How to verify a jar signed with jarsigner programmatically

Question

I\'m wanting to sign a jar using jarsigner, then verify it using a Java application which does not have the signed jar as part of it\'s classpath (i.e. just using a filesyst

Answer 1

You can simply open the JAR with java.util.jar.JarFile and tell it to verify the JAR file. If the JAR is signed, then JarFile has the option to verify it (which is on by default). However, JarFile will also open unsigned JARs happily, therefore you must also check, whether or not the file is signed. You can do so by checking the JAR's manifest for *-Digest attributes: Elements with such an attribute attribute are signed.

Example:

JarFile jar = new JarFile(new File("path/to/your/jar-file"));

// This call will throw a java.lang.SecurityException if someone has tampered
// with the signature of _any_ element of the JAR file.
// Alas, it will proceed without a problem if the JAR file is not signed at all
InputStream is = jar.getInputStream(jar.getEntry("META-INF/MANIFEST.MF"));
Manifest man = new Manifest(is);
is.close();

Set signed = new HashSet();
for(Map.Entry entry: man.getEntries().entrySet()) {
    for(Object attrkey: entry.getValue().keySet()) {
        if (attrkey instanceof Attributes.Name && 
           ((Attributes.Name)attrkey).toString().indexOf("-Digest") != -1)
            signed.add(entry.getKey());
    }
}

Set entries = new HashSet();
for(Enumeration entry = jar.entries(); entry.hasMoreElements(); ) {
    JarEntry je = entry.nextElement();
    if (!je.isDirectory())
        entries.add(je.getName());
}

// contains all entries in the Manifest that are not signed.
// Ususally, this contains:
//  * MANIFEST.MF itself
//  * *.SF files containing the signature of MANIFEST.MF
//  * *.DSA files containing public keys of the signer

Set unsigned = new HashSet(entries);
unsigned.removeAll(signed);

// contains all the entries with a signature that are not present in the JAR
Set missing = new HashSet(signed);
missing.removeAll(entries);