Computer security is hard. Software, computer and network security are all ongoing battles between attacker and defender. And in many cases the attacker has an inherent advantage: He only has to find one network flaw, while the defender has to find and fix every flaw.
Bruce Schneier says Unfortunately, cryptography can't solve most computer-security problems. The one problem cryptography can solve is the security of data when it's not in use. Encrypting files, archives -- even entire disks -- is easy.