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From: Chris Stewart <stewart.chris1234@gmail.com>
Date: Tue, 13 May 2025 04:03:00 -0500
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Subject: Re: [bitcoindev] [Proposal] 64-bit arithmetic in Script
To: =?UTF-8?Q?Martin_Habov=C5=A1tiak?= <martin.habovstiak@gmail.com>
Cc: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
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Hi Martin

Thanks for your interest in the topic.

>It mentions upgrading existing opcodes, which is a hardfork, not soft
fork, at least without using a different leaf version. But it also mentions
OP_SUCCESSX which are different opcodes

I view 64-bit arithmetic as a feature of a wider set of consensus changes.
Here is what I think the most likely deployment story is

>This proposal could be deployed in conjunction with any of the new opcode
proposals in the Motivation section using Tapscript's OP_SUCCESSx semantics=
.
[18
<https://github.com/Christewart/bips/blob/2025-03-17-64bit-pt2/bip-XXXX.med=
iawiki#cite_note-18>
]

The opcodes mentioned in the motivation section are OP_{IN,OUT}_AMOUNT,
OP_VAULT, OP_CHECKCONTRACTVERIFY, OP_CTV

As an example, this proposal could (hypothetically) be deployed in
conjunction with OP_CCV. OP_CCV would take advantage of the OP_SUCCESSx
semantics allowing us to redefine existing opcode's (OP_ADD,OP_SUB, etc)
semantics.

There=E2=80=99s been some discussion
<https://delvingbitcoin.org/t/ctv-csfs-can-we-reach-consensus-on-a-first-st=
ep-towards-covenants/1509/48?u=3Dchris_stewart_5>
around deploying OP_CTV via a NOP opcode instead of OP_SUCCESSx. I think
that would be a poor choice, as it wouldn=E2=80=99t allow new Script featur=
es to be
shipped in parallel with the new opcode.

I found this StackExchange post <https://bitcoin.stackexchange.com/q/125237=
>
helpful in thinking through various deployment strategies for new
Tapscript-based consensus upgrades.

>I'd also love to see analysis why stop at 64 bits and not go all the way
to 256 which could be useful for cryptography.

In my view, there=E2=80=99s still a lot of uncertainty about cryptographic =
features
being added to Script. There's increasing discussion around quantum
computing, which raises the question of how much numerical precision we may
eventually need. I'm not opposed to extending precision further=E2=80=94but=
 if we
go beyond 64 bits, why stop at 256? With OP_SUCCESSx, arbitrary precision
becomes a real option <https://bitcoin.stackexchange.com/a/126153/20835>.

I chose 64 bits because it covers what=E2=80=99s needed for amount locks. I=
f
someone strongly believes that 64 bits isn't enough, I=E2=80=99d welcome a
competing BIP and would be happy to review and discuss it with the author.

>Anyway, pushing amounts on the stack would be great. Though I'm surprised
you're only proposing the sum, not individual outputs. Why?

Good question=E2=80=94and slightly out of scope for this BIP. Script doesn=
=E2=80=99t
support looping, so it=E2=80=99s not straightforward to iterate over and su=
m all
elements unless the transaction structure (i.e., number of inputs or
outputs) is known in advance.
You can measure the number of stack elements with OP_DEPTH, but there=E2=80=
=99s no
way to write something like OP_ADD [num-stack-elements-from-OP_DEPTH] to
sum them all. I=E2=80=99m definitely open to hearing other approaches, thou=
gh.

-Chris



On Mon, May 12, 2025 at 2:32=E2=80=AFPM Martin Habov=C5=A1tiak <
martin.habovstiak@gmail.com> wrote:

> Hi,
>
> the proposal seems to be quite confused about how it's going to do that.
> It mentions upgrading existing opcodes, which is a hardfork, not soft for=
k,
> at least without using a different leaf version. But it also mentions
> OP_SUCCESSX which are different opcodes. I think it needs some analysis.
> (leaf version seems better intuitively)
>
> I'd also love to see analysis why stop at 64 bits and not go all the way
> to 256 which could be useful for cryptography.
>
> Anyway, pushing amounts on the stack would be great. Though I'm surprised
> you're only proposing the sum, not individual outputs. Why?
>
> Good luck!
>
> Martin
>
> D=C5=88a po 12. 5. 2025, 14:21 Chris Stewart <stewart.chris1234@gmail.com=
>
> nap=C3=ADsal(a):
>
>> This soft fork proposal extends the range of numeric operands in Script
>> from -2^31+1 to 2^31-1, to -2^63+1 to 2^63-1. It further expands the
>> result range for arithmetic operations from -2^63 to 2^63-1, to -2^127
>> to 2^127- 1.
>>
>> All existing opcodes[1
>> <https://github.com/Christewart/bips/blob/2025-03-17-64bit-pt2/bip-XXXX.=
mediawiki#cite_note-1>
>> ] that interpret stack elements as numbers are upgraded to support
>> 64-bit parameters.
>>
>> The existing number encoding format[2
>> <https://github.com/Christewart/bips/blob/2025-03-17-64bit-pt2/bip-XXXX.=
mediawiki#cite_note-2>
>> ] and arithmetic semantics[3
>> <https://github.com/Christewart/bips/blob/2025-03-17-64bit-pt2/bip-XXXX.=
mediawiki#cite_note-3>
>> ] from the original Bitcoin implementation are preserved, while
>> enhancing the supported precision.
>>
>>
>> https://github.com/Christewart/bips/blob/2025-03-17-64bit-pt2/bip-XXXX.m=
ediawiki
>>
>> The purpose for this BIP is to lay the groundwork for introducing amount=
s
>> into Script. This document takes no opinion on how this is done.
>>
>> I've prototyped a few different proposals now introducing amount locks
>> into Script[0][1] and feel like this proposal is stable enough for serio=
us
>> review.
>>
>> -Chris
>>
>> [0] -
>> https://delvingbitcoin.org/t/op-inout-amount/549/4?u=3Dchris_stewart_5
>>
>> [1] -
>> https://delvingbitcoin.org/t/op-inout-amount/549/5?u=3Dchris_stewart_5
>>
>>
>>
>> --
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>> .
>>
>

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<div dir=3D"ltr"><div>Hi Martin</div><div><br></div><div>Thanks for your in=
terest in the topic.</div><div><br></div><div>&gt;It mentions upgrading exi=
sting opcodes, which is a hardfork, not soft=20
fork, at least without using a different leaf version. But it also=20
mentions OP_SUCCESSX which are different opcodes</div><div><br></div><div>I=
 view 64-bit arithmetic as a feature of a wider set of consensus changes. H=
ere is what I think the most likely deployment story is<br><br>&gt;This pro=
posal could be deployed in conjunction with any of the new=20
opcode proposals in the Motivation section using Tapscript&#39;s OP_SUCCESS=
x
 semantics.<sup id=3D"m_-3858691108396401021gmail-user-content-cite_ref-18-=
0">[<a href=3D"https://github.com/Christewart/bips/blob/2025-03-17-64bit-pt=
2/bip-XXXX.mediawiki#cite_note-18" target=3D"_blank">18</a>]</sup></div><di=
v><sup id=3D"m_-3858691108396401021gmail-user-content-cite_ref-18-0"><br></=
sup></div><div>The opcodes mentioned in the motivation section are OP_{IN,O=
UT}_AMOUNT, OP_VAULT, OP_CHECKCONTRACTVERIFY, OP_CTV</div><div><br></div><d=
iv>As an example, this proposal could (hypothetically) be deployed in conju=
nction with OP_CCV. OP_CCV would take advantage of the OP_SUCCESSx semantic=
s allowing us to redefine existing opcode&#39;s (OP_ADD,OP_SUB, etc) semant=
ics.=C2=A0</div><div><br></div><div>There=E2=80=99s been some <a href=3D"ht=
tps://delvingbitcoin.org/t/ctv-csfs-can-we-reach-consensus-on-a-first-step-=
towards-covenants/1509/48?u=3Dchris_stewart_5" target=3D"_blank">discussion=
</a> around deploying <code>OP_CTV</code> via a <code>NOP</code> opcode ins=
tead of <code>OP_SUCCESSx</code>. I think that would be a poor choice, as i=
t wouldn=E2=80=99t allow new Script features to be shipped in parallel with=
 the new opcode.</div><div><br></div><div>I found <a rel=3D"noopener" href=
=3D"https://bitcoin.stackexchange.com/q/125237" target=3D"_blank">this Stac=
kExchange post</a> helpful in thinking through various deployment strategie=
s for new Tapscript-based consensus upgrades.</div><div><br></div><div>&gt;=
I&#39;d also love to see analysis why stop at 64 bits and not go all the wa=
y to 256 which could be useful for cryptography.</div><div><br></div><div>I=
n my view, there=E2=80=99s still a lot of uncertainty about cryptographic f=
eatures being added to Script. There&#39;s increasing discussion around qua=
ntum computing, which raises the question of how much numerical precision w=
e may eventually need. I&#39;m not opposed to extending precision further=
=E2=80=94but if we go beyond 64 bits, why stop at 256? With <code>OP_SUCCES=
Sx</code>, <a href=3D"https://bitcoin.stackexchange.com/a/126153/20835" tar=
get=3D"_blank">arbitrary precision becomes a real option</a>.</div><div><br=
></div><div>I chose 64 bits because it covers what=E2=80=99s needed for amo=
unt locks. If someone strongly believes that 64 bits isn&#39;t enough, I=E2=
=80=99d welcome a competing BIP and would be happy to review and discuss it=
 with the author.</div><div><br></div><div>&gt;Anyway, pushing amounts on t=
he stack would be great. Though I&#39;m=20
surprised you&#39;re only proposing the sum, not individual outputs. Why?</=
div><div><br></div><div>Good question=E2=80=94and slightly out of scope for=
 this BIP. Script doesn=E2=80=99t support looping, so it=E2=80=99s not stra=
ightforward to iterate over and sum all elements unless the transaction str=
ucture (i.e., number of inputs or outputs) is known in advance.<br>
You can measure the number of stack elements with <code>OP_DEPTH</code>, bu=
t there=E2=80=99s no way to write something like <code>OP_ADD [num-stack-el=
ements-from-OP_DEPTH]</code> to sum them all. I=E2=80=99m definitely open t=
o hearing other approaches, though.</div><div><br></div><div>-Chris</div><d=
iv><br></div><div><br></div></div><br><div class=3D"gmail_quote"><div dir=
=3D"ltr" class=3D"gmail_attr">On Mon, May 12, 2025 at 2:32=E2=80=AFPM Marti=
n Habov=C5=A1tiak &lt;<a href=3D"mailto:martin.habovstiak@gmail.com" target=
=3D"_blank">martin.habovstiak@gmail.com</a>&gt; wrote:<br></div><blockquote=
 class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px so=
lid rgb(204,204,204);padding-left:1ex"><div dir=3D"auto">Hi,<div dir=3D"aut=
o"><br></div><div dir=3D"auto">the proposal seems to be quite confused abou=
t how it&#39;s going to do that. It mentions upgrading existing opcodes, wh=
ich is a hardfork, not soft fork, at least without using a different leaf v=
ersion. But it also mentions OP_SUCCESSX which are different opcodes. I thi=
nk it needs some analysis. (leaf version seems better intuitively)</div><di=
v dir=3D"auto"><br></div><div dir=3D"auto">I&#39;d also love to see analysi=
s why stop at 64 bits and not go all the way to 256 which could be useful f=
or cryptography.</div><div dir=3D"auto"><br></div><div dir=3D"auto">Anyway,=
 pushing amounts on the stack would be great. Though I&#39;m surprised you&=
#39;re only proposing the sum, not individual outputs. Why?</div><div dir=
=3D"auto"><br></div><div dir=3D"auto">Good luck!</div><div dir=3D"auto"><br=
></div><div dir=3D"auto">Martin</div></div><br><div class=3D"gmail_quote"><=
div dir=3D"ltr" class=3D"gmail_attr">D=C5=88a po 12. 5. 2025, 14:21 Chris S=
tewart &lt;<a href=3D"mailto:stewart.chris1234@gmail.com" target=3D"_blank"=
>stewart.chris1234@gmail.com</a>&gt; nap=C3=ADsal(a):<br></div><blockquote =
class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px sol=
id rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr"><p dir=3D"auto">This=
 soft fork proposal extends the range of numeric operands in Script from -2=
^<sup>31</sup>+1 to 2^<sup>31</sup>-1, to -2^<sup>63</sup>+1 to 2^<sup>63</=
sup>-1.
It further expands the result range for arithmetic operations from -2^<sup>=
63</sup> to 2^<sup>63</sup>-1, to -2^<sup>127</sup> to 2^<sup>127</sup>- 1.
</p>
<p dir=3D"auto">All existing opcodes<sup id=3D"m_-3858691108396401021m_-116=
2971288865938356m_1405182730796312002gmail-user-content-cite_ref-1-0">[<a h=
ref=3D"https://github.com/Christewart/bips/blob/2025-03-17-64bit-pt2/bip-XX=
XX.mediawiki#cite_note-1" rel=3D"noreferrer" target=3D"_blank">1</a>]</sup>=
=20
that interpret stack elements as numbers are upgraded to support 64-bit par=
ameters.
</p>
<p dir=3D"auto">The existing number encoding format<sup id=3D"m_-3858691108=
396401021m_-1162971288865938356m_1405182730796312002gmail-user-content-cite=
_ref-2-0">[<a href=3D"https://github.com/Christewart/bips/blob/2025-03-17-6=
4bit-pt2/bip-XXXX.mediawiki#cite_note-2" rel=3D"noreferrer" target=3D"_blan=
k">2</a>]</sup>=20
and arithmetic semantics<sup id=3D"m_-3858691108396401021m_-116297128886593=
8356m_1405182730796312002gmail-user-content-cite_ref-3-0">[<a href=3D"https=
://github.com/Christewart/bips/blob/2025-03-17-64bit-pt2/bip-XXXX.mediawiki=
#cite_note-3" rel=3D"noreferrer" target=3D"_blank">3</a>]</sup>=20
from the original Bitcoin implementation are preserved, while enhancing the=
 supported precision.</p><p dir=3D"auto"><a href=3D"https://github.com/Chri=
stewart/bips/blob/2025-03-17-64bit-pt2/bip-XXXX.mediawiki" rel=3D"noreferre=
r" target=3D"_blank">https://github.com/Christewart/bips/blob/2025-03-17-64=
bit-pt2/bip-XXXX.mediawiki</a></p><p>The purpose for this BIP is to lay the=
 groundwork for introducing amounts into Script. This document takes no opi=
nion on how this is done.</p><p>I&#39;ve prototyped a few different proposa=
ls now introducing amount locks into Script[0][1] and feel like this propos=
al is stable enough for serious review.</p><p>-Chris</p><p>[0] - <a href=3D=
"https://delvingbitcoin.org/t/op-inout-amount/549/4?u=3Dchris_stewart_5" re=
l=3D"noreferrer" target=3D"_blank">https://delvingbitcoin.org/t/op-inout-am=
ount/549/4?u=3Dchris_stewart_5</a></p><p>[1] - <a href=3D"https://delvingbi=
tcoin.org/t/op-inout-amount/549/5?u=3Dchris_stewart_5" rel=3D"noreferrer" t=
arget=3D"_blank">https://delvingbitcoin.org/t/op-inout-amount/549/5?u=3Dchr=
is_stewart_5</a></p><p><br></p><br></div>

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