how to read private key for digital signature

Question

I am building application for signing files digitally, I done tow parts signing and verification, but I want to build method to read private key, and than just sign the file. Because i do not want to use different key every time when I am signing something.

    // Here is my method for reading private key
class PK implements Serializable{
    PrivateKey pk = null;

    public PK(String nazivFajla) throws FileNotFoundException, IOException {
        this.pk = privatni;
        FileOutputStream fos = new FileOutputStream(nazivFajla);
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(this.pk);
        oos.close();
        fos.close();
    }

    public PrivateKey getPk(String nazivFajla) throws FileNotFoundException, IOException, ClassNotFoundException {

        FileInputStream fis = new FileInputStream(nazivFajla);
        ObjectInputStream ois = new ObjectInputStream(fis);
        this.pk = (PrivateKey) ois.readObject();
        return pk;
    }

And here is method in my button field:

    private void ucitajPrivatniActionPerformed(java.awt.event.ActionEvent evt) {                                               
try {
        jfc.showOpenDialog(jfc);
        String fajlZaprivatniKljuc = jfc.getSelectedFile().getAbsolutePath();
        PK pk = new PK(fajlZaprivatniKljuc);
        privatni = pk.getPk(fajlZaprivatniKljuc);
        this.tick_UF2.setVisible(true);
    } catch (IOException | ClassNotFoundException ex) {
        Logger.getLogger(GUI.class.getName()).log(Level.SEVERE, null, ex);
    }    
} 

This is my signing method:

    private void Potpisi1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    try {
        //Alisa bira algoritme - izabrala je RSA sa SHA-1

        Signature dsa = Signature.getInstance("SHA1withRSA");

        try{
        dsa.initSign(privatni);
        }catch(Exception e){
            System.out.println("Crashed here!");

        }    

        FileInputStream poruka = new FileInputStream(fajlZaPotpis);
        BufferedInputStream bufporuka = new BufferedInputStream(poruka);
        byte[] buffer = new byte[1024];
        int len;
        while (bufporuka.available() != 0) {
            len = bufporuka.read(buffer);
            dsa.update(buffer, 0, len);
        }
        bufporuka.close();
        poruka.close();
        //Alisa generiše digitalni potpis pozivajući metod sign()
        byte[] digitalniPotpis = dsa.sign();
        jfc.showSaveDialog(jfc);
        sacuvaj(digitalniPotpis, jfc.getSelectedFile().getAbsolutePath());
        this.tick_UF3.setVisible(true);

    } catch (NoSuchAlgorithmException ex) {
        Logger.getLogger(GUI.class.getName()).log(Level.SEVERE, null, ex);
    } catch (FileNotFoundException ex) {
        Logger.getLogger(GUI.class.getName()).log(Level.SEVERE, null, ex);
    } catch (IOException ex) {
        Logger.getLogger(GUI.class.getName()).log(Level.SEVERE, null, ex);
    } catch (SignatureException ex) {
        Logger.getLogger(GUI.class.getName()).log(Level.SEVERE, null, ex);
    }

And here is the mistake after trying to sign:

мај 14, 2015 8:09:55 PM DP.GUI Potpisi1ActionPerformed
SEVERE: null
java.security.SignatureException: object not initialized for signature or verification
    at java.security.Signature.update(Signature.java:738)
    at DP.GUI.Potpisi1ActionPerformed(GUI.java:553)
    at DP.GUI.access$1500(GUI.java:46)
    at DP.GUI$16.actionPerformed(GUI.java:306)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6525)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6290)
    at java.awt.Container.processEvent(Container.java:2234)
    at java.awt.Component.dispatchEventImpl(Component.java:4881)
    at java.awt.Container.dispatchEventImpl(Container.java:2292)
    at java.awt.Component.dispatchEvent(Component.java:4703)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
    at java.awt.Container.dispatchEventImpl(Container.java:2278)
    at java.awt.Window.dispatchEventImpl(Window.java:2739)
    at java.awt.Component.dispatchEvent(Component.java:4703)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:746)
    at java.awt.EventQueue.access$400(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:697)
    at java.awt.EventQueue$3.run(EventQueue.java:691)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
    at java.awt.EventQueue$4.run(EventQueue.java:719)
    at java.awt.EventQueue$4.run(EventQueue.java:717)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:716)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Please if anyone know the answer please help me, I am a junior programmer, and I really need help about this.. Thanks in advance!

Answer 1

Private keys should not be encoded using object serialization. Private keys can be encoded using the getEncoded() method and read in using PKCS8EncodedKeySpec supplied to an "RSA" KeyFactory instance.