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Java in a Nutshell

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6.9 Signed Applets

In Java 1.1 it is possible to circumvent these applet security restrictions by attaching a digital signature to a JAR file. When a Web browser or applet viewer loads a JAR file that has been signed by a trusted entity (the user specifies whom she trusts), the browser may grant the applet contained in the JAR file special privileges, such as the ability to read and write local files, that are not available to untrusted applets.

Signing an applet with the javakey tool provided by the JDK is a somewhat cumbersome task. First, of course, you must have a security database set up. The database must contain the certificate and the public and private keys that you want to use to sign the applet. See the javakey documentation in Chapter 16, JDK Tools for details on this process.

Once you have a properly configured security database, you must create a simple "directive file" that gives javakey the information it needs to sign your JAR file. A directive file might look like this:

# The entity doing the signing
signer=david
# The certificate number to use
cert=1
# Currently unused
chain=0
# The base name of the signature files to be added to the JAR manifest
signature.file=DAVIDSIG

Having created a directive file named mysig, for example, you could then sign a JAR file like this:

% javakey -gs mysig soundmap.jar

This command creates a new JAR file named soundmap.jar.sig that you can use in an HTML archive attribute just as you would use an unsigned JAR file.

The javakey tool is used for all aspects of administering the Java system security database. One of the other things you can do with it is to declare which entities are trusted. You do this with the -t option. For example, you might declare your trust for the author as follows:

% javakey -t DavidFlanagan true

Or you could revoke your trust like this:

% javakey -t DavidFlanagan false

The appletviewer program makes use of any trust values declared in this way. Note that javakey and appletviewer support only untrusted entities and fully-trusted entities, without any gradations in between. We may see additional levels of trust in the future.

Bear in mind that the javakey techniques described here apply only to the JDK. Other vendors may provide other mechanisms for signing applets, and Web browsers may provide other ways of declaring trusted entities.


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