Further Java Practical Class
In order to gain a star in the mark sheet you must complete this exercise. Completing the exercise does not gain you any credit in the examination. In this exercise you will implement a custom security manager for a new chat client called
SafeChatClient. Your new version of the chat client should allow only the following network and file system actions for any code your program downloads:
www.cam.ac.uk on port 80 (i.e. web traffic).
Read (but not modify) the home directory of the user running the chat client.
Start by copying your implementation of
ChatClient from Ticklet 2 into a new class called
SafeChatClient inside the package
uk.ac.cam.crsid.fjava.tick2star. Take a copy of the source code for
uk.ac.cam.cl.fjava.messages.DynamicObjectInputStream and use it to produce a new class in your Ticklet 2* package called
SafeObjectInputStream. Your task for this Ticklet is to modify the code for
SafeObjectInputStream so that any code downloaded and invoked by the client can only perform the file system and network actions as described as above.
Replace the use of
SafeChatClient and check your code works in an identical way to
ChatClient by connecting to
java-1b.cl.cam.ac.uk on port 15004. The class which is downloaded from the server will print
DONE if it has worked successfully.
By default, the JVM runs without an instance of the
SecurityManager object. You'll need one in order to be able to define security policies for the class loader. Therefore your first task is to create an instance of
SecurityManager and add it to the virtual machine (hint: try
System.setSecurityManager). Your chat client should not work with the default
SecurityManager, since the default policy does not permit class loaders.
You should use the following
String, representing a URL, as the source of the security policy:
"http://www.cl.cam.ac.uk/teaching/1011/FJava/all.policy" (hint: try
System.setProperty("java.security.policy", ...)). Your implementation of
SafeChatClient should now run as before (i.e. with the security flaws). Why?
addClass method in
SafeObjectInputStream to use
java.security.SecureClassLoader instead of
java.lang.ClassLoader. The class
SecureClassLoader provides a definition of the method
defineClass which takes a fifth argument of type
java.security.ProtectionDomain. (You will need to create an instance of
ProtectionDomain which supports only the permissions the downloaded class should be given. Apple users may find that additional permissions are needed and should investigate the need for read and write permissions within
apple.* as well as
The home directory of the current user can be found using the following expression
Test your implementation of
SafeChatClient by connecting to
java-1b.cl.cam.ac.uk on port 15004. The new class file should load and execute and the "wholesome content" should download, but the "dubious content" should fail to download. Similarly, any modifications to the filesystem should fail. It should not print
DONE at the end.
Please put the source code and byte code of
SafeObjectInputStream, together with any other classes you wrote to support your application (including the contents of
messages.jar), into a jar file called
crsid-tick2star.jar. Please email the jar file to
firstname.lastname@example.org. You should receive a response via email within an hour. If you do not, please send an email to