Unauthorized copying of a transaction barcode is prevented by including a sensed condition or other publicly-accessible data with the transaction barcode for use as a comparison with the publicly accessible data determined at a barcode reader. If the sensed condition included in the transaction barcode indicates that the transaction barcode was generated for a different transaction, then the barcode reader invalidates the transaction. For instance, if the barcode was generated too distant in time, position, or sequential transactions, then the barcode reader invalidates the transaction barcode as an unauthorized copy of a transaction barcode generated for a different transaction.