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 , but was never formally proposed, so I wrote a proposal .
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  https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-February/012460.html  https://github.com/cdecker/bips/blob/noinput/bip-xyz.mediawiki  https://blockstream.com/2018/04/30/eltoo-next-lightning.html  https://blockstream.com/eltoo.pdf _______________________________________________ Lightning-dev mailing list Lightningemail@example.com https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev