The easiest way to protect information from being stolen or tampered with is to prevent any unauthorized person from having physical access to it. While no network can be completely protected in this way, most early systems used separate circuits, often separate wires, and derived much of their security from this simple process of playing keep-away. This is no longer possible with Internet-based systems. The biggest single issue limiting the scale of the Internet's impact on commerce is security. We'll briefly discuss the problem of preventing someone from stealing your data while it is being communicated and, to a lesser extent, the challenges involved in stopping someone from breaking into your system.