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From: Antoine Riard <antoine.riard@gmail.com>
Date: Fri, 13 Jun 2025 02:18:57 +0100
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Message-ID: <CALZpt+E5o3wLW6VMk5boyYBstM7AYt2PUaMPYR5jHt9EKXvvow@mail.gmail.com>
Subject: Re: [bitcoindev] CTV + CSFS: a letter
To: "James O'Beirne" <james.obeirne@gmail.com>
Cc: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
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Hi James,

Thanks for the additional thoughts.

> In your example script, you're not making use of the template hash or
OP_NOP4.

I think this is where we're talking past each other as in example of the
script previosuly given, there is a CTV opcode used in the first OP_IF
branch.

Here again the script:

        OP_IF
                <my_little_vault_hash> OP_CHECKTEMPLATEVERIFY
        OP_ELSE
                <alice_bob_their_family_aggregated_pubkey> OP_CHECKSIG
        OP_ENDIF

Correct if I'm wrong, but in my understanding of BIP119, if the first
path is taken, the templating will be checked on the spending transaction
from the <my_little_vault_hash> stack element.

Of course, this is not a concern specific to OP_CTV and it's concerning
all the non bitcoin witness v1 traffic. Though, apart of the additional
work to change BIP119 and its code, I don't see why it's not technically
rational to make BIP119 a bitcoin witness v1 only.

Reducing the attack surface now, it's always less attack surface for
funds locked in the future thanks to CTV. Indeed, if you see technical
rational not to do CTV a segwit v1 and keep it as a legacy or you would
like I explain better such "blocksig overflow attack", I'm all hear.

The letter was asking for technical review. So here some "troubleshoot"
review of CTV, which I believe it's worthy to fix in its design. I don't
think it's a lot of work to make CTV a segwit v1, though I can suggest
pseudo-code if you wish so.

Re-iterating my previous commitment to advance on the review of CTV+ CSFS
(and BIP54) during the next 6 months. Your letter was asking for some kind
of goodwill signaling, here mine.

Thanks for the degree of professionalism you're upholding in the wish
to move the lines forward.

Best,
Antoine
OTS hash: 03eedd0ff78d4417c53cb0eb5660c89d5d13f6e1c4fc55a8d7f2bb83f209ce5b

Le jeu. 12 juin 2025 =C3=A0 04:34, James O'Beirne <james.obeirne@gmail.com>=
 a
=C3=A9crit :

> Hey Antoine,
>
> Thanks for the post. Based on my read of what you're describing
> nothing in particular in your attack is specific to CTV. In your example
> script, you're not making use of the template hash or OP_NOP4.
>
> As far as I can tell, the DoS you're describing basically affects all non
> witness v1 activity on bitcoin - i.e. some malicious user filling blocks
> up to their sigops limit to deny other users service.
>
> Given that probably most activity on bitcoin is not witness v1,
> I don't see how this is a CTV-specific issue.
>
> Thanks,
> James
>
>

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<div dir=3D"ltr">Hi James,<br><br>Thanks for the additional thoughts.<br><b=
r>&gt; In your example script, you&#39;re not making use of the template ha=
sh or OP_NOP4.<br><br>I think this is where we&#39;re talking past each oth=
er as in example of the<br>script previosuly given, there is a CTV opcode u=
sed in the first OP_IF<br>branch.<br><br>Here again the script:<br><br>=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 OP_IF<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0 =C2=A0 &lt;my_little_vault_hash&gt; OP_CHECKTEMPLATEVERIFY<br>=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 OP_ELSE<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0 &lt;alice_bob_their_family_aggregated_pubkey&gt; OP_CHECK=
SIG<br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 OP_ENDIF<br><br>Correct if I&#39;m wrong=
, but in my understanding of BIP119, if the first<br>path is taken, the tem=
plating will be checked on the spending transaction<br>from the &lt;my_litt=
le_vault_hash&gt; stack element.<br><br>Of course, this is not a concern sp=
ecific to OP_CTV and it&#39;s concerning<br>all the non bitcoin witness v1 =
traffic. Though, apart of the additional<br>work to change BIP119 and its c=
ode, I don&#39;t see why it&#39;s not technically<br>rational to make BIP11=
9 a bitcoin witness v1 only.<br><br>Reducing the attack surface now, it&#39=
;s always less attack surface for<br>funds locked in the future thanks to C=
TV. Indeed, if you see technical<br>rational not to do CTV a segwit v1 and =
keep it as a legacy or you would<br>like I explain better such &quot;blocks=
ig overflow attack&quot;, I&#39;m all hear.<br><br>The letter was asking fo=
r technical review. So here some &quot;troubleshoot&quot;<br>review of CTV,=
 which I believe it&#39;s worthy to fix in its design. I don&#39;t<br>think=
 it&#39;s a lot of work to make CTV a segwit v1, though I can suggest<br>ps=
eudo-code if you wish so.<br><br>Re-iterating my previous commitment to adv=
ance on the review of CTV+ CSFS<br>(and BIP54) during the next 6 months. Yo=
ur letter was asking for some kind<br>of goodwill signaling, here mine.<br>=
<br>Thanks for the degree of professionalism you&#39;re upholding in the wi=
sh<br>to move the lines forward.<br><br>Best,<br>Antoine<br>OTS hash: 03eed=
d0ff78d4417c53cb0eb5660c89d5d13f6e1c4fc55a8d7f2bb83f209ce5b</div><br><div c=
lass=3D"gmail_quote gmail_quote_container"><div dir=3D"ltr" class=3D"gmail_=
attr">Le=C2=A0jeu. 12 juin 2025 =C3=A0=C2=A004:34, James O&#39;Beirne &lt;<=
a href=3D"mailto:james.obeirne@gmail.com">james.obeirne@gmail.com</a>&gt; a=
 =C3=A9crit=C2=A0:<br></div><blockquote class=3D"gmail_quote" style=3D"marg=
in:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1e=
x"><div dir=3D"ltr"><div dir=3D"ltr"><div>Hey Antoine,</div><div><br></div>=
<div>Thanks for the post. Based on my read of what you&#39;re describing</d=
iv><div>nothing in particular in your attack is specific to CTV. In your ex=
ample</div><div>script, you&#39;re not making use of the template hash or O=
P_NOP4.</div><div><br></div><div>As far as I can tell, the DoS you&#39;re d=
escribing basically affects all non</div><div>witness v1 activity on bitcoi=
n - i.e. some malicious user filling blocks</div><div>up to their sigops li=
mit to deny other users service.</div><div><br></div><div>Given that probab=
ly most=C2=A0activity on bitcoin is not witness v1,</div><div>I don&#39;t s=
ee how this is a CTV-specific issue.</div><div><br></div><div>Thanks,</div>=
<div>James</div></div><br></div>
</blockquote></div>

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