[bitcoin-dev] BIP-341: Committing to all scriptPubKeys in the signature message

[Andrew Kozlik <andrew.kozlik@satoshilabs.com>]

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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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