From: "Kenshiro []" <tensiam@hotmail.com>
To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
	ZmnSCPxj <ZmnSCPxj@protonmail.com>
Thread-Topic: [bitcoin-dev] Implementing Confidential Transactions in
	extension blocks
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Date: Mon, 11 Feb 2019 10:19:15 +0000
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Subject: Re: [bitcoin-dev] Implementing Confidential Transactions in
 extension blocks
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Good morning ZmnSCPxj,

Thank you for your answer.

There is a position that fullnodes must be able to get a view of the UTXO s=
et, and extension blocks (which are invisible to pre-extension-block fullno=
des) means that fullnodes no longer have an accurate view of the UTXO set.

I think old nodes don't need to know the CT part of the UTXO set. It would =
be possible to move coins from normal address to CT address and the opposit=
e, it would be written as "anyone-can-spend" transactions in the main block=
 so old nodes are fully aware of these transactions. Miners would enforce t=
hat "anyone-can-spend" transactions are true. The full details of the trans=
actions involving CT would be in the extension block. CT to CT transactions=
 don't need to be written in the main block. Maybe I'm missing some technic=
al detail here but it looks good for me.


> - Capacity increase: the CT signature is stored in the extension block, s=
o CT transactions  increase the maximum number of transactions per block

This is not an unalloyed positive: block size increase, even via extension =
block, translates to greater network capacity usage globally on all fullnod=
es.

Yes, there is an increase in block size and network usage but I think it wo=
uld still be possible for people with regular computers to run a full node,=
 an people in developing countries could use light wallets.

Regards


________________________________
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
Sent: Monday, February 11, 2019 5:29
To: Kenshiro \[\]; Bitcoin Protocol Discussion
Subject: Re: [bitcoin-dev] Implementing Confidential Transactions in extens=
ion blocks

Good morning Kenshiro,

> - Soft fork: old nodes see CT transactions as "sendtoany" transactions

There is a position that fullnodes must be able to get a view of the UTXO s=
et, and extension blocks (which are invisible to pre-extension-block fullno=
des) means that fullnodes no longer have an accurate view of the UTXO set.
SegWit still provides pre-SegWit fullnodes with a view of the UTXO set, alt=
hough pre-SegWit fullnodes could be convinced that a particular UTXO is any=
one-can-spend even though they are no longer anyone-can-spend.

Under this point-of-view, then, extension block is "not" soft fork.
It is "evil" soft fork since older nodes are forced to upgrade as their int=
ended functionality becomes impossible.
In this point-of-view, it is no better than a hard fork, which at least is =
very noisy about how older fullnode versions will simply stop working.

> - Safe: if there is a software bug in CT it's impossible to create new co=
ins because the coins move from normal block to normal block as public tran=
sactions

I think more relevant here is the issue of a future quantum computing breac=
h of the algorithms used to implement confidentiality.

I believe this is also achievable with a non-extension-block approach by im=
plementing a globally-verified publicly-visible counter of the total amount=
 in all confidential transaction outputs.
Then it becomes impossible to move from confidential to public transactions=
 with a value more than this counter, thus preventing inflation even if a f=
uture QC breach allows confidential transaction value commitments to be ope=
ned to any value.

(do note that a non-extension-block approach is a definite hardfork)

> - Capacity increase: the CT signature is stored in the extension block, s=
o CT transactions increase the maximum number of transactions per block

This is not an unalloyed positive: block size increase, even via extension =
block, translates to greater network capacity usage globally on all fullnod=
es.

Regards,
ZmnSCPxj

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<style type=3D"text/css" style=3D"display:none;"> P {margin-top:0;margin-bo=
ttom:0;} </style>
</head>
<body dir=3D"ltr">
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
Good morning&nbsp;ZmnSCPxj,</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
Thank you for your answer.</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<i>There is a position that fullnodes must be able to get a view of the UTX=
O set, and extension blocks (which are invisible to pre-extension-block ful=
lnodes) means that fullnodes no longer have an accurate view of the UTXO se=
t.</i><br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
I think old nodes don't need to know the CT part of the UTXO set. It would =
be possible to move coins from normal address to CT address and the opposit=
e, it would be written as &quot;anyone-can-spend&quot; transactions in the =
main block so old nodes are fully aware of
 these transactions. Miners would enforce that &quot;anyone-can-spend&quot;=
 transactions are true. The full details of the transactions involving CT w=
ould be in the extension block. CT to CT transactions don't need to be writ=
ten in the main block. Maybe I'm missing some
 technical detail here but it looks good for me.</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<span><i>&gt; - Capacity increase: the CT signature is stored in the extens=
ion block, so CT transactions&nbsp; increase the maximum number of transact=
ions per block<br>
</i></span>
<div><i><br>
</i></div>
<span><i>This is not an unalloyed positive: block size increase, even via e=
xtension block, translates to greater network capacity usage globally on al=
l fullnodes.</i></span><br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<span><br>
</span></div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
Yes, there is an increase in block size and network usage but I think it wo=
uld still be possible for people with regular computers to run a full node,=
 an people in developing countries could use light wallets.</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
Regards</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div>
<div id=3D"appendonsend"></div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<br>
</div>
<hr tabindex=3D"-1" style=3D"display:inline-block; width:98%">
<div id=3D"divRplyFwdMsg" dir=3D"ltr"><font face=3D"Calibri, sans-serif" co=
lor=3D"#000000" style=3D"font-size:11pt"><b>From:</b> ZmnSCPxj &lt;ZmnSCPxj=
@protonmail.com&gt;<br>
<b>Sent:</b> Monday, February 11, 2019 5:29<br>
<b>To:</b> Kenshiro \[\]; Bitcoin Protocol Discussion<br>
<b>Subject:</b> Re: [bitcoin-dev] Implementing Confidential Transactions in=
 extension blocks</font>
<div>&nbsp;</div>
</div>
<div class=3D"BodyFragment"><font size=3D"2"><span style=3D"font-size:11pt"=
>
<div class=3D"PlainText">Good morning Kenshiro,<br>
<br>
&gt; - Soft fork: old nodes see CT transactions as &quot;sendtoany&quot; tr=
ansactions<br>
<br>
There is a position that fullnodes must be able to get a view of the UTXO s=
et, and extension blocks (which are invisible to pre-extension-block fullno=
des) means that fullnodes no longer have an accurate view of the UTXO set.<=
br>
SegWit still provides pre-SegWit fullnodes with a view of the UTXO set, alt=
hough pre-SegWit fullnodes could be convinced that a particular UTXO is any=
one-can-spend even though they are no longer anyone-can-spend.<br>
<br>
Under this point-of-view, then, extension block is &quot;not&quot; soft for=
k.<br>
It is &quot;evil&quot; soft fork since older nodes are forced to upgrade as=
 their intended functionality becomes impossible.<br>
In this point-of-view, it is no better than a hard fork, which at least is =
very noisy about how older fullnode versions will simply stop working.<br>
<br>
&gt; - Safe: if there is a software bug in CT it's impossible to create new=
 coins because the coins move from normal block to normal block as public t=
ransactions<br>
<br>
I think more relevant here is the issue of a future quantum computing breac=
h of the algorithms used to implement confidentiality.<br>
<br>
I believe this is also achievable with a non-extension-block approach by im=
plementing a globally-verified publicly-visible counter of the total amount=
 in all confidential transaction outputs.<br>
Then it becomes impossible to move from confidential to public transactions=
 with a value more than this counter, thus preventing inflation even if a f=
uture QC breach allows confidential transaction value commitments to be ope=
ned to any value.<br>
<br>
(do note that a non-extension-block approach is a definite hardfork)<br>
<br>
&gt; - Capacity increase: the CT signature is stored in the extension block=
, so CT transactions increase the maximum number of transactions per block<=
br>
<br>
This is not an unalloyed positive: block size increase, even via extension =
block, translates to greater network capacity usage globally on all fullnod=
es.<br>
<br>
Regards,<br>
ZmnSCPxj<br>
</div>
</span></font></div>
</div>
</body>
</html>

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