Date: Thu, 21 Jun 2018 03:09:14 -0400
To: Gregory Maxwell <greg@xiph.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Should Graftroot be optional?
Precedence: list

Good morning Greg,


> On Wed, Jun 20, 2018 at 12:12 PM, ZmnSCPxj via bitcoin-dev
>=20
> bitcoin-dev@lists.linuxfoundation.org wrote:
>=20
> > This has the advantage that the Graftroot signature commits to a single=
 outpoint and cannot be used to spend all outpoints that happen to pay to t=
he same `P` public key.
>=20
> If it isn't possible to make a graftroot signature independent of the
>=20
> outpoint then the functionality is greatly reduced to the point of
>=20
> largely mooting it-- because you could no longer prepare the grafts
>=20
> before the coins to be spent existed, and meaning you must stay online
>=20
> and sign new grafts as coins show up. In my view graft's two main
>=20
> gains are being able to delegate before coins exist and making the
>=20
> conditional transfer atomic (e.g. compared to just pre-signing a
>=20
> transaction). Making outpoint binding optional, so that you could
>=20
> choose to either sign for particular outputs or in a blanket way would
>=20
> be a lot more useful.
>=20

Perhaps `SIGHASH_NOINPUT` can do this? One can argue that the option to not=
 commit a signature to refer to a specific outpoint is orthogonal to the op=
tion to Graftroot, so having a separate flag for that makes sense.

The proposal could then be:

1. Define a transaction `nVersion` reserved for Graftroot. Transactions wit=
h that `nVersion` are disallowed in blocks.
2. If a next-SegWit-version P2WPKH (or P2WPK) is spent, and the top witness=
 stack item is a signature with `SIGHASH_GRAFTROOT` flag, then this is a Gr=
aftroot spend.
3. The signature signs an imaginary 1-input 1-output tx, with the input cop=
ied from the spending tx, the output value being the entire output being sp=
ent, and the output `scriptPubKey` being the Graftroot script (second to to=
p witness stack). The imaginary tx has the Graftroot-reserved `nVersion`.
4. The Graftroot signature has its other flags `SIGHASH_NOINPUT` evaluated =
also when verifying it signs the imaginary tx.
5. The Graftroot signature and the Graftroot script are popped and the scri=
pt executed in the context of the original Graftroot-spending tx.


This lets users select whether committing to a specific outpoint is needed =
or not, independently of Graftroot.

Regards,
ZmnSCPxj