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<TITLE>Understanding Digital Signatures: Inside the Java Virtual Machine
by Bill Venners - Beta Version</TITLE>
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<P><H1>Chapter Three</H1></P>
<P><H2>Security</H2></P>
<P>Aside from platform independence, discussed in the previous chapter, the other major technical challenge a network-oriented software technology must deal with is security. Networks, because they allow computers to share data and distribute processing, can potentially serve as a way to break into a computer system, enabling someone to steal information, destroy information, or steal computing resources. As a consequence, connecting a computer to a network raises many security issues.</P>
<P>To address the security concerns raised by networks, Java韘 architecture comes with an extensive built-in security model. This chapter gives an overview of the security model built into Java韘 core architecture.</P>
<H3><EM><P>Why Security?</P>
</EM></H3><P>Java韘 security model is one of the key architectural features that makes it an appropriate technology for networked environments. Security is important because networks represent a potential avenue of attack to any computer hooked to them. This concern becomes especially strong in an environment in which software is downloaded across the network and executed locally, as is done, for example, with Java applets. Because the class files for an applet are automatically downloaded when a user goes to the containing web page in a browser, it is likely that a user will encounter applets from untrusted sources. Without any security, this would be a convenient way to spread viruses. Thus, Java韘 security mechanisms help make Java suitable for networks because they establish a needed trust in the safety of network-mobile code.</P>
<P>Java韘 security model is focused on protecting end-users from hostile programs downloaded across a network from untrusted sources. To accomplish this goal, Java provides a customizable "sandbox" in which Java programs run. A Java program must play only inside its sandbox. It can do anything within the boundaries of its sandbox, but can韙 take any action outside those boundaries. The sandbox for untrusted Java applets, for example, prohibits many activities, including:</P>
<UL><LI> reading or writing to the local disk,
<LI> making a network connection to any but the host from which the applet came,
<LI> creating a new process, and
<LI> loading a new dynamic library and directly calling a native method.</UL>
<P>By making it impossible for downloaded code to perform certain actions, Java韘 security model protects the end-user from the threat of hostile code.</P>
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