At best, a firewall protects a network from undesired access from the
rest of the Internet; it cannot provide security to legitimate communication
between the inside and the outside of the firewall. In contrast,
the cryptography-based security mechanisms described in this chapter
are capable of providing secure communication between any participants
anywhere. This being the case, why are firewalls so common? One
reason is that firewalls can be deployed unilaterally, using mature commercial
products, while cryptography-based security requires support at
both endpoints of the communication. A more fundamental reason for
the dominance of firewalls is that they encapsulate security in a centralized
place, in effect factoring security out of the rest of the network.
A system administrator can manage the firewall to provide security, freeing
the users and applications inside the firewall fromsecurity concerns—
at least some kinds of security concerns.