nds the identity and password in an encrypted message using the symmetric key. Here the intruder does not have any idea about the identity and password because it does not know the symmetric key. But the intruder can cause replay attack by intercepting both the authentica- tion message and the data message and storing and resending them to the receiver again. In another approach, replay attack can be avoided by using a large ran- dom number called nonce that is used only once SO as to distinguish a fresh authentication from a repeated one. Here the sender identity in plaintext is sent to the receiver who sends a nonce in plaintext. The sender then encrypts it using the symmetric key and sends back the nonce to the receiver. The intruder cannot replay the message as the nonce is valid only once. Secure Communication User authentication with public key cryptography: Here the sender encrypts the message with its private key and the receiver can decrypt the message using the sender's public key and authenticate the sender. But the intruder can either announce its public key to the receiver or encrypt the message containing a nonce with its private key, which the receiver can decrypt with the intruder's public key (assumed to be that of the sender). 31.5 Key Management Key management includes both symmetric key distribution and public key certification. Symmetric keys are extremely useful when they serve a dynamic func- tion between two parties such that they are created for one session and destroyed when the session is over. These keys are called session keys. Some of the key management protocols are: Diffie-Hellman method: In the Diffie-Hellman protocol two parties are provided with a one-time session key, SO that they can exchange data with- out remembering or storing it for future use. Without meeting, the two par- ties can agree on the key through the Internet. The disadvantage is that an intruder can easily intercept the messages sent. Kerberos: This is a very popular authentication protocol that is also a key d