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From: Tim Ruffing <crypto@timruffing.de>
To: Pieter Wuille <pieter.wuille@gmail.com>, Bitcoin Protocol Discussion
	<bitcoin-dev@lists.linuxfoundation.org>
Date: Wed, 06 Jun 2018 23:25:33 +0200
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Subject: Re: [bitcoin-dev] Should Graftroot be optional?
Precedence: list

What you're saying makes sense.

By the way, an even stronger reason why you shouldn't be able to
"repurpose" just a Graftroot signature as a transaction: You may want
to reveal to others that you've delegated. But if an observer sees the
delegation (literally the Graftroot signature), this observer could
send the Graftroot signature to the network (and lock out the other
delegates and the initial owner). So you would need to keep the
signature itself secret, otherwise we can't call this delegation.

So it may sense to consider the idea of an implicit transaction for the
case when one really solves the delegated script (as you mentioned) but
only in this case.

Tim


On Wed, 2018-06-06 at 10:04 -0700, Pieter Wuille wrote:
> On Wed, Jun 6, 2018 at 5:48 AM, Tim Ruffing via bitcoin-dev
> <bitcoin-dev@lists.linuxfoundation.org> wrote:
> > On Thu, 2018-05-31 at 17:25 -0700, Pieter Wuille via bitcoin-dev
> > wrote:
> > > The best argument for why Graftroot does not need to be optional
> > > I
> > > think was how Greg put it: "since the signer(s) could have signed
> > > an
> > > arbitrary transaction instead, being able to delegate is strictly
> > > less
> > > powerful.".
> 
> ...
> 
> > So
> > I think Graftroot delegation is not "strictly less powerful" than
> > just
> > using a normal transaction: Graftroot enables to delegate in a way
> > such
> > that the delegation itself cannot be fixed in the chain. I think
> > this
> > is not possible currently. (Okay, you can just pass around the
> > secret
> > keys but has other problems obviously).
> > 
> > Does this have practical implications?
> > I don't see any but maybe this helps someone to identify an
> > undesirable
> > implication.
> 
> Interesting point; I don't see any relevant implications to this
> either, but it's indeed good to point out this as a distinction.
> 
> > One way to be on the safe side and probably make Greg's argument go
> > through is to just define the semantics such that (*) is allowed,
> > i.e.,
> > call g-sig a "Graftroot transaction" and give it transaction
> > semantics.
> > This provides a new perspective on Graftroot: Then Graftroot does
> > not
> > introduce new semantics but (*) is just an optimized version of
> > (**)
> > that uses fewer bytes and may be better for privacy.
> 
> So you're saying: the Graftroot signature data could be made
> identical
> to the signature hash of an implicit 1-input-1-output transaction
> spending the coin and creating a new output with the delegated script
> as sPK, and the same amount.
> 
> I like that idea, but I don't think it can be *exactly* that. If it's
> possible to take a Graftroot signature and instead construct a
> transaction with it, you have inherently introduced a malleability.
> The created outpoint will be different in both cases (different
> txid),
> meaning that a chain of dependent unconfirmed transactions may be
> broken by giving one participant the ability to choose between
> Graftroot delegation or actual spending.
> 
> Two points here: (1) the implicit transaction would be 0 fee (unless
> we somehow assign a portion of the fee to the delegation itself for
> purposes of sighash computing), and (2) this sounds very similar to
> the issue SIGHASH_NOINPUT is intended to solve. About that...
> 
> > Interestingly Andrew's blind-sig example and Johnson's fix (g-sig
> > signs
> > the outpoint) are just a special case. If g-sig has transaction
> > semantics, it must sign the outpoint (and other stuff).
> 
> You're right when you're comparing with existing transaction sighash
> semantics, but not when SIGHASH_NOINPUT would exist. If that were the
> case, the only real difference is your point above of not being able
> to commit the implicit transaction separately. In other words, we're
> back to something Johnson pointed out earlier: some of the perceived
> problems with Graftroot are also issues with SIGHASH_NOINPUT.
> 
> I wonder if we can make this explicit: Graftroot spending becomes a
> special sighash flag (which possibly is only allowed at the top
> level)
> - it builds an implicit transaction which moves all the coins to a
> newly provided script, computes the sighash of that transaction
> (taking all of the Graftroot signature's sighash flags into account -
> including potentially SIGHASH_NOINPUT), and requires a signature with
> that. The delegated script is then evaluated in the context of that
> implicit transaction.
> 
> However, in order to avoid the malleability issue I think the actual
> signature should still be different - possibly by simply passing
> through the Graftroot sighash flag into the sighash being computed.
> 
> Cheers,
>