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From: Greg Sanders <gsanders87@gmail.com>
Date: Thu, 10 Jul 2025 08:05:07 -0400
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Subject: Re: [bitcoindev] What's a good stopping point? Making the case for
 the capabilities enabled by CTV+CSFS
To: Josh Doman <joshsdoman@gmail.com>
Cc: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
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Hi Josh,

For one, MuHash doesn't have a compact membership proof, for one, making it
unlikely to be useful for anything we're likely thinking of. It's used in
Bitcoin Core for equivalency of UTXO sets in snapshots. To validate
membership, the entire population has to be iterated.

Best,
Greg

On Wed, Jul 9, 2025 at 5:54=E2=80=AFPM Josh Doman <joshsdoman@gmail.com> wr=
ote:

> I tend to agree. It's hard to justify the leap in expressivity of OP_TX /
> OP_TXHASH solely on the basis of enabling commitments to sibling prevouts=
.
> A more targeted approach would be better.
>
> In that vein, I think there's a way to use MuHash to generalize CTV /
> TEMPLATEHASH and commit to sibling prevouts in constant time.
>
> The idea is to precompute a MuHash accumulator containing SHA256(index ||
> prevout) for each input in the transaction.
>
> Then, to compute the sibling commitment for input i, we simply copy the
> accumulator and remove the SHA256 hash for that input. Thanks to MuHash,
> this takes constant time. Finally, we include the sibling commitment in t=
he
> existing proposed commitment scheme.
>
> This would represent a low-cost way to commit to the next txid, providing
> predictability regardless of how many inputs are spent (unlike existing
> proposals). Given that MuHash is already in the codebase, I'm inclined to
> believe this wouldn't be a heavy lift and would better achieve the goal o=
f
> a primitive that "commits to the next transaction."
>
> Thoughts?
>
> Best,
> Josh
>
> On Friday, July 4, 2025 at 9:08:48=E2=80=AFAM UTC-4 Antoine Poinsot wrote=
:
>
>> I agree the BitVM/CTV idea suggests inspection of other inputs can be
>> useful for applications
>> leveraging connector outputs.
>>
>> While it is potentially compelling, the BitVM use case was only briefly
>> presented, with no
>> demonstration or even detailed description of how it would work in
>> practice. This makes it hard to
>> assess the costs and benefits of this approach. Furthermore, it's hard t=
o
>> assess how much of an
>> improvement it brings to Bitcoin users as BitVM has yet to be delivered
>> and see any meaningful
>> adoption.
>>
>> As Greg responded when it was raised earlier in this thread[^0], as
>> things stand today i don't think
>> this idea justifies the leap in expressivity.
>>
>> Best,
>> Antoine
>>
>> [^0]:
>> https://gnusha.org/pi/bitcoindev/8d37b779-bf2e-4f63...@googlegroups.com
>> <https://gnusha.org/pi/bitcoindev/8d37b779-bf2e-4f63-a51c-9953434d7553n@=
googlegroups.com>
>>
>>
>> On Thursday, July 3rd, 2025 at 4:54 AM, Anthony Towns <
>> a...@erisian.com.au> wrote:
>>
>> >
>> >
>> > On Tue, Jun 24, 2025 at 11:54:02AM -0400, Matt Corallo wrote:
>> >
>> > > > which
>> > > > warrants a compelling demonstration that arbitrary transaction
>> introspection
>> > > > does enable important use cases not achievable with more minimal
>> capabilities.
>> > > > I'm somewhat skeptical that showing this isn't rather simple,
>> >
>> >
>> > I think the BitVM/CTV idea posted on delving [0] is one such simple
>> demo?
>> >
>> > I gave an example in that thread of how you'd implement the desired
>> > construct using bllsh's introspection primitives, but the same could
>> > equally well be done with Rusty's as-yet unpublished OP_TX, something
>> > like:
>> >
>> > DUP 0x1011 TX 0x00000002 EQUALVERIFY 0x1009 TX 0x0809 TX EQUALVERIFY
>> >
>> > where:
>> >
>> > * "0x1011 TX" pops an input index from the stack and gives the
>> four-byte
>> > vout index of that input's prevout
>> > * "0x1009 TX" pops an input index from the stack and gives the txid of
>> that input's
>> > prevout
>> > * "0x0809 TX" gives the txid of the current input's prevout
>> >
>> > (this encodes "this utxo can only be spent (via this path) if its
>> sibling
>> > output at index 2 is also being spent in the same transaction")
>> >
>> > Cheers,
>> > aj
>> >
>> > [0]
>> https://delvingbitcoin.org/t/how-ctv-csfs-improves-bitvm-bridges/1591
>> >
>> > --
>> > You received this message because you are subscribed to the Google
>> Groups "Bitcoin Development Mailing List" group.
>> > To unsubscribe from this group and stop receiving emails from it, send
>> an email to bitcoindev+...@googlegroups.com.
>> > To view this discussion visit
>> https://groups.google.com/d/msgid/bitcoindev/aGX_MNORQVQT_lp4%40erisian.=
com.au.
>>
>>
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<div dir=3D"ltr">Hi Josh,<div><br></div><div>For one, MuHash doesn&#39;t ha=
ve a compact membership proof, for one, making it unlikely to be useful for=
 anything we&#39;re likely thinking of. It&#39;s used in Bitcoin Core for e=
quivalency=C2=A0of UTXO sets in snapshots. To validate membership, the enti=
re population has to be iterated.</div><div><br></div><div>Best,</div><div>=
Greg</div></div><br><div class=3D"gmail_quote gmail_quote_container"><div d=
ir=3D"ltr" class=3D"gmail_attr">On Wed, Jul 9, 2025 at 5:54=E2=80=AFPM Josh=
 Doman &lt;<a href=3D"mailto:joshsdoman@gmail.com">joshsdoman@gmail.com</a>=
&gt; wrote:<br></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">I te=
nd to agree. It&#39;s hard to justify the leap in expressivity of OP_TX / O=
P_TXHASH solely on the basis of enabling commitments to sibling prevouts. A=
 more targeted approach would be better.<div><br></div><div>In that vein, I=
 think there&#39;s a way to use MuHash to generalize CTV / TEMPLATEHASH and=
 commit to sibling prevouts in constant time.<br><div><br></div><div><div>T=
he idea is to precompute a MuHash accumulator containing SHA256(index || pr=
evout) for each input in the transaction.<br></div><div><br></div><div>Then=
, to compute the sibling commitment for input i, we simply copy the accumul=
ator and remove the SHA256 hash for that input. Thanks to MuHash, this take=
s constant time. Finally, we include the sibling commitment in the existing=
 proposed commitment scheme.</div><div><br></div></div></div><div>This woul=
d represent a low-cost way to commit to the next txid, providing predictabi=
lity regardless of how many inputs are spent (unlike existing proposals). G=
iven that MuHash is already in the codebase, I&#39;m inclined to believe th=
is wouldn&#39;t be a heavy lift and would better achieve the goal of a prim=
itive that &quot;commits to the next transaction.&quot;</div><div><br></div=
><div>Thoughts?</div><div><br></div><div>Best,</div><div>Josh</div><br><div=
 class=3D"gmail_quote"><div dir=3D"auto" class=3D"gmail_attr">On Friday, Ju=
ly 4, 2025 at 9:08:48=E2=80=AFAM UTC-4 Antoine Poinsot wrote:<br></div><blo=
ckquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left=
:1px solid rgb(204,204,204);padding-left:1ex">I agree the BitVM/CTV idea su=
ggests inspection of other inputs can be useful for applications
<br>leveraging connector outputs.
<br>
<br>While it is potentially compelling, the BitVM use case was only briefly=
 presented, with no
<br>demonstration or even detailed description of how it would work in prac=
tice. This makes it hard to
<br>assess the costs and benefits of this approach. Furthermore, it&#39;s h=
ard to assess how much of an
<br>improvement it brings to Bitcoin users as BitVM has yet to be delivered=
 and see any meaningful
<br>adoption.
<br>
<br>As Greg responded when it was raised earlier in this thread[^0], as thi=
ngs stand today i don&#39;t think
<br>this idea justifies the leap in expressivity.
<br>
<br>Best,
<br>Antoine
<br>
<br>[^0]: <a href=3D"https://gnusha.org/pi/bitcoindev/8d37b779-bf2e-4f63-a5=
1c-9953434d7553n@googlegroups.com" rel=3D"nofollow" target=3D"_blank">https=
://gnusha.org/pi/bitcoindev/8d37b779-bf2e-4f63...@googlegroups.com</a>
<br>
<br>
<br>On Thursday, July 3rd, 2025 at 4:54 AM, Anthony Towns &lt;<a rel=3D"nof=
ollow">a...@erisian.com.au</a>&gt; wrote:
<br>
<br>&gt;=20
<br>&gt;=20
<br>&gt; On Tue, Jun 24, 2025 at 11:54:02AM -0400, Matt Corallo wrote:
<br>&gt;=20
<br>&gt; &gt; &gt; which
<br>&gt; &gt; &gt; warrants a compelling demonstration that arbitrary trans=
action introspection
<br>&gt; &gt; &gt; does enable important use cases not achievable with more=
 minimal capabilities.
<br>&gt; &gt; &gt; I&#39;m somewhat skeptical that showing this isn&#39;t r=
ather simple,
<br>&gt;=20
<br>&gt;=20
<br>&gt; I think the BitVM/CTV idea posted on delving [0] is one such simpl=
e demo?
<br>&gt;=20
<br>&gt; I gave an example in that thread of how you&#39;d implement the de=
sired
<br>&gt; construct using bllsh&#39;s introspection primitives, but the same=
 could
<br>&gt; equally well be done with Rusty&#39;s as-yet unpublished OP_TX, so=
mething
<br>&gt; like:
<br>&gt;=20
<br>&gt; DUP 0x1011 TX 0x00000002 EQUALVERIFY 0x1009 TX 0x0809 TX EQUALVERI=
FY
<br>&gt;=20
<br>&gt; where:
<br>&gt;=20
<br>&gt; * &quot;0x1011 TX&quot; pops an input index from the stack and giv=
es the four-byte
<br>&gt; vout index of that input&#39;s prevout
<br>&gt; * &quot;0x1009 TX&quot; pops an input index from the stack and giv=
es the txid of that input&#39;s
<br>&gt; prevout
<br>&gt; * &quot;0x0809 TX&quot; gives the txid of the current input&#39;s =
prevout
<br>&gt;=20
<br>&gt; (this encodes &quot;this utxo can only be spent (via this path) if=
 its sibling
<br>&gt; output at index 2 is also being spent in the same transaction&quot=
;)
<br>&gt;=20
<br>&gt; Cheers,
<br>&gt; aj
<br>&gt;=20
<br>&gt; [0] <a href=3D"https://delvingbitcoin.org/t/how-ctv-csfs-improves-=
bitvm-bridges/1591" rel=3D"nofollow" target=3D"_blank">https://delvingbitco=
in.org/t/how-ctv-csfs-improves-bitvm-bridges/1591</a>
<br>&gt;=20
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