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Java Applet for Signing with a Smart Card

  • February 24, 2006
  • By Svetlin Nakov
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Listing 1: SmartCardSignerApplet.java

import java.applet.Applet;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.ByteArrayInputStream;
import java.util.Arrays;
import java.util.Enumeration;
import java.util.List;
import java.security.*;
import java.security.cert.CertPath;
import java.security.cert.Certificate;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.lang.reflect.Constructor;

import netscape.javascript.JSException;
import netscape.javascript.JSObject;

 * Applet for digital signing documents with a smart card. The
 * applet is intended to be placed in a HTML document containing
 * a single HTML form that is used for applet input/output.
 * The applet accepts several parameters - the name of the field
 * in the HTML form that contains the file name to be signed
 * and the names of the fields in the HTML form, where the
 * certification chain and signature should be stored.
 * If the signing process is successful, the signature and
 * certification chain fields in the HTML form are filled.
 * Otherwise, an error message explaining the failure reason
 * is shown.
 * The applet asks the user to locate in his local file system
 * the PKCS#11 implementation library that is part of software
 * that come with the smart card and the smart card reader.
 * Usually this is a Windows .DLL file located in Windows
 * system32 directory or .so library
 * (e.g. C:\windows\system32\pkcs201n.dll).
 * The applet also asks the user to enter his PIN code for
 * accessing the smart card. If the smart card contains a
 * certificate and a corresponding private key, the signature of
 * the file is calculated and is placed in the HTML form.
 * In addition to the calculated signature the certificate with
 * its full certification chain is extracted from the smart card
 * and is placed in the HTML form too. The digital signature is
 * placed as Base64-encoded sequence of bytes. The certification
 * chain is placed as ASN.1 DER-encoded sequence of bytes,
 * additionally encoded in Base64. In case the smart card contains
 * only one certificate without its full certification chain, a
 * chain consisting of this single certificate is extracted and
 * stored in the HTML form instead of a full certification chain.
 * Digital signature algorithm used is SHA1withRSA. The length of
 * the calculated signature depends on the length of the private
 * key on the smart card.
 * The applet should be able to access the local machine's file
 * system for reading and writing. Reading the local file system
 * is required for the applet to access the file that should be
 * signed. Writing the local file system is required for the applet
 * to save its settings in the user's home directory. Accessing
 * the local file system is not possible by default, but if the
 * applet is digitally signed (with jarsigner), it runs with no
 * security restrictions and can do anything. This applet should
 * be signed in order to run.
 * Java Plug-In version 1.5 or higher is required for accessing
 * the PKCS#11 smart card functionality, so the applet will not
 * run in any other Java runtime environment.
 * This file is part of NakovDocumentSigner digital document
 * signing framework for Java-based Web applications:
 * http://www.nakov.com/documents-signing/
 * Copyright (c) 2005 by Svetlin Nakov - http://www.nakov.com
 * All rights reserved. This code is freeware. It can be used
 * for any purpose as long as this copyright statement is not
 * removed or modified.
public class SmartCardSignerApplet extends Applet {

   private static final String FILE_NAME_FIELD_PARAM =
   private static final String CERT_CHAIN_FIELD_PARAM =
   private static final String SIGNATURE_FIELD_PARAM =
   private static final String SIGN_BUTTON_CAPTION_PARAM =

   private static final String PKCS11_KEYSTORE_TYPE =
   private static final String X509_CERTIFICATE_TYPE =
   private static final String CERTIFICATION_CHAIN_ENCODING =
   private static final String DIGITAL_SIGNATURE_ALGORITHM_NAME =
   private static final String SUN_PKCS11_PROVIDER_CLASS =

   private Button mSignButton;

    * Initializes the applet - creates and initializes its graphical
    * user interface. Actually the applet consists of a single
    * button, that fills its all surface. The button's caption is
    * taken from the applet parameter SIGN_BUTTON_CAPTION_PARAM.

   public void init() {
      String signButtonCaption =
      mSignButton = new Button(signButtonCaption);
      mSignButton.setLocation(0, 0);
      Dimension appletSize = this.getSize();
      mSignButton.addActionListener(new ActionListener(){
         public void actionPerformed(ActionEvent e) {

    * Signs the selected file. The file name comes from a field
    * in the HTML document. The result consists of the calculated
    * digital signature and certification chain, both placed in
    * fields in the HTML document, encoded in Base64 format.
    * The HTML document should contain only one HTML form.
    * The name of the field, that contains the name of the file
    * to be signed is obtained from FILE_NAME_FIELD_PARAM applet
    * parameter. The names of the output fields for the signature
    * and the certification chain are obtained from the parameters
    * is asked to choose a PKCS#11 implementation library and a
    * PIN code for accessing the smart card.

   private void signSelectedFile() {
      try {
         // Get the file name to be signed from the form in the
         // HTML document
         JSObject browserWindow = JSObject.getWindow(this);
         JSObject mainForm =
            (JSObject) browserWindow.eval("document.forms[0]");
         String fileNameFieldName =
         JSObject fileNameField =
           (JSObject) mainForm.getMember(fileNameFieldName);
         String fileName = (String) fileNameField.getMember("value");

         // Perform the actual file signing
         CertificationChainAndSignatureBase64 signingResult =
         if null) {
            // Document signed. Fill the certificate and
            // signature fields
            String certChainFieldName =
            JSObject certChainField =
               (JSObject) mainForm.getMember(certChainFieldName);
            String signatureFieldName =
            JSObject signatureField =
               (JSObject) mainForm.getMember(signatureFieldName);
         else {
            // User canceled signing
      catch (DocumentSignException dse) {
         // Document signing failed. Display error message
         String errorMessage = dse.getMessage();
         JOptionPane.showMessageDialog(this, errorMessage);
      catch (SecurityException se) {
            "Unable to access the local file system.\n" +
            "This applet should be started with full security
             permissions.\n" +
            "Please accept to trust this applet when the Java
             Plug-In asks you.");
      catch (JSException jse) {
            "Unable to access some of the fields of the\n" +
            "HTML form. Please check the applet parameters.");
      catch (Exception e) {
            "Unexpected error: " + e.getMessage());

    * Signs given local file. The certificate and private key to
    * be used for signing come from the locally attached smart
    * card. The user is requested to provide a PKCS#11
    * implementation library and the PIN code for accessing the
    * smart card. @param aFileName the name of the file to be
    * signed.
    * @return the digital signature of the given file and the
    * certification chain of the certificatie used for signing the
    * file, both Base64-encoded or null if the signing process is
    * canceled by the user.
    * @throws DocumentSignException when a problem arised during
    * the singing process (e.g. smart card access problem,
    * invalid certificate, invalid PIN code, etc.)

   private CertificationChainAndSignatureBase64 signFile(String
   throws DocumentSignException {

      // Load the file for signing
      byte[] documentToSign = null;
      try {
         documentToSign = readFileInByteArray(aFileName);
      } catch (IOException ioex) {
         String errorMsg = "Cannot read the file for signing " +
            aFileName + ".";
         throw new DocumentSignException(errorMsg, ioex);

      // Show a dialog for choosing PKCS#11 implementation library
      // and smart card PIN
      PKCS11LibraryFileAndPINCodeDialog pkcs11Dialog =
         new PKCS11LibraryFileAndPINCodeDialog();
      boolean dialogConfirmed;
      try {
         dialogConfirmed = pkcs11Dialog.run();
      } finally {

      if (dialogConfirmed) {
         String oldButtonLabel = mSignButton.getLabel();
         try {
            String pkcs11LibraryFileName =
            String pinCode = pkcs11Dialog.getSmartCardPINCode();

            // Do the actual signing of the document with the
            // smart card
            CertificationChainAndSignatureBase64 signingResult =
               signDocument(documentToSign, pkcs11LibraryFileName,
            return signingResult;
         finally {
      else {
         return null;

   private CertificationChainAndSignatureBase64 signDocument(
      byte[] aDocumentToSign, String aPkcs11LibraryFileName,
         String aPinCode)
   throws DocumentSignException {
      if (aPkcs11LibraryFileName.length() == 0) {
         String errorMessage = "It is mandatory to choose " +
            "a PCKS#11 native implementation library for " + 
            "smart card (.dll or .so file)!";
         throw new DocumentSignException(errorMessage);

      // Load the keystore from the smart card using the specified
      // PIN code
      KeyStore userKeyStore = null;
         userKeyStore = loadKeyStoreFromSmartCard(
             aPkcs11LibraryFileName, aPinCode);
      catch (Exception ex) {
         String errorMessage = "Cannot read the keystore from "
            "the smart card.\n" +
            "Possible reasons:\n" +
            " - The smart card reader in not connected.\n" +
            " - The smart card is not inserted.\n" +
            " - The PKCS#11 implementation library is invalid.\n" +
            " - The PIN for the smart card is incorrect.\n" +
            "Problem details: " + ex.getMessage();
         throw new DocumentSignException(errorMessage, ex);

      // Get the private key and its certification chain from the
      // keystore
      PrivateKeyAndCertChain privateKeyAndCertChain = null;
      try {
         privateKeyAndCertChain =
      } catch (GeneralSecurityException gsex) {
         String errorMessage = "Cannot extract the private key " +
            "and certificate from the smart card. Reason: " +
         throw new DocumentSignException(errorMessage, gsex);

      // Check if the private key is available
      PrivateKey privateKey = privateKeyAndCertChain.mPrivateKey;
      if (privateKey == null) {
         String errorMessage = "Cannot find the private key on " +
            "the smart card.";
         throw new DocumentSignException(errorMessage);

      // Check if X.509 certification chain is available
      Certificate[] certChain =
      if (certChain == null) {
         String errorMessage = "Cannot find the certificate on " +
            "the smart card.";
         throw new DocumentSignException(errorMessage);

      // Create the result object
      CertificationChainAndSignatureBase64 signingResult =
         new CertificationChainAndSignatureBase64();

      // Save X.509 certification chain in the result encoded in
      // Base64
      try {
         signingResult.mCertificationChain =
      catch (CertificateException cee) {
         String errorMessage = "Invalid certificate on the "
            "smart card.";
         throw new DocumentSignException(errorMessage);

      // Calculate the digital signature of the file,
      // encode it in Base64 and save it in the result
      try {
         byte[] digitalSignature =
            signDocument(aDocumentToSign, privateKey);
         signingResult.mSignature =
      catch (GeneralSecurityException gsex) {
         String errorMessage = "File signing failed.\n" +
            "Problem details: " + gsex.getMessage();
         throw new DocumentSignException(errorMessage, gsex);

      return signingResult;

    * Loads the keystore from the smart card using its PKCS#11
    * implementation library and the Sun PKCS#11 security provider.
    * The PIN code for accessing the smart card is required.

   private KeyStore loadKeyStoreFromSmartCard(String
      aPKCS11LibraryFileName, String aSmartCardPIN)
   throws GeneralSecurityException, IOException {
      // First configure the Sun PKCS#11 provider. It requires a
      // stream (or file) containing the configuration parameters -
      // "name" and "library".
      String pkcs11ConfigSettings =
         "name = SmartCard\n" + "library = " + aPKCS11LibraryFileName;
      byte[] pkcs11ConfigBytes = pkcs11ConfigSettings.getBytes();
      ByteArrayInputStream confStream =
         new ByteArrayInputStream(pkcs11ConfigBytes);

      // Instantiate the provider dynamically with Java reflection
      try {
         Class sunPkcs11Class =
         Constructor pkcs11Constr = sunPkcs11Class.getConstructor(
         Provider pkcs11Provider =
           (Provider) pkcs11Constr.newInstance(confStream);
      catch (Exception e) {
         throw new KeyStoreException("Can initialize " +
            "Sun PKCS#11 security provider. Reason: " + 

      // Read the keystore form the smart card
      char[] pin = aSmartCardPIN.toCharArray();
      KeyStore keyStore = KeyStore.getInstance(PKCS11_KEYSTORE_TYPE);
      keyStore.load(null, pin);
      return keyStore;

    * @return private key and certification chain corresponding
    * to it, extracted from the given keystore. The keystore is
    * considered to have only one entry that contains both
    * certification chain and its corresponding private key.
    * If the keystore has no entries, an exception is thrown.

   private PrivateKeyAndCertChain getPrivateKeyAndCertChain(
      KeyStore aKeyStore)
   throws GeneralSecurityException {
      Enumeration aliasesEnum = aKeyStore.aliases();
      if (aliasesEnum.hasMoreElements()) {
         String alias = (String)aliasesEnum.nextElement();
         Certificate[] certificationChain =
         PrivateKey privateKey =
            (PrivateKey) aKeyStore.getKey(alias, null);
         PrivateKeyAndCertChain result = new PrivateKeyAndCertChain();
         result.mPrivateKey = privateKey;
         result.mCertificationChain = certificationChain;
         return result;
      else {
         throw new KeyStoreException("The keystore is empty!");

    * @return Base64-encoded ASN.1 DER representation of given
    * X.509 certification chain.

   private String encodeX509CertChainToBase64
      (Certificate[] aCertificationChain)
   throws CertificateException {
      List certList = Arrays.asList(aCertificationChain);
      CertificateFactory certFactory =
      CertPath certPath = certFactory.generateCertPath(certList);
      byte[] certPathEncoded =
      String base64encodedCertChain =
      return base64encodedCertChain;

    * Reads the specified file into a byte array.

   private byte[] readFileInByteArray(String aFileName)
   throws IOException {
      File file = new File(aFileName);
      FileInputStream fileStream = new FileInputStream(file);
      try {
         int fileSize = (int) file.length();
         byte[] data = new byte[fileSize];
         int bytesRead = 0;
         while (bytesRead < fileSize) {
            bytesRead += fileStream.read(
               data, bytesRead, fileSize-bytesRead);
         return data;
      finally {

    * Signs given document with a given private key.

   private byte[] signDocument(byte[] aDocument,
                               PrivateKey aPrivateKey)
   throws GeneralSecurityException {
      Signature signatureAlgorithm =
      byte[] digitalSignature = signatureAlgorithm.sign();
      return digitalSignature;

    * Data structure that holds a pair of private key and
    * certification chain corresponding to this private key.

   static class PrivateKeyAndCertChain {
      public PrivateKey mPrivateKey;
      public Certificate[] mCertificationChain;

    * Data structure that holds a pair of Base64-encoded
    * certification chain and digital signature.

   static class CertificationChainAndSignatureBase64 {
      public String mCertificationChain = null;
      public String mSignature = null;

    * Exception class used for document signing errors.

   static class DocumentSignException extends Exception {
      public DocumentSignException(String aMessage) {

      public DocumentSignException(String aMessage,
            Throwable aCause) {
         super(aMessage, aCause);


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