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From: Salvatore Ingala <salvatore.ingala@gmail.com>
Date: Mon, 1 May 2023 23:15:20 +0200
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To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Merkleize All The Things
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Hi all,

I apologize for a couple of oversights in my last e-mail.

The first is that m_B can't be committed as-is in the contract's
embedded data, with the current semantics of OP_COCV, which
only allows 32-byte values. A solution could be to store its
hash SHA256(m_B), instead.

(I didn't test the Scripts, so there could be other bugs =E2=88=92 hopefull=
y the
general idea is clear, anyway)

On Mon, 1 May 2023 at 15:11, Salvatore Ingala <salvatore.ingala@gmail.com>
wrote:

> If the internal_pubkey is a musig-aggregated key of Alice and Bob,
> the game can be settled entirely offline after the first transaction.
> Simply, Bob communicates his move to Alice, Alice reveals her move to
> Bob, and they can settle the bet. The game would be played without
> any script being executed, therefore all transactions could look like
> any other P2TR, with the only possible fingerprinting being due to the
> input amounts.
>

This is incomplete: Alice can't trust Bob by revealing her move, as
he could then cheat on-chain and play a different move.

The fix should be straightforward, after adding the requirement that the
internal pubkey of [S1] is a musig2 of both players.
After Bob reveals his move (say, Rock), Alice will only agree to continue
the game off-chain if Bob pre-signs transactions for the state [S1] (where
m_B =3D Paper, and m_B =3D Scissors) that send all the money to Alice.
This guarantees that a cheating Bob is punished.

Best,
Salvatore Ingala

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<div dir=3D"ltr"><div>Hi all,</div><div><br></div><div>I apologize for a co=
uple of oversights in my last e-mail.</div><div><br></div><div>The first is=
 that m_B can&#39;t be committed as-is in the contract&#39;s</div><div>embe=
dded data, with the current semantics of OP_COCV, which</div><div>only allo=
ws 32-byte values. A solution could be to store its</div><div>hash SHA256(m=
_B), instead.</div><div><br></div><div>(I didn&#39;t test the Scripts, so t=
here could be other bugs =E2=88=92 hopefully the</div><div>general idea is =
clear, anyway)</div><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=
=3D"gmail_attr">On Mon, 1 May 2023 at 15:11, Salvatore Ingala &lt;<a href=
=3D"mailto:salvatore.ingala@gmail.com">salvatore.ingala@gmail.com</a>&gt; w=
rote:</div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.=
8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"lt=
r"><div dir=3D"ltr"><div>If the internal_pubkey is a musig-aggregated key o=
f Alice and Bob,<br>the game can be settled entirely offline after the firs=
t transaction.<br>Simply, Bob communicates his move to Alice, Alice reveals=
 her move to<br>Bob, and they can settle the bet. The game would be played =
without<br>any script being executed, therefore all transactions could look=
 like</div><div>any other P2TR,=C2=A0with the only possible fingerprinting =
being due to the</div><div>input amounts.<br></div></div></div></blockquote=
><div><br></div><div>This is incomplete: Alice can&#39;t trust Bob by revea=
ling her move, as</div><div>he could then cheat on-chain and play a differe=
nt move.</div><div><br></div><div>The fix should be straightforward, after =
adding the requirement that the</div><div>internal pubkey of [S1] is a musi=
g2 of both players.</div><div>After Bob reveals his move (say, Rock), Alice=
 will only agree to continue</div><div>the game off-chain if Bob pre-signs =
transactions for the state [S1] (where</div><div>m_B =3D Paper, and m_B =3D=
 Scissors) that send all the money to Alice.</div><div>This guarantees that=
 a=C2=A0cheating Bob is punished.<br></div><div><br></div><div>Best,</div><=
div>Salvatore Ingala</div><div><br></div></div></div>

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