Understanding Protection Domains and Permissions in Font

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Understanding Protection Domains and Permissions
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Java s built-in security is based on a Java application that s run by a virtual machine that prevents the application from breaking certain security rules. The virtual machine allows each Java application access to its own data only, unlike a C program that can more or less read and modify any data anywhere on the machine. The virtual machine s bytecode verifier ensures that the Java application won t get out of its memory area by cheating and adding or removing the wrong data from the stack. In addition to restricting access to memory, the virtual machine protects the real machine from a potentially malicious application by restricting access to protected resources. A MIDlet (like an Applet) is run in a sandbox, which is essentially a place where it has enough room to run around and have fun but can t get out and make trouble. The application management software that runs the MIDlet decides which protected resources the MIDlet can access. MIDP 1 security was entirely based on the sandbox model. The only part of memory a MIDP 1 MIDlet can access is the RMS area set aside for that MIDlet s suite (see 5), and the MIDlet couldn t perform any network operations without the AMS first consulting the user. A weakness to the MIDP 1 system was that MIDlets known to the manufacturer, operator, or user to be safe were made to jump through the same annoying hoops as unknown MIDlets downloaded from the Internet. So with MIDP 2 a system of protection domains and permissions was introduced, based on the security model used by other versions of Java such as Java SE and the Connected Device Configuration (CDC) of Java ME.
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CHAPTER 8 SECURING YOUR APPLICATIONS
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A protection domain is essentially a mapping between a set of permissions and a set of certificates that can be used to authenticate a given MIDlet. A permission is an object that represents the right to perform a restricted operation (see the Requesting Permissions section). When a MIDlet is installed, the AMS places it in a protection domain based on which certificate was used to digitally sign the MIDlet (or into a special domain for untrusted third-party MIDlets if the MIDlet is a MIDP 1 MIDlet and/or not signed). If the MIDlet attempts to perform an action that requires permission, the result depends on whether the permission is one that is granted to the MIDlet s protection domain. If it is (and if the MIDlet correctly requested the permission in its JAD file; see Requesting Permissions ), then the action is allowed without consulting the user. If not, the AMS will take over the display and show the user a system screen stating what restricted action the MIDlet would like to perform (such as sending an SMS to a given phone number; see Figure 8-1) and asking the user s permission. If the permission is not granted, the method call that required permission throws a SecurityException.
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Figure 8-1. The AMS shows a warning screen if an untrusted MIDlet attempts to perform a restricted action. A MIDP 2 or greater handset generally has four protection domains: the manufacturer domain, the operator domain, the identified third-party domain, and the unidentified thirdparty domain. (It is possible for the manufacturer to define other protection domains as well.) The manufacturer domain is the domain for all of those MIDlets signed by the handset manufacturer, and the operator domain is the domain for all of the MIDlets signed by the mobile network operator who provides the user s phone service. Both of these parties are implicitly trusted, so these two domains are granted the widest range of permissions. (The digital certificates for the operator domain may be stored in the device s smart card to allow the user to switch the device from one operator s network to another.) The identified third-party domain is the domain for MIDlets signed by a certificate that is granted by a recognized Certificate Authority (see the section Using Digital Certificates ). In MIDP 2, this domain was called the Trusted Third-Party Domain but the name was misleading because the only thing the Certificate
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