Date: Thu, 20 Aug 2020 21:38:16 +0000
To: Nadav Kohen <nadav@suredbits.com>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Detailed protocol design for routed
	multi-transaction CoinSwap
Precedence: list

Good morning Nadav,

> Hey Chris and all,
>
> Looking good :) I have one major concern though
>
> >=C2=A0 =C2=A0 q =3D EC privkey generated by maker
> >=C2=A0 =C2=A0 Q =3D q.G =3D EC pubkey published by maker
> >
> >=C2=A0 =C2=A0 p =3D nonce generated by taker
> >=C2=A0 =C2=A0 P =3D p.G =3D nonce point calculated by taker
> >
> >=C2=A0 =C2=A0 R =3D Q + P =3D pubkey used in bitcoin transaction
> >=C2=A0 =C2=A0 =C2=A0 =3D (q + p).G
>
> If I'm understanding this correctly (which I'm not sure I ame), it seems =
like the plan is to put R on-chain as the key to an output? As stated this =
is completely=C2=A0insecure as Q is known in advance so the taker can alway=
s choose a nonce p but then claim that their nonce point is p.G - Q so that=
 the key that goes on-chain is (p.G - Q=C2=A0+ Q) =3D p.G allowing them to =
steal the funds.

My reading from this is that nonce `p` has to be given by the taker to the =
maker outright.
In original post:

> Taker sends unsigned transaction which pays to multisig using pubkey Q,
> and also sends nonce p.

Thus, taker provides a proof-of-knowledge, i.e. the actual `p` scalar itsel=
f (not zero-knowledge, but what the maker needs is proof-of-knowledge, and =
could not care less if the proof is zero-knowledge or not).

On the other hand, I do not see the point of this tweak if you are going to=
 use 2p-ECDSA, since my knowledge is that 2p-ECDSA uses the pubkey that is =
homomorphic to the product of the private keys.
And that pubkey is already tweaked, by the fresh privkey of the maker (and =
the maker is buying privacy and wants security of the swap, so is incentivi=
zed to generate high-entropy temporary privkeys for the actual swap operati=
on).

Not using 2p-ECDSA of some kind would remove most of the privacy advantages=
 of CoinSwap.
You cannot hide among `2 <A> <B> 2 OP_CHECKMULTISIG` scripts of Lightning, =
because:

* Lightning channel closes tend to be weeks at least after the funding outp=
oint creation, whereas CoinSwap envisions hours or days.
* Lightning mutual channel closes have a very high probability of spending =
to two P2WPKH addresses.

You need to hide among the much larger singlesig anonymity set, which means=
 using a single signature (created multiparty by both participants), not tw=
o signatures (one from each participant).

Or is this intended for HTLCs in open-coded SCRIPTs `OP_DUP OP_IF OP_HASH16=
0 <hash> OP_EQUAL <A> OP_ELSE <time> OP_CHECKSEQUENCEVERIFY OP_DROP <B> OP_=
ENDIF OP_CHECKSIG`?
This provides a slight privacy boost in a case (contract transaction public=
ation) where most of the privacy is lost anyway.

Regards,
ZmnSCPxj