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Programmatically Signing JAR Files
Pages: 1, 2, 3

JARSigner

Now that we have described an ideal JAR signing utility, let's create one. The following JARSigner class takes a name for the signing key, a java.security.PrivateKey, and an array of java.security.cert.X509Certificate objects which act as the certificate chain, the first element of which is the certificate containing the public key corresponding to the private key used in the JAR signing, and which ends in the certificate of the root signing authority. We then provide a signJarFile method which takes as parameters the java.util.jar.JarFile to sign and an output stream to which the signed jar file will be written. The signing method, as desired, also propagates java.security.NoSuchAlgorithmException, java.security.InvalidKeyException, java.security.SignatureException, and java.io.IOException -- all of which can be handled individually by any application which wishes to wrap the JARSigner.



import java.io.*;
import java.security.*;
import java.security.cert.*;
import java.util.*;
import sun.misc.BASE64Encoder;
import sun.security.util.ManifestDigester;
import sun.security.util.SignatureFile;

public class JARSigner
extends Object {
  // the alias for the signing key, the private key to sign with,
  // and the certificate chain
  private String alias;
  private PrivateKey privateKey;
  private X509Certificate[] certChain;

  public JARSigner( String alias, PrivateKey privateKey, X509Certificate[] certChain ) {
    this.alias = alias;
    this.privateKey = privateKey;
    this.certChain = certChain;

  }

Getting a valid Manifest

If a JAR has a MANIFEST.MF file, then we need to extract it from the JAR and verify its contents. All entries must be validated; entries in the Manifest that do not map to actual entries in the JAR must be removed and JAR entries that do not have corresponding entries in the Manifest must be added. To do this, three methods are provided: one to extract a Manifest if one exists, another to validate the contents, and a third, a higher level method, to create both the Manifest and to verify the contents.

// retrieve the manifest from a jar file -- this will either
// load a pre-existing META-INF/MANIFEST.MF, or create a new
// one
private static Manifest getManifestFile( JarFile jarFile )
throws IOException {
JarEntry je = jarFile.getJarEntry( "META-INF/MANIFEST.MF" );
if( je != null ) {
Enumeration entries = jarFile.entries();
while( entries.hasMoreElements() ) {
je = (JarEntry)entries.nextElement();
if( "META-INF/MANIFEST.MF".equalsIgnoreCase( je.getName() ) )
break;

else
je = null;

}

}

// create the manifest object
Manifest manifest = new Manifest();
if( je != null )
manifest.read( jarFile.getInputStream( je ) );
return manifest;

}

// given a manifest file and given a jar file, make sure that
// the contents of the manifest file is correct and return a
// map of all the valid entries from the manifest
private static Map pruneManifest( Manifest manifest, JarFile jarFile )
throws IOException {
Map map = manifest.getEntries();
Iterator elements = map.keySet().iterator();
while( elements.hasNext() ) {
String element = (String)elements.next();
if( jarFile.getEntry( element ) == null )
elements.remove();

}
return map;

}

// make sure that the manifest entries are ready for the signed
// JAR manifest file. if we already have a manifest, then we
// make sure that all the elements are valid. if we do not
// have a manifest, then we create a new signed JAR manifest
// file by adding the appropriate headers
private static Map createEntries( Manifest manifest, JarFile jarFile )
throws IOException {
Map entries = null;
if( manifest.getEntries().size() > 0 )
entries = pruneManifest( manifest, jarFile );

else {
// if there are no pre-existing entries in the manifest,
// then we put a few default ones in
Attributes attributes = manifest.getMainAttributes();
attributes.putValue( Attributes.Name.MANIFEST_VERSION.toString(), "1.0" );
attributes.putValue( "Created-By", System.getProperty( "java.version" ) + " (" + System.getProperty( "java.vendor" ) + ")" );
entries = manifest.getEntries();

}
return entries;

}

Inserting file signatures into the Manifest

Each entry must have an associated cryptographic message digest in the Manifest. As described above, we need to enumerate through all the JAR entries and record the base-64 encoding of the signature of the "running" contents of the JAR, i.e. the signature of the first file is the digest of the contents of the first entry, the signature of the second file is the digest of the contents of the second file appended to the first, and so on. The digest is recorded as an attribute under the key SHA1-Digest for the entry in the Manifest.

Here is an example of a Manifest for a signed JAR

Manifest-Version: 1.0
Created-By: 1.3.0_02 (Sun Microsystems Inc.)

Name: a/b/c1.class
SHA1-Digest: fcav7ShIG6i86xPepmitOVo4vWY=

Name: a/b/c2.class
SHA1-Digest: kdHbE7kL9ZHLgK7akHttYV4XIa0=

The SHA1-Digest for a/b/c1.class is the base-64 version of the signature block of c1.class, and the SHA1-Digest of a/b/c2.class is the signature block of the bytes of c1.class and c2.class. The files in the META-INF directory do not make it into the Manifest.

// helper function to update the digest
private static BASE64Encoder b64Encoder = new BASE64Encoder();
private static String updateDigest( MessageDigest digest, InputStream inputStream )
throws IOException {
  byte[] buffer = new byte[2048];
  int read = 0;
  while( ( read = inputStream.read( buffer ) ) > 0 )
  digest.update( buffer, 0, read );
  inputStream.close();

  return b64Encoder.encode( digest.digest() );

}

// update the attributes in the manifest to have the
// appropriate message digests. we store the new entries into
// the entries Map and return it (we do not compute the digests
// for those entries in the META-INF directory)
private static Map updateManifestEntries( Manifest manifest, JarFile jarFile, MessageDigest messageDigest, Map entries )
throws IOException {
  Enumeration jarElements = jarFile.entries();
  while( jarElements.hasMoreElements() ) {
    JarEntry jarEntry = (JarEntry)jarElements.nextElement();
    if( jarEntry.getName().startsWith( "META-INF" ) )
    continue;

    else if( manifest.getAttributes( jarEntry.getName() ) != null ) {
    // update the digest and record the base 64 version of
    // it into the attribute list
    Attributes attributes = manifest.getAttributes( jarEntry.getName() );
    attributes.putValue( "SHA1-Digest", updateDigest( messageDigest, jarFile.getInputStream( jarEntry ) ) );

    } else if( !jarEntry.isDirectory() ) {
    // store away the digest into a new Attribute
    // because we don't already have an attribute list
    // for this entry. we do not store attributes for
    // directories within the JAR
    Attributes attributes = new Attributes();
    attributes.putValue( "SHA1-Digest", updateDigest( messageDigest, jarFile.getInputStream( jarEntry ) ) );
    entries.put( jarEntry.getName(), attributes );

    }

  }
  return entries;

}

Creating the signature and signature block file

The signature file is the record that contains the signed version of the Manifest file, preventing any files from being added to the JAR file without the signed JAR verifier noticing. The signature file contains a digest of both the entire Manifest in the SHA1-Digest-Manifest header as well as digest values for all the entries that are present in the Manifest. It is similar to the Manifest, except taht the digests are computed from the corresponding values in the Manifest rather than the contents of the file itself.

Signature-Version: 1.0
SHA1-Digest-Manifest: h1yS+K9T7DyHtZrtI+LxvgqaMYM=
Created-By: SignatureFile JDK 1.2

Name: a/b/c1.class
SHA1-Digest: fcav7ShIG6i86xPepmitOVo4vWY=

Name: a/b/c2.class
SHA1-Digest: xrQem9snnPhLySDiZyclMlsFdtM=

This is an example of the final SF file in the META-INF directory of the signed JAR. Thankfully, there are already classes in the Java library to generate the signature file for us, so we do not have to manually create it. Using the sun.security.util package, we can easily use a serialized version of the Manifest along with a ManifestDigester and a SignatureFile object. The SignatureFile object can write itself to an output stream, so we can use that later to write the file out into the JAR.

// a small helper function that will convert a manifest into an
// array of bytes
private byte[] serialiseManifest( Manifest manifest )
throws IOException {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
manifest.write( baos );
baos.flush();
baos.close();
return baos.toByteArray();

}

// create a signature file object out of the manifest and the
// message digest
private SignatureFile createSignatureFile( Manifest manifest, MessageDigest messageDigest )
throws IOException {
// construct the signature file and the signature block for
// this manifest
ManifestDigester manifestDigester = new ManifestDigester( serialiseManifest( manifest ) );
return new SignatureFile( new MessageDigest[] { messageDigest }, manifest, manifestDigester, this.alias, true );

}

A signature block must be created along with the signature. It contains the public key and the certificate signing chain in a non-human readable format used to verify the contents of the signed JAR. Using the sun.security.util package, we can generate the signature block from the SignatureFile object that we just created.

SignatureFile.Block block = signatureFile.generateBlock( this.privateKey, this.certChain, true );

Pages: 1, 2, 3

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