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From: "Swambo, Jacob" <jacob.swambo@kcl.ac.uk>
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Thread-Topic: ANYPREVOUT in place of CTV
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Subject: Re: [bitcoin-dev] ANYPREVOUT in place of CTV
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Thanks Darosior for your response.

I see now that APOAS (e.g. with ANYONECANPAY and/or SINGLE) and CTV (with l=
ess restrictive templates) fall prey to the same trade-off between flexibil=
ity and safety. So I retract my statement about that 'point in favour of OP=
_CTV'. It would be nice to by-pass the trade-off, but it seems to be unavoi=
dable. That begs the question, why would we want to have a way to commit to=
 less restrictive templates?

Firstly, I posit that if a transaction does not allow RBF, then it would be=
 very difficult for an attacker to repackage parts of the transaction into =
a malicious alternative and rebroadcast it before it reaches the mempool of=
 the majority of nodes, who would then reject the malicious alternative.

Secondly, some covenant-based applications aren't as critical as others, an=
d it may well be acceptable to take the risk of using something like ANYONE=
CANPAY|ALL even with RBF enabled.

Third, in a trusted multi-party context you can safely make use of flexible=
 signature messages. Let's say there are 3 people and a UTXO with the follo=
wing locking script as a single leaf in the tapscript:

<pk1> OP_CHECKSIG <pk2> OP_CHECKSIGADD <pk3> OP_CHECKSIGADD 2 OP_EQUAL <APO=
AS|SINGLE:signature_covenant_tx> <covenant_PK> OP_CHECKSIG

And they produce this witness:

<SINGLE:sig_1> <ALL:sig_2>

The second participant can, for example, add a change output before signing=
. <sig_1> is not sufficient and so can't be repackaged without the authoris=
ation of participant 2.


The additional flexibility through composing APOAS with other SIGHASH modes=
, and the ability to re-bind covenant transactions to different UTXOs allow=
s protocol designers to do more with APOAS covenants than with CTV covenant=
s (as currently spec'd). I'm not yet convinced that BIP-118 is totally safe=
, but I think the debate recently is part of that maturation process and I'=
m glad for it.


Jacob Swambo


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<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">Thanks Darosior for your response.
<o:p></o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">I see now that APOAS (e.g. with ANYONECANPAY and/or S=
INGLE) and CTV (with less restrictive templates) fall prey to the same trad=
e-off between flexibility and safety. So
 I retract my statement about that 'point in favour of OP_CTV'. It would be=
 nice to by-pass the trade-off, but it seems to be unavoidable. That begs t=
he question, why would we want to have a way to commit to less restrictive =
templates?
<o:p></o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">Firstly, I posit that if a transaction does not allow=
 RBF, then it would be very difficult for an attacker to repackage parts of=
 the transaction into a malicious alternative
 and rebroadcast it before it reaches the mempool of the majority of nodes,=
 who would then reject the malicious alternative.<o:p></o:p></span></span><=
/p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">Secondly, some covenant-based applications aren't as =
critical as others, and it may well be acceptable to take the risk of using=
 something like ANYONECANPAY|ALL even with
 RBF enabled.<o:p></o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">Third, in a trusted multi-party context you can safel=
y make use of flexible signature messages. Let's say there are 3 people and=
 a UTXO with the following locking script
 as a single leaf in the tapscript:<o:p></o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal" style=3D"text-indent:36.0pt"><span class=3D"DefaultF=
ontHxMailStyle"><span style=3D"font-size:16.0pt">&lt;pk1&gt; OP_CHECKSIG &l=
t;pk2&gt; OP_CHECKSIGADD &lt;pk3&gt; OP_CHECKSIGADD 2 OP_EQUAL &lt;APOAS|SI=
NGLE:signature_covenant_tx&gt; &lt;covenant_PK&gt; OP_CHECKSIG<o:p></o:p></=
span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">And they produce this witness:<o:p></o:p></span></spa=
n></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal" style=3D"text-indent:36.0pt"><span class=3D"DefaultF=
ontHxMailStyle"><span style=3D"font-size:16.0pt">&lt;SINGLE:sig_1&gt; &lt;A=
LL:sig_2&gt;<o:p></o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">The second participant can, for example, add a change=
 output before signing. &lt;sig_1&gt; is not sufficient and so can't be rep=
ackaged without the authorisation of participant
 2.<o:p></o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">The additional flexibility through composing APOAS wi=
th other SIGHASH modes, and the ability to re-bind covenant transactions to=
 different UTXOs allows protocol designers
 to do more with APOAS covenants than with CTV covenants (as currently spec=
'd). I'm not yet convinced that BIP-118 is totally safe, but I think the de=
bate recently is part of that maturation process and I'm glad for it.<o:p><=
/o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt">Jacob Swambo</span></span></p>
<p class=3D"MsoNormal"><span class=3D"DefaultFontHxMailStyle"><span style=
=3D"font-size:16.0pt"><o:p>&nbsp;</o:p></span></span></p>
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