Why should I sign my JAR files?
I know that I need to sign my client-side JAR files (containing Applets) so that special things like filesystem access can be done, and so that the annoying bit at the bottom of windows doesn't show, but why else? And do I need to sign my server-side JAR files containing Servlets, etc.?
Some basic rules for when and when not to sign JARs would be appreciated - thanks!
Signing a jar file, just like using certificates in other contexts, is done so that people using it know where it came from. People may trust that Chris Carruthers isn't going to write malicious code, and so they're willing to allow your applet access to their file system. The signature gives them some guarantee that the jar really was created by you, and not by an impostor or someone they don't trust.
In the case of server-side or library jars, there's usually no need to provide that kind of guarantee to anybody. If it's your server, then you know what jars you're using and where they came from, and you probably trust your own code not to be malicious.
The short answer - don't, unless your company policy forces you to.
The long answer
Signing jars is effectively telling your customer "I made this, and I guarantee it won't mess up your system. If it does, come to me for retribution". This is why signed jars in client-side solution deployed from remote servers (applets / webstart) enjoy higher privileges than non-signed solutions do.
On server-side solutions, where you don't have to to placate the JVM security demands, this guarantee is only for your customer peace of mind.
The bad thing about signed jars is that they load slower than unsigned jars. How much slower? it's CPU-bound, but I've noticed more than a 100% increase in loading time. Also, patches are harder (you have to re-sign the jar), class-patches are impossible (all classes in a single package must have the same signature source) and splitting jars becomes a chore. Not to mention your build process is longer, and that proper certificates cost money (self-signed is next to useless).
So, unless your company policy forces you to, don't sign jars on the server side, and keep common jars in signed and non-signed versions (signed go to the client-side deployment, non-signed go to server-side codebase).
A good reason could be if you never wanted anybody to be able to sneak in modfied classes to be called by your code.
Unfortunately that includes yourself :-D So this is only to be done if you really need it. Check the "sealed jar" concept.
In terms of applets: From 6u10, the Sun JRE replace the warning banner with less obtrusive (from 6u12, IIRC) warning triangle (necessary to support shaped and transparent windows). 6u10 also allows controlled file access through the JNLP services API.
The principle of least privilege says that you should not sign the classes of your jar files. Security is not necessarily easy.
Simply showing a certificate dialog box should not be construed to mean that the entire contents of a web page is to be trusted.