Pitfalls of verifying signed jar files

In the Java world, it’s possible to digitally sign a jar file using ‘keytool’ to generate or import a digital signature, and ‘jarsigner’ to do the signing. What isn’t so obvious is that when we use ‘jarsigner’ to verify a signed jar, it doesn’t verify that we trust the signature that signed the file. It simply tells us whether the contents were signed by a public key that was included with the jar file.

Surprisingly, there’s no option to tell jarsigner to check for trusted signatures.

In code, we can use java.util.jar.JarFile to check the validity of a jar file. By default, the constructor to JarFile says we want to check the validity. Code must then iterate through each entry in the JarFile and seek to the end of each input stream, otherwise, the integrity isn’t checked. In other words, the java.util.jar.JarFile doesn’t give us the integrity checking with a simple method call such as isValid(), and it doesn’t give us an easy way to check that we trust the signature that the entries were signed with.

Anyone, anywhere, can create their own certificate, and sign a jar file — so if we want to establish trust for a signed jar, we get to do extra work. On stackoverflow.com, Jarek Przygódzki linked to code that shows how to check for trusted signatures.

I wonder why establishing trust for a signed jar isn’t easier. Could it be that signed jar files originated in the bygone era when we ran Java applets in our web browsers? Did web browsers use their certificate authority database to verify some level of trust for the signature contained in a jar file?

Verifying trust is a delicate issue, as demonstrated by the recently hacked certificate authorities including Diginotar and Comodo. Perhaps it’s a good thing that Java’s libraries and command line tools don’t make it deceptively simple to check jar files based on certificates trusted by Certificate Authorities.

Still, I wish the documentation for jarsigner and JarFile would shed more light on the limits of their default verification. I’d call it “hash checking” or “integrity checking based on hashing”.