e in the processing of data the security check is made; what’s more important is that it is made at all. Because they do not allow any direct communication between the protected network and outside world, proxy firewall inherently provide network address translation. Whenever an outside site gets a connection from the firewall’s proxy address, it in turn hides and translates the addresses of system behind the firewall. Prior to the invention of firewalls, routers were often pressed into service to provide security and network isolation. Many sites connecting to the Internet in the early days relied on ordinary routers to filter the types of traffic allowed into or out of the network. Routers operate on each packet as a unique event unrelated to previous packets, filtered on IP source, IP destination, IP port number, and a f few other basic data contained in the packet header. Filtering, strictly speaking, does not constitute a firewall because it does not have quite enough detailed control over data flow to permit building highly secure connections. The biggest problem with using filtering routers for security is the FTP protocol, which, as part of its specification, makes a callback connection in which the remote system initiates a connection to the client, over which data is transmitted. Cryptography is at the heart of computer and network security. The important cryptographic functions are encryption, decryption, one-way hashing, and digital signatures. Ciphers are divided into two categories, symmetric and asymmetric, or public-key systems. Symmetric ciphers are functions where the same key is used for encryption and decryption. Public-key systems can be used for encryption, but they are also useful for key agreement and digital signatures. Key-agreement protocols enable two parties to compute a secret key, even in the face of an eavesdropper.Symmetric ciphers are the most efficient way to encrypt data so that...
