Good evening, > I didn't think this was the design. The update transaction can spend any prior, with a fixed script, due to NOINPUT.
>From my reading of the final construction, each update transaction has a unique script to bind settlement transactions to exactly one update. > My understanding is that this is not logically possible? The update transaction has no fixed txid until it commits to a particular output-to-be-spent, which is either the funding/kickoff txout, or a lower-`nLockTime` update transaction output. > Thus a settlement transaction *must* use `NOINPUT` as well, as it has no txid it can spend, if it is constrained to spend a particular update transaction. This is also my understanding. Any presigned descendants of a NOINPUT txn must also use NOINPUT as well. This chain must continue until a signer is online to bind a txn to a confirmed input. The unique settlement keys thus prevent rebinding of settlement txns since NOINPUT with a shared script would be too liberal. Cheers, Conner On Mon, Dec 2, 2019 at 18:55 ZmnSCPxj <zmnsc...@protonmail.com> wrote: > Good morning Rusty, > > > > Hi all, > > > I recently revisited the eltoo paper and noticed some things related > > > watchtowers that might affect channel construction. > > > Due to NOINPUT, any update transaction can spend from any other, so > > > in theory the tower only needs the most recent update txn to resolve > > > any dispute. > > > In order to spend, however, the tower must also produce a witness > > > script which when hashed matches the witness program of the input. To > > > ensure settlement txns can only spend from exactly one update txn, > > > each update txn uses unique keys for the settlement clause, meaning > > > that each state has a unique witness program. > > > > I didn't think this was the design. The update transaction can spend > > any prior, with a fixed script, due to NOINPUT. > > > > The settlement transaction does not use NOINPUT, and thus can only > > spend the matching update. > > My understanding is that this is not logically possible? > The update transaction has no fixed txid until it commits to a particular > output-to-be-spent, which is either the funding/kickoff txout, or a > lower-`nLockTime` update transaction output. > Thus a settlement transaction *must* use `NOINPUT` as well, as it has no > txid it can spend, if it is constrained to spend a particular update > transaction. > > Unless I misunderstand how update transactions work, or what settlement > transactions are. > > Regards, > ZmnSCPxj > -- —Sent from my Spaceship
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