b as
> PoP). An interesting note is that under this scheme neither party is forced
> to pay extra on-chain fees in the case of a counter-party who doesn't
> cooperate whilst in the wrong.
> One wrinkle with this scheme is that setup isn't trustless. Although it is
> generally true that one party must sign the funding transaction for a DLC
> before the other party for on-chain DLCs, at least there is the mitigation
> that when your counter-party goes silent, you can move your input funds
> invalidating the funding transaction you signed (at a cost because fees).
> So what can we do here to ensure that both payments are setup at the same
> time in the case that Alice and Bob don't trust each other?
> Say that although they don't trust each other, they're both willing to
> trust some escrow entity who generates some point E for their payment.
> Alice's payment point to Bob becomes B + S_B + E and Bob's to Alice becomes
> A + S_A + E. The escrow now waits to hear from Alice and Bob that they have
> incoming payments setup and only once both of them attest to this (using
> signatures, for example) does the escrow release the scalar to E to them
> both. The escrow can also have a timeout at which it will never reveal the
> scalar to E: forcing both parties to commit to the contract well before the
> DLC event. In this way, trust has been moved from counter-party to
> trustworthy (hopefully) escrow in such a way that the Escrow learns nothing
> about the contract itself (other than that there is one of some kind).
> I believe that this scheme can be extended to more events through the use
> of multiple payments being setup (usually in both directions) but this
> seems complicated and I've rambled for long enough. One last note is that
> DLC oracles can be composed in the usual way under this scheme (by
> addition) and potentially even threshold multi-oracles can be supported in
> this way, although this would require the oracles to attest to some shared
> key's points with other oracles which isn't necessarily optimal.
> Best,
> Nadav
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