Code signing is a security measure that you can use to identify the originator of a downloaded class. The m-let service will enforce code signatures if it is instantiated in secure mode. One of the constructors of the MLet class takes a boolean parameter that specifies the security mode. For obvious security reasons, the security mode cannot be modified once the m-let service is instantiated.
When the m-let service is running in secure mode, it will only load classes and native libraries that are signed by a trusted party. A trusted party is identified by a key: this key was used to sign the code and a copy of the key is given to all parties that want to download the signed class. Therefore, you must identify trusted keys in your agent before attempting to download their signed classes.
Downloading native libraries always requires a custom security manager, regardless of whether they are trusted or not.
In the MLet class, security is not determined when you instantiate the m-let service. Rather, security is enabled or disabled for your entire agent application, including any class loaders used by the m-let service.
To enable security, start your agent applications with the java.lang.SecurityManager property on the command line. Then, when the m-let service loads a class through one of its class loaders, the class loader will check the origin and signature of the class against the list of trusted origins and signatures.
The tools involved in signing a class file are the jar, keytool, and jarsigner utilities. On the host where the agent application will download a class, you define a set of permissions for signatures and URL origins. Then, you need to use the policytool utility to generate a java.policy file containing the trusted signatures. Refer to the Java 2 Platform Standard Edition (J2SE) documentation for the description of these utilities.
When the agent application is started with a security manager, it will check this policy file to ensure that the origin and signature of a downloaded class match a trusted origin and a trusted signature. If they do not match, the code is not trusted and cannot be loaded.
When the agent application is started without the security manager, all classes and native libraries can be downloaded and instantiated, regardless of their origin and signature, or lack thereof.