More Public Key Cryptography...

• Obtaining Public Keys: Use an authentication server that has a list of public keys. Upon request, the server sends message (PK,U) encrypted with the server's private key.
• Performing Handshakes: When two users communicate, they first confirm each other's identity using preliminary messages with nonce identifiers.

...and KERBEROS - a Private Key System.

• Users must first be authenticated by Kerberos and then obtain tickets to use services.
• Kerberos generates temporary conversation keys used by a client and server to communicate.
• Three-phase authentication process:
  1. Getting the initial ticket (i.e., credentials).
  2. Getting the server ticket.
  3. Requesting the service.

All of these authentication schemes are designed to foil attacks based on replay of messages and/or transmission of bogus messages.