trezor-firmware/core/src/apps/bitcoin/sign_tx/matchcheck.py

from trezor import wire
from trezor.utils import ensure

from .. import multisig
from ..common import BIP32_WALLET_DEPTH

if False:
    from typing import Any, Generic, TypeVar

    from trezor.messages.TxInput import TxInput
    from trezor.messages.TxOutput import TxOutput

    T = TypeVar("T")
else:
    # mypy cheat: Generic[T] will be `object` which is a valid parent type
    Generic = [object]  # type: ignore
    T = 0  # type: ignore


class MatchChecker(Generic[T]):
    """
    MatchCheckers are used to identify the change-output in a transaction. An output is
    a change-output if it has a certain matching attribute with all inputs.
    1. When inputs are first processed, add_input() is called on each one to determine
       if they all match.
    2. Outputs are tested using output_matches() to tell whether they are admissible as
       a change-output.
    3. Before signing each input, check_input() is used to ensure that the attribute has
       not changed.

    There are two possible paths:

    (a) If all inputs match on the attribute, the matching value is stored. Every output
        that matches the stored value is admissible as a change-output.

    (b) If some inputs do not match, a special value MISMATCH is stored. When the
        matcher is in this state, _no outputs_ are admissible as change-outputs.
        check_input() is a no-op in this case: if there is no matching attribute to
        check against, we cannot detect modifications.
    """

    MISMATCH = object()
    UNDEFINED = object()

    def __init__(self) -> None:
        self.attribute: object | T = self.UNDEFINED
        self.read_only = False  # Failsafe to ensure that add_input() is not accidentally called after output_matches().

    def attribute_from_tx(self, txio: TxInput | TxOutput) -> T:
        # Return the attribute from the txio, which is to be used for matching.
        # If the txio is invalid for matching, then return an object which
        # evaluates as a boolean False.
        raise NotImplementedError

    def add_input(self, txi: TxInput) -> None:
        ensure(not self.read_only)

        if self.attribute is self.MISMATCH:
            return  # There was a mismatch in previous inputs.

        added_attribute = self.attribute_from_tx(txi)
        if not added_attribute:
            self.attribute = self.MISMATCH  # The added input is invalid for matching.
        elif self.attribute is self.UNDEFINED:
            self.attribute = added_attribute  # This is the first input.
        elif self.attribute != added_attribute:
            self.attribute = self.MISMATCH

    def check_input(self, txi: TxInput) -> None:
        if self.attribute is self.MISMATCH:
            return  # There was already a mismatch when adding inputs, ignore it now.

        # All added inputs had a matching attribute, allowing a change-output.
        # Ensure that this input still has the same attribute.
        if self.attribute != self.attribute_from_tx(txi):
            raise wire.ProcessError("Transaction has changed during signing")

    def output_matches(self, txo: TxOutput) -> bool:
        self.read_only = True

        if self.attribute is self.MISMATCH:
            return False

        return self.attribute_from_tx(txo) == self.attribute


class WalletPathChecker(MatchChecker):
    def attribute_from_tx(self, txio: TxInput | TxOutput) -> Any:
        if len(txio.address_n) < BIP32_WALLET_DEPTH:
            return None
        return txio.address_n[:-BIP32_WALLET_DEPTH]


class MultisigFingerprintChecker(MatchChecker):
    def attribute_from_tx(self, txio: TxInput | TxOutput) -> Any:
        if not txio.multisig:
            return None
        return multisig.multisig_fingerprint(txio.multisig)