import sun.misc.BASE64Encoder;
import java.security.cert.Certificate;
import java.security.*;
import java.io.File;
import java.io.FileInputStream;

public final class GetKey {

    private static void usage() {
	System.out.println("Usage: GetKey keystore-absolute-path keystore-passphrase alias");
    }

    public static void main(String[] args) throws Exception {

	if(args.length != 3) { usage(); return; }

	final KeyStore ks = KeyStore.getInstance("JKS");
	final String fileName = args[0];

	char[] passPhrase = args[1].toCharArray();
	final BASE64Encoder myB64 = new BASE64Encoder();
	
	final File certificateFile = new File(fileName);
	ks.load(new FileInputStream(certificateFile), passPhrase);
	final String alias = args[2];
	final KeyPair kp = getPrivateKey(ks, alias, passPhrase);

	final PrivateKey privKey = kp.getPrivate();

	final String b64 = myB64.encode(privKey.getEncoded());

	System.out.println("-----BEGIN PRIVATE KEY-----");
	System.out.println(b64);
	System.out.println("-----END PRIVATE KEY-----");
    }

    // From http://javaalmanac.com/egs/java.security/GetKeyFromKs.html
    public static KeyPair getPrivateKey(KeyStore keystore, String alias, char[] password) 
    throws Exception {
	// Get private key
	final Key key = keystore.getKey(alias, password);
	if (key instanceof PrivateKey) {
	    // Get certificate of public key
	    final Certificate cert = keystore.getCertificate(alias);

	    // Get public key
	    final PublicKey publicKey = cert.getPublicKey();

	    // Return a key pair
	    return new KeyPair(publicKey, (PrivateKey)key);
	}
        return null;
    }
}