Re: [bitcoin-dev] BIP sighash_noinput

["Russell O'Connor" <roconnor@blockstream.io>]

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At the risk of bikeshedding, shouldn't NOINPUT also zero out the
hashSequence so that its behaviour is consistent with ANYONECANPAY?

On Mon, Apr 30, 2018 at 12:29 PM, Christian Decker via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hi all,
>
> I'd like to pick up the discussion from a few months ago, and propose a new
> sighash flag, `SIGHASH_NOINPUT`, that removes the commitment to the
> previous
> output. This was previously mentioned on the list by Joseph Poon [1], but
> was
> never formally proposed, so I wrote a proposal [2].
>
> We have long known that `SIGHASH_NOINPUT` would be a great fit for
> Lightning.
> They enable simple watch-towers, i.e., outsource the need to watch the
> blockchain for channel closures, and react appropriately if our
> counterparty
> misbehaves. In addition to this we just released the eltoo [3,4] paper
> which
> describes a simplified update mechanism that can be used in Lightning, and
> other
> off-chain contracts, with any number of participants.
>
> By not committing to the previous output being spent by the transaction,
> we can
> rebind an input to point to any outpoint with a matching output script and
> value. The binding therefore is no longer explicit through a reference, but
> through script compatibility, and the transaction ID reference in the
> input is a
> hint to validators. The sighash flag is meant to enable some off-chain
> use-cases
> and should not be used unless the tradeoffs are well-known. In particular
> we
> suggest using contract specific key-pairs, in order to avoid having any
> unwanted
> rebinding opportunities.
>
> The proposal is very minimalistic, and simple. However, there are a few
> things
> where we'd like to hear the input of the wider community with regards to
> the
> implementation details though. We had some discussions internally on
> whether to
> use a separate opcode or a sighash flag, some feeling that the sighash flag
> could lead to some confusion with existing wallets, but given that we have
> `SIGHASH_NONE`, and that existing wallets will not sign things with unknown
> flags, we decided to go the sighash way. Another thing is that we still
> commit
> to the amount of the outpoint being spent. The rationale behind this is
> that,
> while rebinding to outpoints with the same value maintains the value
> relationship between input and output, we will probably not want to bind to
> something with a different value and suddenly pay a gigantic fee.
>
> The deployment part of the proposal is left vague on purpose in order not
> to
> collide with any other proposals. It should be possible to introduce it by
> bumping the segwit script version and adding the new behavior.
>
> I hope the proposal is well received, and I'm looking forward to discussing
> variants and tradeoffs here. I think the applications we proposed so far
> are
> quite interesting, and I'm sure there are many more we can enable with this
> change.
>
> Cheers,
> Christian
>
> [1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/
> 2016-February/012460.html
> [2] https://github.com/cdecker/bips/blob/noinput/bip-xyz.mediawiki
> [3] https://blockstream.com/2018/04/30/eltoo-next-lightning.html
> [4] https://blockstream.com/eltoo.pdf
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