Analyze the following custom authentication protocol handshake description for security vulnerabilities: 1. Client sends ClientHello with nonce_Nc. 2. Server responds with ServerHello, nonce_Ns, and a digital signature over (Nc || Ns) using its private key. 3. Client verifies signature, derives a session key using KDF(Nc, Ns, PreSharedSecret), and sends a Finished message encrypted with the session key. Identify potential replay attacks, man-in-the-middle vulnerabilities, or forward secrecy issues. Propose a modified protocol that mitigates these risks using modern AEAD ciphers.