Return code doesn't help this time because there is more than one kind of correct input. There can be valid encrypted message, valid signed message or combinati

Return code doesn't help this time because there is more than one kind of correct input. There can be valid encrypted message, valid signed message or combination of both. In the attack valid encrypted message was made to look like valid signed message. For both verification would succeed unless you know what the message is supposed to contain before processing it with gpg.