Sure, you need a way to verify that the package has been locked by who you thought you were sending it to. Your attack would work by removing Bob completely, y

Sure, you need a way to verify that the package has been locked by who you thought you were sending it to. Your attack would work by removing Bob completely, you just play his part in the protocol. This is true for any protocol though. If you don't know who you're talking to then you could be talking to anyone. In practice most protocols get around this by either pre-sharing a private key, or using a public/private key pair with a challenge round to establish identities.