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 filesystem location of the jar)

Now my problem is getting the signature file out of the jar, is there a simple way to do this?

I've had a play with the Inflater and Jar InputStreams with no luck.

Or is this something that can be accomplished in a better way?

Thanks

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<String> signed = new HashSet();
for(Map.Entry<String, Attributes> 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<String> entries = new HashSet<String>();
for(Enumeration<JarEntry> 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<String> unsigned = new HashSet<String>(entries);
unsigned.removeAll(signed);

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

Answer 2

The security Provider implementation guide outlines the process of verifying JARs. Although these instructions are for a JCA cryptographic service provider to verify itself, they should be applicable to your problem.

Specifically, check out the verify(X509Certificate targetCert) method in the sample code, "MyJCE.java".

Answer 3

You can use entry.getCodeSigners() to get the signers for a particular entry in the JAR.

Make sure to open the JarFile with verify=true and to fully read the JAR entry before calling entry.getCodeSigners().

Something like this could be used to verify each entry that is not a signature file:

boolean verify = true;
JarFile jar = new JarFile(signedFile, verify);

// Need each entry so that future calls to entry.getCodeSigners will return anything
Enumeration<JarEntry> entries = jar.entries();
while (entries.hasMoreElements()) {
   JarEntry entry = entries.nextElement();
   IOUtils.copy(jar.getInputStream(entry), new NullOutputStream());
}

// Now check each entry that is not a signature file
entries = jar.entries();
while (entries.hasMoreElements()) {
    JarEntry entry = entries.nextElement();
    String fileName = entry.getName().toUpperCase(Locale.ENGLISH);
    if (!fileName.endsWith(".SF")
       && !fileName.endsWith(".DSA")
       && !fileName.endsWith(".EC")
       && !fileName.endsWith(".RSA")) {

       // Now get code signers, inspect certificates etc here...
       // entry.getCodeSigners();
    }
 }

Answer 4

You can use the jarsigner application to do this. In processbuilder (or Runtime.exec) you can run the command with these arguments

 ProcessBulider pb = new ProcessBuilder("/usr/bin/jarsigner", "-verify", "-certs", f.getAbsolutePath());

and if the output contians verified then the jar is signed

Process p = pb.start();
p.waitFor();
InputStream is = p.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line;
while ((line = br.readLine()) != null)
{
if(line.contains("verified");
...

THere are more complicated things you can do when you have the output of the jarsigner code.