MIME-Version: 1.0
References: <7E4FA664-BBAF-421F-8C37-D7CE3AA5310A@gmail.com>
	<F87D7069-0FDC-4572-B02B-398A2A455935@gmail.com>
	<CAAS2fgT716PiP0ucoASxryM9y+s9H2z06Z0ToaP1xT3BozAtNw@mail.gmail.com>
	<CADZtCSguto2z6Z9CykymxnCokqo1G=sW0Ov0ht+KcD+KMnYyow@mail.gmail.com>
	<CAO3Pvs-YDzfRqmyJ85wTH0ciccjCvkm5stGyP_tVGGna=PMv3A@mail.gmail.com>
	<CAO3Pvs9p5COiS_7Jbj1r2iAKTEdXUcnVTRzL27c3=CeuB9WDTQ@mail.gmail.com>
	<CAAS2fgSyVi0d_ixp-auRPPzPfFeffN=hsWhWT5=EzDO3O+Ue1g@mail.gmail.com>
	<CAO3Pvs_0qCZbRCfL8EJw6gzWjZeXWcJrtg27g_SJ7+PkYTHg6A@mail.gmail.com>
	<CAAS2fgTs+aKyiL8Kg_AZk=Mdh6896MEg=KHa6ANAZO7unsGEsg@mail.gmail.com>
	<CADZtCShyYbgKk2zsKzQniqDw--XKfYWTk3Hk3o50V=MgT6zeuQ@mail.gmail.com>
	<20180602124157.744x7j4u7dqtaa43@email>
	<343A3542-3103-42E9-95B7-640DFE958FFA@gmail.com>
	<CAAS2fgQDdJpzPR9Ve81hhyqU+MO7Ryy125fzK-iv=sfwwORDCw@mail.gmail.com>
	<37BECD1A-7515-4081-85AC-871B9FB57772@gmail.com>
In-Reply-To: <37BECD1A-7515-4081-85AC-871B9FB57772@gmail.com>
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Sat, 2 Jun 2018 23:11:56 -0700
Message-ID: <CAPg+sBjXbwTKW+qbGwJgau-Q2-uJC6N1JH8hH4KThv0Ah3WuqA@mail.gmail.com>
To: Tamas Blummer <tamas.blummer@gmail.com>, 
	Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Content-Type: multipart/alternative; boundary="0000000000007db9d3056db6b28f"
Subject: Re: [bitcoin-dev] BIP 158 Flexibility and Filter Size
Precedence: list

--0000000000007db9d3056db6b28f
Content-Type: text/plain; charset="UTF-8"

On Sat, Jun 2, 2018, 22:56 Tamas Blummer via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Lighter but SPV secure nodes (filter committed) would help the network
> (esp. Layer 2) to grow mesh like, but add more user that blindly follow POW.
>
> On longer term most users' security will be determined by either trusted
> hubs or POW.
> I do not know which is worse, but we should at least offer the choice to
> the user, therefore commit filters.
>

I don't think that's the point of discussion here. Of course, in order to
have filters that verifiably don't lie by omission, the filters need to be
committed to by blocks.

The question is what data that filter should contain.

There are two suggestions:
(a) The scriptPubKeys of the block's outputs, and prevouts of the block's
inputs.
(b) The scriptPubKeys of the block's outputs, and scriptPubKeys of outputs
being spent by the block's inputs.

The advantage of (a) is that it can be verified against a full block
without access to the outputs being spent by it. This allows light clients
to ban nodes that give them incorrect filters, but they do need to actually
see the blocks (partially defeating the purpose of having filters in the
first place).

The advantage of (b) is that it is more compact (scriot reuse, and outputs
spent within the same block as they are created). It also had the advantage
of being more easily usable for scanning of a wallet's transactions. Using
(a) for that in some cases may need to restart and refetch when an output
is discovered, to go test for its spending (whose outpoint is not known
ahead of time). Especially when fetching multiple filters at a time this
may be an issue.

I think both of these potentially good arguments. However, once a committed
filter exists, the advantage of (a) goes away completely - validation of
committed filters is trivial and can be done without needing the full
blocks in the first place.

So I think the question is do we aim for an uncommitted (a) first and a
committed (b) later, or go for (b) immediately?

Cheers,

-- 
Pieter

--0000000000007db9d3056db6b28f
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

<div dir=3D"auto"><div><div class=3D"gmail_quote"><div dir=3D"ltr">On Sat, =
Jun 2, 2018, 22:56 Tamas Blummer via bitcoin-dev &lt;<a href=3D"mailto:bitc=
oin-dev@lists.linuxfoundation.org">bitcoin-dev@lists.linuxfoundation.org</a=
>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0 0=
 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Lighter but SPV secure=
 nodes (filter committed) would help the network (esp. Layer 2) to grow mes=
h like, but add more user that blindly follow POW.<br>
<br>
On longer term most users&#39; security will be determined by either truste=
d hubs or POW.<br>
I do not know which is worse, but we should at least offer the choice to th=
e user, therefore commit filters.<br></blockquote></div></div><div dir=3D"a=
uto"><br></div><div dir=3D"auto">I don&#39;t think that&#39;s the point of =
discussion here. Of course, in order to have filters that verifiably don=
9;t lie by omission, the filters need to be committed to by blocks.</div><d=
iv dir=3D"auto"><br></div><div dir=3D"auto">The question is what data that =
filter should contain.</div><div dir=3D"auto"><br></div><div dir=3D"auto">T=
here are two suggestions:</div><div dir=3D"auto">(a) The scriptPubKeys of t=
he block&#39;s outputs, and prevouts of the block&#39;s inputs.</div><div d=
ir=3D"auto">(b) The scriptPubKeys of the block&#39;s outputs, and scriptPub=
Keys of outputs being spent by the block&#39;s inputs.</div><div dir=3D"aut=
o"><br></div><div dir=3D"auto">The advantage of (a) is that it can be verif=
ied against a full block without access to the outputs being spent by it. T=
his allows light clients to ban nodes that give them incorrect filters, but=
 they do need to actually see the blocks (partially defeating the purpose o=
f having filters in the first place).</div><div dir=3D"auto"><br></div><div=
 dir=3D"auto">The advantage of (b) is that it is more compact (scriot reuse=
, and outputs spent within the same block as they are created). It also had=
 the advantage of being more easily usable for scanning of a wallet&#39;s t=
ransactions. Using (a) for that in some cases may need to restart and refet=
ch when an output is discovered, to go test for its spending (whose outpoin=
t is not known ahead of time). Especially when fetching multiple filters at=
 a time this may be an issue.</div><div dir=3D"auto"><br></div><div dir=3D"=
auto">I think both of these potentially good arguments. However, once a com=
mitted filter exists, the advantage of (a) goes away completely - validatio=
n of committed filters is trivial and can be done without needing the full =
blocks in the first place.</div><div dir=3D"auto"><br></div><div dir=3D"aut=
o">So I think the question is do we aim for an uncommitted (a) first and a =
committed (b) later, or go for (b) immediately?</div><div dir=3D"auto"><br>=
</div><div dir=3D"auto">Cheers,</div><div dir=3D"auto"><br></div><div dir=
=3D"auto">--=C2=A0</div><div dir=3D"auto">Pieter</div><div dir=3D"auto"><br=
></div><div dir=3D"auto"><div class=3D"gmail_quote"><blockquote class=3D"gm=
ail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-le=
ft:1ex">
</blockquote></div></div></div>

--0000000000007db9d3056db6b28f--