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From: "Russell O'Connor" <roconnor@blockstream.io>
Date: Sun, 1 Oct 2017 15:41:46 -0400
Message-ID: <CAMZUoK=heF1FALyGbi7cpzLiQuhLnsq-5Z2-sTgq5b28sjjeUw@mail.gmail.com>
To: Mark Friedenbach <mark@friedenbach.org>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Version 1 witness programs (first draft)
Precedence: list

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On Sun, Oct 1, 2017 at 3:27 PM, Mark Friedenbach <mark@friedenbach.org>
wrote:

> > On Oct 1, 2017, at 12:05 PM, Russell O'Connor <roconnor@blockstream.io>
> wrote:
> >
> > Given the proposed fixed signature size, It seems better to me that we
> create a SIGHASH_WITNESS_WEIGHT flag as opposed to SIGHASH_WITNESS_DEPTH.
>
> For what benefit? If your script actually uses all the items on the stack=
,
> and if your script is not written in such a way as to allow malleability
> (which cannot be prevented in general), then they=E2=80=99re equivalent. =
Using
> weight instead of depth only needlessly restricts other parties to select=
 a
> witness size up-front.
>

Creating a Bitcoin script that does not allow malleability is difficult and
requires wasting a lot of bytes to do so, typically when handling issues
around non-0-or-1 witness values being used with OP_IF, and dealing with
non-standard-zero values, etc.  Adding a witness weight flag cuts through
the worst of all this, and makes script design enormously simpler and makes
scripts smaller and cheaper.


> And to be clear, signing witness weight doesn=E2=80=99t mean the witness =
is not
> malleable. The signer could sign again with a different ECDSA nonce. Or i=
f
> the signer is signing from a 2-of-3 wallet, a common scenario I hope, the=
re
> are 3 possible key combinations that could be used. If using MBV, a
> 3-element tree is inherently unbalanced and the common use case can have =
a
> smaller proof size.
>
> Witnesses are not 3rd party malleable and we will maintain that property
> going forward with future opcodes.
>
> > Mark, you seem to be arguing that in general we still want weight
> malleability even with witness depth fixed, but I don't understand in wha=
t
> scenario we would want that.
>
> Any time all parties are not online at the same time in an interactive
> signing protocol, or for which individual parties have to reconfigure the=
ir
> signing choices due to failures. We should not restrict our script
> signature system to such a degree that it becomes difficult to create
> realistic signing setups for people using best practices (multi-key, 2FA,
> etc.) to sign. If I am a participant in a signing protocol, it would be
> layer violating to treat me as anything other than a black box, such that
> internal errors and timeouts in my signing setup don=E2=80=99t propagate =
upwards to
> the multi-party protocol.
>
> For example, I should be able to try to 2FA sign, and if that fails go
> fetch my backup key and sign with that. But because it=E2=80=99s my infre=
quently
> used backup key, it might be placed deeper in the key tree and therefore
> signatures using it are larger. All the other signers need care is that
> slot #3 in the witness is where my Merkle proof goes. They shouldn=E2=80=
=99t have
> to restart and resign because my proof was a little larger than anticipat=
ed
> =E2=80=94 and maybe they can=E2=80=99t resign because double-spend protec=
tions!
>

I'll argue that I don't want my counter-party going off and using a very
deeply nested key in order to subvert the fee rate we've agreed upon after
I've signed my part of the input.  If we are doing multi-party signing of
inputs we need to communicate anyways to construct the transaction.  I see
no problem with requiring my counter-party to choose their keys before I
sign so that I know up front what our fee rate is going to be.  If they
lose their keys and need a backup, they should have to come back to me to
resign in order that we can negotiate a new fee rate for the transaction
and who is going to be covering how much of the fee and on which inputs.

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<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On S=
un, Oct 1, 2017 at 3:27 PM, Mark Friedenbach <span dir=3D"ltr">&lt;<a href=
=3D"mailto:mark@friedenbach.org" target=3D"_blank">mark@friedenbach.org</a>=
&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0=
 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class=3D"">&gt; =
On Oct 1, 2017, at 12:05 PM, Russell O&#39;Connor &lt;<a href=3D"mailto:roc=
onnor@blockstream.io">roconnor@blockstream.io</a>&gt; wrote:<br>
&gt;<br>
&gt; Given the proposed fixed signature size, It seems better to me that we=
 create a SIGHASH_WITNESS_WEIGHT flag as opposed to SIGHASH_WITNESS_DEPTH.<=
br>
<br>
</span>For what benefit? If your script actually uses all the items on the =
stack, and if your script is not written in such a way as to allow malleabi=
lity (which cannot be prevented in general), then they=E2=80=99re equivalen=
t. Using weight instead of depth only needlessly restricts other parties to=
 select a witness size up-front.<br></blockquote><div><br></div><div>Creati=
ng a Bitcoin script that does not allow malleability is difficult and requi=
res wasting a lot of bytes to do so, typically when handling issues around =
non-0-or-1 witness values being used with OP_IF, and dealing with non-stand=
ard-zero values, etc.=C2=A0 Adding a witness weight flag cuts through the w=
orst of all this, and makes script design enormously simpler and makes scri=
pts smaller and cheaper.<br></div><div>=C2=A0<br></div><blockquote class=3D=
"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding=
-left:1ex">
And to be clear, signing witness weight doesn=E2=80=99t mean the witness is=
 not malleable. The signer could sign again with a different ECDSA nonce. O=
r if the signer is signing from a 2-of-3 wallet, a common scenario I hope, =
there are 3 possible key combinations that could be used. If using MBV, a 3=
-element tree is inherently unbalanced and the common use case can have a s=
maller proof size.<br>
<br>
Witnesses are not 3rd party malleable and we will maintain that property go=
ing forward with future opcodes.<br>
<span class=3D""><br>
&gt; Mark, you seem to be arguing that in general we still want weight mall=
eability even with witness depth fixed, but I don&#39;t understand in what =
scenario we would want that.<br>
<br>
</span>Any time all parties are not online at the same time in an interacti=
ve signing protocol, or for which individual parties have to reconfigure th=
eir signing choices due to failures. We should not restrict our script sign=
ature system to such a degree that it becomes difficult to create realistic=
 signing setups for people using best practices (multi-key, 2FA, etc.) to s=
ign. If I am a participant in a signing protocol, it would be layer violati=
ng to treat me as anything other than a black box, such that internal error=
s and timeouts in my signing setup don=E2=80=99t propagate upwards to the m=
ulti-party protocol.<br>
<br>
For example, I should be able to try to 2FA sign, and if that fails go fetc=
h my backup key and sign with that. But because it=E2=80=99s my infrequentl=
y used backup key, it might be placed deeper in the key tree and therefore =
signatures using it are larger. All the other signers need care is that slo=
t #3 in the witness is where my Merkle proof goes. They shouldn=E2=80=99t h=
ave to restart and resign because my proof was a little larger than anticip=
ated =E2=80=94 and maybe they can=E2=80=99t resign because double-spend pro=
tections!<br></blockquote><div><br></div><div>I&#39;ll argue that I don&#39=
;t want my counter-party going off and using a very deeply nested key in or=
der to subvert the fee rate we&#39;ve agreed upon after I&#39;ve signed my =
part of the input.=C2=A0 If we are doing multi-party signing of inputs we n=
eed to communicate anyways to construct the transaction.=C2=A0 I see no pro=
blem with requiring my counter-party to choose their keys before I sign so =
that I know up front what our fee rate is going to be.=C2=A0 If they lose t=
heir keys and need a backup, they should have to come back to me to resign =
in order that we can negotiate a new fee rate for the transaction and who i=
s going to be covering how much of the fee and on which inputs.<br></div></=
div><br></div></div>

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