Bob has a public-private key pair (pub_Bob, pri_Bob). Alice needs to send some information to Bob. She wants to make sure that when Bob opens the message, he can verify that this is from Alice but not anyone else. So she sends out the message as: [ Alice, Epub_Bob(message) ] to Bob. Basically, she first sends out her name in clear text, then encrypts the message with Bobs public key. Please discuss, can an attacker M impersonate Alice and send out a packet in Alices name? How can he do it? Here we assume that M also has the public key of Bob. For the same question, if Alice sends out [ Epub_Bob(Alice, message) ], can M still impersonate Alice? (Here Alice puts her name in the encryption.)