Date: Sat, 06 Jun 2020 03:59:30 +0000
To: ZmnSCPxj <ZmnSCPxj@protonmail.com>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Subject: Re: [bitcoin-dev] Design for a CoinSwap implementation for
	massively improving Bitcoin privacy and fungibility
Precedence: list

Good morning again Chris,

I am uncertain if you are aware, but some years ago somebody claimed that 2=
p-ECDSA could use Scriptless Script as well over on lightning-dev.

* https://lists.linuxfoundation.org/pipermail/lightning-dev/attachments/201=
80426/fe978423/attachment-0001.pdf
* https://lists.linuxfoundation.org/pipermail/lightning-dev/2018-April/0012=
21.html

I cannot claim to follow the math enough to say it is actually secure, but =
the idea does exist.

If this is sufficiently secure, we can fold the Spilman backout into the sc=
riptless script swap as well.

* Alice creates secret keypairs A[0] =3D a[0] * G, A[1] =3D a[1] * G
* Bob creates secret keypairs B[0] =3D b[0] * G, B[1] =3D b[1] * G
* Alice creates (but does not sign) funding from Alice -> A[0] && B[0]
* Bob provides partial signature for A[0] && B[0] -(nLockTime=3Dlocktime1)-=
> Alice to Alice and Alice completes this signature and stashes it.
* Bob creates (but does not sign) funding from Bob -> A[1] && B[1]
* Alice provides partial signature for A[1] && B[1] -(nLockTime=3DlockTime2=
)-> Bob to Bob and Bob completes this signature and stashes it.
* Alice and Bob sign and broadcast their funding transactions.
  * This can safely be done in any order; Bob will refuse to continue with =
the protocol until it sees Alice funding is confirmed, and will abort if lo=
cktime2 is too near.
* Alice waits for Bob funding tx to confirm.
* Alice provides a 2p-ECDSA adaptor signature for A[1] && B[1] --> Alice; t=
he adaptor signature, when completed, reveals the secret a[0] to Bob.
* Bob waits for Alice funding tx to confirm.
* Bob provides the partial signature for the given adaptor signature for A[=
1] && B[1] --> Alice and  Alice completes this signature and stashes it.
* Alice gives a[0] outright to Bob.
* Bob gives b[1] outright to Alice.
* Alice spends the A[1] && B[1] output before locktime2.
* Bob spends the A[0] && B[0] output before locktime1.

I also pointed out the griefing problem in Lightning also applies to SwapMa=
rket.
Bob can limit the griefing problem by requiring that locktime2 <=3D now + 1=
2, and requiring that locktime1 >=3D now + 60.
This means that Alice has to lock its funds for 10 hours if it forces Bob t=
o lock its funds for 2 hours, making it undesirable as an attack on competi=
ng makers.
This does prevent chaining (no maker is going to accept the outgoing), but =
if Alice wants chaining it can always use the private key handed over to im=
mediately start a funding tx with another Bob.

(This is not a good solution for griefing in the Lightning Network since ch=
annels are intended to be reused there, whereas the Spilman channels in Coi=
nSwap exist only to allow funding transactions to confirm in any order onch=
ain, and are used only for the specific swap; in Lightning the forwarding n=
ode has an incentive to release the incoming HTLC immediately instead of im=
posing the incoming wait time since the funding can be reused for a differe=
nt payment, but in CoinSwap it cannot be reused anyway, so it could just le=
t the incoming timelock lapse instead of releasing that encumbrance as woul=
d be done in Lightning.)

Regards,
ZmnSCPxj