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Date: Sun, 12 Jun 2016 16:40:17 +0200
Message-ID: <CAPg+sBjXctdezfSbi1y7KVpNS4BwD6HESCgZLNaQGqDotVtwGQ@mail.gmail.com>
From: Pieter Wuille <pieter.wuille@gmail.com>
To: Luke Dash-Jr <luke@dashjr.org>,
	Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Content-Type: multipart/alternative; boundary=001a1142c0aa24e348053515c0db
Subject: Re: [bitcoin-dev] BIP141 segwit consensus rule update: extension of
 witness program definition
Precedence: list

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On Jun 8, 2016 18:46, "Luke Dashjr via bitcoin-dev" <
bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> On Wednesday, June 08, 2016 8:23:51 AM Johnson Lau wrote:
> > If someday 32 bytes hash is deemed to be unsafe, the txid would also be
> > unsafe and a hard fork might be needed. Therefore, I don=E2=80=99t see =
how a
> > witness program larger than 40 bytes would be useful in any case (as it
is
> > more expensive and takes more UTXO space). I think Pieter doesn=E2=80=
=99t want
to
> > make it unnecessarily lenient.
>
> There is no harm in being lenient, but it limits the ability to do
softfork
> upgrades in the future. I appreciate Pieter's concern that we'd need to d=
o
> more development and testing to go to this extreme, which is why I am onl=
y
> asking the limit raised to 75 bytes.

No strong opinion, but I'd rather not change it anymore, as I don't see the
point. Any data you would want to encode there can be moved to the witness
at 1/4 the cost and replaced by a 256-bit hash. If the data is 43 bytes or
higher, that is even cheaper. The only thing that cannot be in the hash is
metadata to indicate what hashing/rule scheme itself is used. I think 68
bits (OP_n + 8 bytes) for that is plenty.

--=20
Pieter

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<p dir=3D"ltr">On Jun 8, 2016 18:46, &quot;Luke Dashjr via bitcoin-dev&quot=
; &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-dev@=
lists.linuxfoundation.org</a>&gt; wrote:<br>
&gt;<br>
&gt; On Wednesday, June 08, 2016 8:23:51 AM Johnson Lau wrote:<br>
&gt; &gt; If someday 32 bytes hash is deemed to be unsafe, the txid would a=
lso be<br>
&gt; &gt; unsafe and a hard fork might be needed. Therefore, I don=E2=80=99=
t see how a<br>
&gt; &gt; witness program larger than 40 bytes would be useful in any case =
(as it is<br>
&gt; &gt; more expensive and takes more UTXO space). I think Pieter doesn=
=E2=80=99t want to<br>
&gt; &gt; make it unnecessarily lenient.<br>
&gt;<br>
&gt; There is no harm in being lenient, but it limits the ability to do sof=
tfork<br>
&gt; upgrades in the future. I appreciate Pieter&#39;s concern that we&#39;=
d need to do<br>
&gt; more development and testing to go to this extreme, which is why I am =
only<br>
&gt; asking the limit raised to 75 bytes.</p>
<p dir=3D"ltr">No strong opinion, but I&#39;d rather not change it anymore,=
 as I don&#39;t see the point. Any data you would want to encode there can =
be moved to the witness at 1/4 the cost and replaced by a 256-bit hash. If =
the data is 43 bytes or higher, that is even cheaper. The only thing that c=
annot be in the hash is metadata to indicate what hashing/rule scheme itsel=
f is used. I think 68 bits (OP_n + 8 bytes) for that is plenty.</p>
<p dir=3D"ltr">-- <br>
Pieter<br>
</p>

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