Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

[Johnson Lau <jl2012@xbt.hk>]

> On 23 Nov 2018, at 5:40 PM, Christian Decker via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
> 
> Anthony Towns <aj@erisian.com.au> writes:
>> Commiting to just the sequence numbers seems really weird to me; it
>> only really prevents you from adding inputs, since you could still
>> replace any input that was meant to be there by almost any arbitrary
>> other transaction...
> 
> It's a really roundabout way of committing to the inputs, I
> agree. I'm actually wondering if it makes sense to correct that
> additional blanked field in BIP118 at all since it seems there is no
> real use-case for NOINPUT that doesn't involve blanking the
> `hashSequence` as well.

I think we just make it as simple as this: Always commit to sequence of the same input. Commit to hashSequence if and only if all inputs and all outputs are signed.

The next-generation SIGHASH will introduce not only NOINPUT, but also signing of fees, previous scriptPubKey, and all input values, etc. So it won’t be a simple hack over BIP143. BIP118 might be better changed to be an informational BIP, focus on the rationale and examples of NOINPUT, and be cross-referenced with the consensus BIP.