Re: [bitcoin-dev] Should Graftroot be optional?

[ZmnSCPxj <ZmnSCPxj@protonmail.com>]

Good morning Greg,


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

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 option 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 with 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 Graftroot spend.
3. The signature signs an imaginary 1-input 1-output tx, with the input copied from the spending tx, the output value being the entire output being spent, and the output `scriptPubKey` being the Graftroot script (second to top 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 script 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