Problem with this authentication protocol Consider such authentication protocol:

Alice and Bob share a key.


Bob sends alice a nonce.
Alice encrypts it with shared key and sends to bob.


Now assume Alice was successfully authenticated.

But there is such attack mentioned


  Trudy can hijack connection after authentication, if she can send
  packets with Alice’s source address


This seems to be a general problem with authentication protocols. Even if steps 1 and 2 are secure and authentication protocol is secure, trudy can always mount above mentioned attack after the authentication protocol is finished, isn't it?
How are such problems dealt with?
This is usually solved by creating a session key as part of the authentication process, then using authenticated encryption for the whole session. This is how e.g. TLS does it. Even if an attacker could hijack the connection, they do not have the session key so they cannot produce valid data.

The session key can be constructed in various ways. For example, a very simple, purely symmetric key protocol could have Alice generate a random key, encrypt the nonce using that key and the session key using the longer term key. The rest of the session would be encrypted and authenticated using the session key.

In practice it is a good idea to use something like Diffie-Hellman key exchange, since it gets you forward secrecy, among other things.