Quote from: piotr_n on Today at 05:35:11 PM

May I ask, out of curiosity, what purpose does it serve?
It kind of makes the taproot implementation quite much harder to make.

Requiring all UTXO amounts is there to allow for offline signers to be guaranteed that the amount they are sending is correct. There have been some theorized situations where an offline signer could be tricked into sending more or less Bitcoin to fees than they expect to, which could result either in the loss of funds, or in the transaction not confirming in a timely manner.

Trezor described a potential attack that involve the uncertainty of the amounts in other inputs. This attack could result in the loss of funds by causing an increasing the transaction fee.

So this change to the sighash algorithm resolves such attacks. If the offline signer is lied to about the amount of other inputs, it will produce an invalid signature and so the transaction becomes invalid. Thus it doesn't matter if it is lied to, no funds can be lost.

For requiring all the output scripts, this is again for offline signers, particularly in coinjoins. In this scenario, the offline signer needs to be able to prove that all of the other inputs in the transaction do not belong to it. Otherwise it could lose funds. By requiring all of the scriptPubKeys for the other inputs, the offline signer can be safe to sign even if it is being lied to. Like with the amounts, if it is lied to, it will make an invalid signature and so the transaction is invalid. There is some more information about this on the bitcoin-dev mailing list.