Hi Andrew,

If you use SIGHASH_ALL it shall sign the COutPoints of all inputs which
commit to the scriptPubKeys of the txn.

Thus the 341 hash doesn't need to sign any additional data.

As a metadata protocol you can provide all input transactions to check the
scriptPubKeys.

Best,

Jeremy
--
@JeremyRubin <https://twitter.com/JeremyRubin>


On Thu, Apr 30, 2020 at 1:22 AM Andrew Kozlik via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hi everyone,
>
> In the current draft of BIP-0341 [1] the signature message commits to the
> scriptPubKey of the output being spent by the input. I propose that the
> signature message should commit to the scriptPubKeys of *all* transaction
> inputs.
>
> In certain applications like CoinJoin, a wallet has to deal with
> transactions containing external inputs. To calculate the actual amount
> that the user is spending, the wallet needs to reliably determine for each
> input whether it belongs to the wallet or not. Without such a mechanism an
> adversary can fool the wallet into displaying incorrect information about
> the amount being spent, which can result in theft of user funds [2].
>
> In order to ascertain non-ownership of an input which is claimed to be
> external, the wallet needs the scriptPubKey of the previous output spent by
> this input. It must acquire the full transaction being spent and verify its
> hash against that which is given in the outpoint. This is an obstacle in
> the implementation of lightweight air-gapped wallets and hardware wallets
> in general. If the signature message would commit to the scriptPubKeys of
> all transaction inputs, then the wallet would only need to acquire the
> scriptPubKey of the output being spent without having to acquire and verify
> the hash of the entire previous transaction. If an attacker would provide
> an incorrect scriptPubKey, then that would cause the wallet to generate an
> invalid signature message.
>
> Note that committing only to the scriptPubKey of the output being spent is
> insufficient for this application, because the scriptPubKeys which are
> needed to ascertain non-ownership of external inputs are precisely the ones
> that would not be included in any of the signature messages produced by the
> wallet.
>
> The obvious way to implement this is to add another hash to the signature
> message:
> sha_scriptPubKeys (32): the SHA256 of the serialization of all
> scriptPubKeys of the previous outputs spent by this transaction.
>
> Cheers,
> Andrew Kozlik
>
> [1]
> https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki#common-signature-message
> [2]
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-August/014843.html
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