rich1.0ZmnSCPxj [ARCHIVE] (npub1g5…3ms3l)https://yabu.me/npub1g5zswf6y48f7fy90jf3tlcuwdmjn8znhzaa4vkmtxaeskca8hpss23ms3lnjumphttps://yabu.me📅 Original date posted:2019-12-03 📝 Original message: 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