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From: Zac Greenwood <zachgrw@gmail.com>
Date: Mon, 16 Aug 2021 13:17:19 +0200
Message-ID: <CAJ4-pECSE=by2ag4QmqrXbX_R0sk6HOL1KH0z2h=+915jaEE6Q@mail.gmail.com>
To: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Subject: Re: [bitcoin-dev] Exploring: limiting transaction output amount as
 a function of total input value
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Hi ZmnSCPxj,

Thank you for your counterproposal. I fully agree that as a first step we
must establish whether the proposed functionality can be implemented
without making any changes to consensus.

Your counterproposal is understandably more technical in nature because it
explores an implementation on top of Bitcoin as-is. However I feel that for
a fair comparison of the functionality of both proposals a purely
functional description of your proposal is essential.

If I understand your proposal correctly, then I believe there are some
major gaps between yours and mine:

Keys for unrestricted spending: in my proposal, they never have to come
online unless spending more than the limit is desired. In your proposal,
these keys are required to come online in several situations.

Presigning transactions: not required in my proposal. Wouldn=E2=80=99t such
presigning requirement be detrimental for the usability of your proposal?
Does it mean that for instance the amount and window in which the
transaction can be spent is determined at the time of signing? In my
proposal, there is no limit in the number of transactions per window.

Number of windows: limited in your proposal, unlimited in mine.

There are probably additional gaps that I am currently not technically able
to recognize.

I feel that the above gaps are significant enough to state that your
proposal does not meet the basic requirements of my proposal.

Next to consider is whether the gap is acceptable, weighing the effort to
implement the required consensus changes against the effort and feasibility
of implementing your counterproposal.

I feel that your counterproposal has little chance of being implemented
because of the still considerable effort required and the poor result in
functional terms. I also wonder if your proposal is feasible considering
wallet operability.

Considering all the above, I believe that implementing consensus changes in
order to support the proposed functionality would preferable  over your
counterproposal.

I acknowledge that a consensus change takes years and is difficult to
achieve, but that should not be any reason to stop exploring the appetite
for the proposed functionality and perhaps start looking at possible
technical solutions.

Zac


On Sat, 14 Aug 2021 at 03:50, ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:

> Good morning Zac,
>
>
> > Hi ZmnSCPxj,
> >
> > Thank you for your insightful response.
> >
> > Perhaps I should take a step back and take a strictly functional angle.
> Perhaps the list could help me to establish whether the proposed
> functionality is:
> >
> > Desirable;
> > Not already possible;
> > Feasible to implement.
> >
> > The proposed functionality is as follows:
> >
> > The ability to control some coin with two private keys (or two sets of
> private keys) such that spending is limited over time for one private key
> (i.e., it is for instance not possible to spend all coin in a single
> transaction) while spending is unrestricted for the other private key (no
> limits apply). No limits must apply to coin transacted to a third party.
> >
> > Also, it must be possible never having to bring the unrestricted privat=
e
> key online unless more than the limit imposed on the restrictive private
> key is desired to be spent.
> >
> > Less generally, taking the perspective of a hodler: the user must be
> able to keep one key offline and one key online. The offline key allows
> unrestricted spending, the online key is limited in how much it is allowe=
d
> to spend over time.
> >
> > Furthermore, the spending limit must be intuitive. Best candidate I
> believe would be a maximum spend per some fixed number of blocks. For
> instance, the restrictive key may allow a maximum of 100k sats per any
> window of 144 blocks. Ofcourse the user must be able to set these
> parameters freely.
>
> My proposal does not *quite* implement a window.
> However, that is because it uses `nLockTime`.
>
> With the use of `nSequence` in relative-locktime mode, however, it *does*
> implement a window, sort of.
> More specifically, it implements a timeout on spending --- if you spend
> using a presigned transaction (which creates an unencumbered
> specific-valued TXO that can be arbitrarily spent with your online keyset=
)
> then you cannot get another "batch" of funds until the `nSequence` relati=
ve
> locktime passes.
> However, this *does* implement a window that limits a maximum value
> spendable per any window of the relative timelock you select.
>
> The disadvantage is that `nSequence` use is a lot more obvious and
> discernible than `nLockTime` use.
> Many wallets today use non-zero `nLockTime` for anti-fee-sniping, and tha=
t
> is a good cover for `nLockTime` transactions.
> I believe Dave Harding proposed that wallets should also use, at random,
> (say 50-50) `nSequence`-in-relative-locktime-mode as an alternate
> anti-fee-sniping mechanism.
> This alternate anti-fee-sniping would help cover `nSequence` use.
>
> Note that my proposal does impose a maximum limit on the number of window=
s.
> With `nSequence`-in-relative-locktime-mode the limit is the number of
> times that the online keyset can spend.
> After spending that many windows, the offline keyset has to be put back
> online to generate a new set of transactions.
>
> It has the massive massive advantage that you can implement it today
> without any consensus change, and I think you can expect that consensus
> change will take a LONG time (xref SegWit, Taproot).
>
> Certainly the functionality is desirable.
> But it seems it can be implemented with Bitcoin today.
>
> Regards,
> ZmnSCPxj
>
>

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<div dir=3D"auto">Hi=C2=A0<span style=3D"color:rgb(0,0,0)">ZmnSCPxj,</span>=
</div><div dir=3D"auto"><span style=3D"color:rgb(0,0,0)"><br></span></div><=
div dir=3D"auto"><span style=3D"color:rgb(0,0,0)">Thank you for your counte=
rproposal. I fully agree that as a first step we must establish whether the=
 proposed functionality can be implemented without making any changes to co=
nsensus.</span></div><div dir=3D"auto"><span style=3D"color:rgb(0,0,0)"><br=
></span></div><div style=3D"background-color:rgba(0,0,0,0)!important;border=
-color:rgb(255,255,255)!important;color:rgb(255,255,255)!important" dir=3D"=
auto"><font style=3D"color:rgb(0,0,0)">Your counterproposal is understandab=
ly more technical in nature because it explores an implementation on top of=
 Bitcoin as-is. However I feel that for a fair comparison of the functional=
ity of both proposals a purely functional description of your proposal is e=
ssential.</font></div><div style=3D"background-color:rgba(0,0,0,0);border-c=
olor:rgb(255,255,255)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)"><br></=
font></div><div style=3D"background-color:rgba(0,0,0,0);border-color:rgb(25=
5,255,255)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)">If I understand y=
our proposal correctly, then I believe there are some major gaps between yo=
urs and mine:</font></div><div style=3D"background-color:rgba(0,0,0,0);bord=
er-color:rgb(255,255,255)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)"><b=
r></font></div><div style=3D"background-color:rgba(0,0,0,0)!important;borde=
r-color:rgb(32,33,36)!important;color:rgb(255,255,255)!important" dir=3D"au=
to"><font style=3D"color:rgb(0,0,0)">Keys for unrestricted spending: in my =
proposal, they never have to come online unless spending more than the limi=
t is desired. In your proposal, these keys are required to come online in s=
everal situations.</font></div><div style=3D"background-color:rgba(0,0,0,0)=
;border-color:rgb(32,33,36)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)">=
<br></font></div><div style=3D"background-color:rgba(0,0,0,0)!important;bor=
der-color:rgb(222,223,227)!important;color:rgb(255,255,255)!important" dir=
=3D"auto"><font style=3D"color:rgb(0,0,0)">Presigning transactions: not req=
uired in my proposal. Wouldn=E2=80=99t such presigning requirement be detri=
mental for the usability of your proposal? Does it mean that for instance t=
he amount and window in which the transaction can be spent is determined at=
 the time of signing? In my proposal, there is no limit in the number of tr=
ansactions per window.</font></div><div dir=3D"auto"><span style=3D"color:r=
gb(0,0,0)"><br></span></div><div style=3D"background-color:rgba(0,0,0,0)!im=
portant;border-color:rgb(255,255,255)!important;color:rgb(255,255,255)!impo=
rtant" dir=3D"auto"><font style=3D"color:rgb(0,0,0)">Number of windows: lim=
ited in your proposal, unlimited in mine.</font></div><div dir=3D"auto"><sp=
an style=3D"color:rgb(0,0,0)"><br></span></div><div style=3D"background-col=
or:rgba(0,0,0,0)!important;border-color:rgb(255,255,255)!important;color:rg=
b(255,255,255)!important" dir=3D"auto"><font style=3D"color:rgb(0,0,0)">The=
re are probably additional gaps that I am currently not technically able to=
 recognize.</font></div><div style=3D"background-color:rgba(0,0,0,0);border=
-color:rgb(255,255,255)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)"><br>=
</font></div><div style=3D"background-color:rgba(0,0,0,0);border-color:rgb(=
255,255,255)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)">I feel that the=
 above gaps are significant enough to state that your proposal does not mee=
t the basic requirements of my proposal.</font></div><div style=3D"backgrou=
nd-color:rgba(0,0,0,0);border-color:rgb(255,255,255)" dir=3D"auto"><font st=
yle=3D"color:rgb(0,0,0)"><br></font></div><div style=3D"background-color:rg=
ba(0,0,0,0)!important;border-color:rgb(32,33,36)!important;color:rgb(255,25=
5,255)!important" dir=3D"auto"><font style=3D"color:rgb(0,0,0)">Next to con=
sider is whether the gap is acceptable, weighing the effort to implement th=
e required consensus changes against the effort and feasibility of implemen=
ting your counterproposal.</font></div><div style=3D"background-color:rgba(=
0,0,0,0);border-color:rgb(32,33,36)" dir=3D"auto"><font style=3D"color:rgb(=
0,0,0)"><br></font></div><div style=3D"background-color:rgba(0,0,0,0);borde=
r-color:rgb(32,33,36)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)">I feel=
 that your counterproposal has little chance of being implemented because o=
f the still considerable effort required and the poor result in functional =
terms. I also wonder if your proposal is feasible considering wallet operab=
ility.</font></div><div style=3D"background-color:rgba(0,0,0,0);border-colo=
r:rgb(32,33,36)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)"><br></font><=
/div><div style=3D"background-color:rgba(0,0,0,0);border-color:rgb(32,33,36=
)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)">Considering all the above,=
 I believe that implementing consensus changes in order to support the prop=
osed functionality would preferable =C2=A0over your counterproposal.</font>=
</div><div style=3D"background-color:rgba(0,0,0,0);border-color:rgb(32,33,3=
6)" dir=3D"auto"><font style=3D"color:rgb(0,0,0)"><br></font></div><div sty=
le=3D"background-color:rgba(0,0,0,0);border-color:rgb(32,33,36)" dir=3D"aut=
o"><font style=3D"color:rgb(0,0,0)">I acknowledge that a consensus change t=
akes years and is difficult to achieve, but that should not be any reason t=
o stop exploring the appetite for the proposed functionality and perhaps st=
art looking at possible technical solutions.</font></div><div style=3D"back=
ground-color:rgba(0,0,0,0);border-color:rgb(32,33,36)" dir=3D"auto"><font s=
tyle=3D"color:rgb(0,0,0)"><br></font></div><div style=3D"background-color:r=
gba(0,0,0,0);border-color:rgb(32,33,36)" dir=3D"auto"><font style=3D"color:=
rgb(0,0,0)">Zac</font></div><div style=3D"background-color:rgba(0,0,0,0);bo=
rder-color:rgb(32,33,36)" dir=3D"auto"><br></div><div><br><div class=3D"gma=
il_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Sat, 14 Aug 2021 at 03:5=
0, ZmnSCPxj &lt;<a href=3D"mailto:ZmnSCPxj@protonmail.com">ZmnSCPxj@protonm=
ail.com</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"=
margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padd=
ing-left:1ex;border-left-color:rgb(204,204,204)">Good morning Zac,<br>
<br>
<br>
&gt; Hi=C2=A0ZmnSCPxj,<br>
&gt;<br>
&gt; Thank you for your insightful response.<br>
&gt;<br>
&gt; Perhaps I should take a step back and take a strictly functional angle=
. Perhaps the list could help me to establish whether=C2=A0the proposed fun=
ctionality is:<br>
&gt;<br>
&gt; Desirable;<br>
&gt; Not already possible;<br>
&gt; Feasible to implement.<br>
&gt;<br>
&gt; The proposed functionality is as follows:<br>
&gt;<br>
&gt; The ability to control some coin with two private keys (or two sets of=
 private keys) such that spending is limited over time for one private key =
(i.e., it is for instance not possible to spend all coin in a single transa=
ction) while spending is unrestricted for the other private key (no limits =
apply). No limits must apply to coin transacted to a third party.<br>
&gt;<br>
&gt; Also, it must be possible never having to bring the unrestricted priva=
te key online unless more than the limit imposed on the restrictive private=
 key is desired to be spent.<br>
&gt;<br>
&gt; Less generally, taking the perspective of a hodler: the user must be a=
ble to keep one key offline and one key online. The offline key allows unre=
stricted spending, the online key is limited in how much it is allowed to s=
pend over time.<br>
&gt;<br>
&gt; Furthermore, the spending limit must be intuitive. Best candidate I be=
lieve would be a maximum spend per some fixed number of blocks. For instanc=
e, the restrictive key may allow a maximum of 100k sats per any window of 1=
44 blocks. Ofcourse the user must be able to set these parameters freely.<b=
r>
<br>
My proposal does not *quite* implement a window.<br>
However, that is because it uses `nLockTime`.<br>
<br>
With the use of `nSequence` in relative-locktime mode, however, it *does* i=
mplement a window, sort of.<br>
More specifically, it implements a timeout on spending --- if you spend usi=
ng a presigned transaction (which creates an unencumbered specific-valued T=
XO that can be arbitrarily spent with your online keyset) then you cannot g=
et another &quot;batch&quot; of funds until the `nSequence` relative lockti=
me passes.<br>
However, this *does* implement a window that limits a maximum value spendab=
le per any window of the relative timelock you select.<br>
<br>
The disadvantage is that `nSequence` use is a lot more obvious and discerni=
ble than `nLockTime` use.<br>
Many wallets today use non-zero `nLockTime` for anti-fee-sniping, and that =
is a good cover for `nLockTime` transactions.<br>
I believe Dave Harding proposed that wallets should also use, at random, (s=
ay 50-50) `nSequence`-in-relative-locktime-mode as an alternate anti-fee-sn=
iping mechanism.<br>
This alternate anti-fee-sniping would help cover `nSequence` use.<br>
<br>
Note that my proposal does impose a maximum limit on the number of windows.=
<br>
With `nSequence`-in-relative-locktime-mode the limit is the number of times=
 that the online keyset can spend.<br>
After spending that many windows, the offline keyset has to be put back onl=
ine to generate a new set of transactions.<br>
<br>
It has the massive massive advantage that you can implement it today withou=
t any consensus change, and I think you can expect that consensus change wi=
ll take a LONG time (xref SegWit, Taproot).<br>
<br>
Certainly the functionality is desirable.<br>
But it seems it can be implemented with Bitcoin today.<br>
<br>
Regards,<br>
ZmnSCPxj<br>
<br>
</blockquote></div></div>

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