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From: Johnson Lau <jl2012@xbt.hk>
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To: ZmnSCPxj <ZmnSCPxj@protonmail.com>,
	bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] [Lightning-dev] More thoughts on NOINPUT safety
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> On 20 Mar 2019, at 4:07 PM, ZmnSCPxj via bitcoin-dev =
<bitcoin-dev@lists.linuxfoundation.org> wrote:
>=20
> Hi aj,
>=20
> Re-reading again, I think perhaps I was massively confused by this:
>=20
>> - alternatively, we could require every script to have a valid =
signature
>> that commits to the input. In that case, you could do eltoo with a
>> script like either:
>>=20
>> <A> CHECKSIGVERIFY <B> CHECKSIG
>> or <P> CHECKSIGVERIFY <Q> CHECKSIG
>>=20
>>=20
>> where A is Alice's key and B is Bob's key, P is muSig(A,B) and Q is
>> a key they both know the private key for. In the first case, Alice
>> would give Bob a NOINPUT sig for the tx, and when Bob wanted to =
publish
>> Bob would just do a SIGHASH_ALL sig with his own key. In the second,
>> Alice and Bob would share partial NOINPUT sigs of the tx with P, and
>> finish that when they wanted to publish.
>=20
> Do you mean that *either* of the above two scripts is OK, *or* do you =
mean they are alternatives within a single MAST or `OP_IF`?
>=20

It means either.

If you use <A> CHECKSIGVERIFY <B> CHECKSIG style, A and B will exchange =
the NOINPUT sig, and they will add the required non-NOINPUT sig when =
needed.

If you use <muSig(A,B)> CHECKVERIFY <Q> CHECKSIG, A and B will co-sign =
the muSig(A,B) with NOINPUT. They will also share the private key of Q, =
so they could produce a non-NOINPUT sig when needed.

The first style is slightly easier as it doesn=E2=80=99t need muSig. But =
with 3 or more parties, the second style is more efficient.

However, if you use watchtower, you have to use the second style. That =
means you need to share the private key for Q with the watchtower, That =
also means the watchtower will have the ability to reply the NOINPU =
muSig. But it is still strictly better than anyone-can-replay.


--Apple-Mail=_1BEAD8F9-AF34-4F09-BDF0-F25AE6715A03
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<html><head><meta http-equiv=3D"Content-Type" content=3D"text/html; =
charset=3Dutf-8"></head><body style=3D"word-wrap: break-word; =
-webkit-nbsp-mode: space; line-break: after-white-space;" class=3D""><br =
class=3D""><div><br class=3D""><blockquote type=3D"cite" class=3D""><div =
class=3D"">On 20 Mar 2019, at 4:07 PM, ZmnSCPxj via bitcoin-dev &lt;<a =
href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" =
class=3D"">bitcoin-dev@lists.linuxfoundation.org</a>&gt; wrote:</div><br =
class=3D"Apple-interchange-newline"><div class=3D""><span =
style=3D"caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: =
12px; font-style: normal; font-variant-caps: normal; font-weight: =
normal; letter-spacing: normal; text-align: start; text-indent: 0px; =
text-transform: none; white-space: normal; word-spacing: 0px; =
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; =
display: inline !important;" class=3D"">Hi aj,</span><br =
style=3D"caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: =
12px; font-style: normal; font-variant-caps: normal; font-weight: =
normal; letter-spacing: normal; text-align: start; text-indent: 0px; =
text-transform: none; white-space: normal; word-spacing: 0px; =
-webkit-text-stroke-width: 0px; text-decoration: none;" class=3D""><br =
style=3D"caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: =
12px; font-style: normal; font-variant-caps: normal; font-weight: =
normal; letter-spacing: normal; text-align: start; text-indent: 0px; =
text-transform: none; white-space: normal; word-spacing: 0px; =
-webkit-text-stroke-width: 0px; text-decoration: none;" class=3D""><span =
style=3D"caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: =
12px; font-style: normal; font-variant-caps: normal; font-weight: =
normal; letter-spacing: normal; text-align: start; text-indent: 0px; =
text-transform: none; white-space: normal; word-spacing: 0px; =
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; =
display: inline !important;" class=3D"">Re-reading again, I think =
perhaps I was massively confused by this:</span><br style=3D"caret-color: =
rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: =
normal; font-variant-caps: normal; font-weight: normal; letter-spacing: =
normal; text-align: start; text-indent: 0px; text-transform: none; =
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; =
text-decoration: none;" class=3D""><br style=3D"caret-color: rgb(0, 0, =
0); font-family: Helvetica; font-size: 12px; font-style: normal; =
font-variant-caps: normal; font-weight: normal; letter-spacing: normal; =
text-align: start; text-indent: 0px; text-transform: none; white-space: =
normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; =
text-decoration: none;" class=3D""><blockquote type=3D"cite" =
style=3D"font-family: Helvetica; font-size: 12px; font-style: normal; =
font-variant-caps: normal; font-weight: normal; letter-spacing: normal; =
orphans: auto; text-align: start; text-indent: 0px; text-transform: =
none; white-space: normal; widows: auto; word-spacing: 0px; =
-webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; =
text-decoration: none;" class=3D"">- alternatively, we could require =
every script to have a valid signature<br class=3D"">that commits to the =
input. In that case, you could do eltoo with a<br class=3D"">script like =
either:<br class=3D""><br class=3D"">&lt;A&gt; CHECKSIGVERIFY &lt;B&gt; =
CHECKSIG<br class=3D"">or &lt;P&gt; CHECKSIGVERIFY &lt;Q&gt; CHECKSIG<br =
class=3D""><br class=3D""><br class=3D"">where A is Alice's key and B is =
Bob's key, P is muSig(A,B) and Q is<br class=3D"">a key they both know =
the private key for. In the first case, Alice<br class=3D"">would give =
Bob a NOINPUT sig for the tx, and when Bob wanted to publish<br =
class=3D"">Bob would just do a SIGHASH_ALL sig with his own key. In the =
second,<br class=3D"">Alice and Bob would share partial NOINPUT sigs of =
the tx with P, and<br class=3D"">finish that when they wanted to =
publish.<br class=3D""></blockquote><br style=3D"caret-color: rgb(0, 0, =
0); font-family: Helvetica; font-size: 12px; font-style: normal; =
font-variant-caps: normal; font-weight: normal; letter-spacing: normal; =
text-align: start; text-indent: 0px; text-transform: none; white-space: =
normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; =
text-decoration: none;" class=3D""><span style=3D"caret-color: rgb(0, 0, =
0); font-family: Helvetica; font-size: 12px; font-style: normal; =
font-variant-caps: normal; font-weight: normal; letter-spacing: normal; =
text-align: start; text-indent: 0px; text-transform: none; white-space: =
normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; =
text-decoration: none; float: none; display: inline !important;" =
class=3D"">Do you mean that *either* of the above two scripts is OK, =
*or* do you mean they are alternatives within a single MAST or =
`OP_IF`?</span><br style=3D"caret-color: rgb(0, 0, 0); font-family: =
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: =
normal; font-weight: normal; letter-spacing: normal; text-align: start; =
text-indent: 0px; text-transform: none; white-space: normal; =
word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: =
none;" class=3D""><br style=3D"caret-color: rgb(0, 0, 0); font-family: =
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: =
normal; font-weight: normal; letter-spacing: normal; text-align: start; =
text-indent: 0px; text-transform: none; white-space: normal; =
word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: =
none;" class=3D""></div></blockquote><div><br class=3D""></div><div>It =
means either.</div><div><br class=3D""></div><div>If you use &lt;A&gt; =
CHECKSIGVERIFY &lt;B&gt; CHECKSIG style, A and B will exchange the =
NOINPUT sig, and they will add the required non-NOINPUT sig when =
needed.</div><div><br class=3D""></div><div>If you use =
&lt;muSig(A,B)&gt; CHECKVERIFY &lt;Q&gt; CHECKSIG, A and B will co-sign =
the muSig(A,B) with NOINPUT. They will also share the private key of Q, =
so they could produce a non-NOINPUT sig when needed.</div><div><br =
class=3D""></div><div>The first style is slightly easier as it doesn=E2=80=
=99t need muSig. But with 3 or more parties, the second style is more =
efficient.</div><div><br class=3D""></div><div>However, if you use =
watchtower, you have to use the second style. That means you need to =
share the private key for Q with the watchtower, That also means the =
watchtower will have the ability to reply the NOINPU muSig. But it is =
still strictly better than anyone-can-replay.</div></div><br =
class=3D""></body></html>=

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