A firewall is a system or group of systems that enforces an access control policy between two networks. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy. As the computing environment became more distributed, it became clear that some techniques would be required on the network, and that steps would have to be taken to secure privileged information traveling across the network. Cryptography allows users to disguise data that attackers gain no information from listening to the information as transmitted.
Authentication allows clients and servers to securely determine each other s identities.
In an open network computing environment, a workstation cannot be trusted to identify its users correctly to network services. Many techniques have been developed to provide the different services necessary to operate large groups of computers in an efficient and secure manner. Cryptography provides that ability to send information between participants in a way that prevents others from reading it. Authentication basically establishes who you are. There are two cases of authenticating a device across a network.