The program is in Java, so the solution is to run it under a security manager. This is really easy - just pass "-Djava.security.manager" to the JVM and any access to a security sensitive resource will throw an exception. Because this is a verification done by the JVM, there is no possibility for the program to bypass this. When the program runs without security manager, I just display a warning message like this:
You should not run random Java programs downloaded from the Internet without security manager.
Use the --help-security option to know how to do this.
Note that running the program under a security manager only one or two times to check that it is not a Trojan horse is not better than doing nothing; if I wanted to do something bad in my program, I would be sure to not do it when a security manager is installed, to not be detected.
Then comes the tedious task of writing a policy file to authorize the program to access the resources it is supposed to access and no more than this. One way of doing this is to add the "-Djava.security.debug=access" to the JVM command line and to add (or not) to the policy file a permission each time an access denied is found. The problem is that one needs to run absolutely all the options in the program to be sure to trigger all the security accesses, which is not very practical. The solution I chose for my program is to add an option that generates the policy file that is needed to run the program with the specific parameters under the security manager. Here's an example:
$ java -Djava.security.manager -jar id2kindle.jar --generate-policy draft-ietf-behave-turn-uri-01.txt
permission java.io.FilePermission "draft-ietf-behave-turn-uri-01.txt", "read";
permission java.io.FilePermission "draft-ietf-behave-turn-uri-01.zip", "write";
The standard output can be redirected to the policy file, which is then used to run the program:
$ java -Djava.security.manager -jar id2kindle.jar --generate-policy draft-ietf-behave-turn-uri-01.txt >id2kindle.policy
$ cat id2kindle.policy
$ java -Djava.security.manager -Djava.security.policy=id2kindle.policy -jar id2kindle.jar draft-ietf-behave-turn-uri-01.txt
As long as the content of the policy file is verified before running the program, nothing bad can happen. Well, excepted that most people will run it at least one time without security manager and one time is all that is needed to do bad things.