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 > _______________________________________________ > bitcoin-dev mailing list > bitcoin-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >
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