RSAEncryption

Question

Hello,
i want to use the RSA-Encryption to encryt the communication between the host and the javacard.
On the host side i use the following code to generate the private and public key and to create the cipher:
KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
kpg.initialize(512);
KeyPair kp = kpg.genKeyPair();
publicKey = kp.getPublic();
privateKey = kp.getPrivate();
Cipher cipher = Cipher.getInstance("RSA");Then I send the exponent and the modulus to the card using normal apdus.
In the constructor of the applet i use the following code to init the key and the cipher:
protected WalletApplication(byte[] bArray, short bOffset, byte bLength) {
        serverPublicKey = (RSAPublicKey) KeyBuilder.buildKey(
                KeyBuilder.TYPE_RSA_PUBLIC,
                KeyBuilder.LENGTH_RSA_512, false);
        cipher = Cipher.getInstance(ALG_RSA_PKCS1 ,false);
        register();
    }But if i encrypt a message on the javacard and send it back to the host in order to decrypt it. I got an BadPaddingException. Can someone explain how to fix this problem.
Thanks.

Answer 1

Thanks for your answer, changing the parameter in the getInstance does not solve so the problem, so here is the complete code:
public class Client {
    private byte[] message;
    private Socket socketCard;
    private InputStream is;
    private OutputStream os;
    private CadClientInterface cad;
    private PrivateKey privateKey;
    private RSAPublicKey publicKey;
    public Client() {
        initRSA();
        connectToSmartCardSimulator();
    public void connectToSmartCardSimulator() {
        try {
            socketCard = new Socket("localhost", 9025);
            is = socketCard.getInputStream();
            os = socketCard.getOutputStream();
            cad = CadDevice.getCadClientInstance(CadDevice.PROTOCOL_T1, is, os);
            cad.powerUp();
            cad.exchangeApdu(selectAppletApdu());
            // send the public key to the card
            cad.exchangeApdu(sendExponentApdu(publicKey.getPublicExponent()));
            cad.exchangeApdu(sendModulusApdu(publicKey.getModulus()));
            // send a message to the card and get it back encrypted
            message = encryptMessage();
            cad.powerDown();
            socketCard.close();
            System.out.println(new String(message));
            System.out.println(new String(rsaDecrypt(message)));
        } catch (IOException ex) {
            ex.printStackTrace();
        } catch (CadTransportException ex) {
            ex.printStackTrace();
    private Apdu selectAppletApdu() {
        Apdu apdu = new Apdu();
        apdu.command[Apdu.CLA] = (byte) 0x00;
        apdu.command[Apdu.INS] = (byte) 0xa4;
        apdu.command[Apdu.P1] = (byte) 0x04;
        apdu.command[Apdu.P2] = (byte) 0x00;
        byte[] aid = {(byte) 0x20, (byte) 0xDC, (byte) 0xA1, (byte) 0xCA,
            (byte) 0xC9, (byte) 0xF5};
        apdu.setDataIn(aid);
        return apdu;
    private Apdu sendExponentApdu(BigInteger exponent) {
        Apdu apdu = new Apdu();
        apdu.command[Apdu.CLA] = (byte) 0xB0;
        apdu.command[Apdu.INS] = (byte) 0x20;
        apdu.command[Apdu.P1] = (byte) 0x02;
        apdu.command[Apdu.P2] = (byte) 0x00;
        apdu.setDataIn(exponent.toByteArray());
        return apdu;
    private Apdu sendModulusApdu(BigInteger modulus) {
        Apdu apdu = new Apdu();
        apdu.command[Apdu.CLA] = (byte) 0xB0;
        apdu.command[Apdu.INS] = (byte) 0x30;
        apdu.command[Apdu.P1] = (byte) 0x01;
        apdu.command[Apdu.P2] = (byte) 0x00;
        apdu.setDataIn(modulus.toByteArray());
        return apdu;
    public byte[] rsaDecrypt(byte[] data) {
        try {
            Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
            cipher.init(Cipher.DECRYPT_MODE, privateKey);
            byte[] cipherData = cipher.doFinal(data);
            return cipherData;
        } catch (InvalidKeyException ex) {
            ex.printStackTrace();
        } catch (IllegalBlockSizeException ex) {
            ex.printStackTrace();
        } catch (BadPaddingException ex) {
            ex.printStackTrace();
        } catch (NoSuchAlgorithmException ex) {
            ex.printStackTrace();
        } catch (NoSuchPaddingException ex) {
            ex.printStackTrace();
        return null;
   private void initRSA() {
        try {
            KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
            kpg.initialize(512);
            KeyPair kp = kpg.genKeyPair();
            publicKey = (RSAPublicKey) kp.getPublic();
            privateKey = kp.getPrivate();
        } catch (NoSuchAlgorithmException ex) {
            ex.printStackTrace();
   public static void main(String[] args) {
        Client client = new Client();
    private byte[] encryptMessage() throws CadTransportException, IOException {
        Apdu apdu = new Apdu();
        apdu.command[Apdu.CLA] = (byte) 0xB0;
        apdu.command[Apdu.INS] = (byte) 0x40;
        apdu.command[Apdu.P1] = (byte) 0x02;
        apdu.command[Apdu.P2] = (byte) 0x00;
        apdu.setDataIn("This is a test".getBytes());
        cad.exchangeApdu(apdu);
        return apdu.getDataOut();
public class WalletApplication extends Applet {
    // codes of CLA byte in the command APDUs
    final static byte Wallet_CLA = (byte) 0xB0;
    final static byte INS_SET_PUBLIC_KEY = (byte) 0x20;
    final static byte P1_SET_PUBLIC_KEY_MODULUS = (byte) 0x01;
    final static byte P1_SET_PUBLIC_KEY_EXPONENT = (byte) 0x02;
    final static byte INS_CRYPTION_MODE = (byte) 0x40;
    final static byte P1_DECRYPTION_MODE = (byte) 0x01;
    final static byte P1_ENCRYPTION_MODE = (byte) 0x02;
    private RSAPublicKey publicKey;
    private Cipher cipher;
    public static void install(byte[] bArray, short bOffset, byte bLength) {
        new WalletApplication(bArray, bOffset, bLength);
    protected WalletApplication(byte[] bArray, short bOffset, byte bLength) {
        publicKey = (RSAPublicKey) KeyBuilder.buildKey(
                KeyBuilder.TYPE_RSA_PUBLIC,
                KeyBuilder.LENGTH_RSA_512, false);
        cipher = Cipher.getInstance(Cipher.ALG_RSA_PKCS1,false);
        // register the applet instance with the JCRE
        register();
    public boolean select() {
        return true;
    public void deselect() {
   public void process(APDU apdu) {
        byte[] buffer = apdu.getBuffer();
        if (selectingApplet()) {
            return;
        // verify the CLA byte
        if (buffer[ISO7816.OFFSET_CLA] != Wallet_CLA) {
            ISOException.throwIt(ISO7816.SW_CLA_NOT_SUPPORTED);
        // check the INS byte to decide which service method to call
        switch (buffer[ISO7816.OFFSET_INS]) {
           case INS_SET_PUBLIC_KEY:
                switch (buffer[ISO7816.OFFSET_P1]) {
                    case P1_SET_PUBLIC_KEY_EXPONENT:
                        setServerKeyExp(apdu);
                        break;
                    case P1_SET_PUBLIC_KEY_MODULUS:
                        setServerKeyMod(apdu);
                        break;
                return;
            case INS_CRYPTION_MODE:
                switch (buffer[ISO7816.OFFSET_P1]) {
                    case P1_ENCRYPTION_MODE:
                        rsaEncrypt(apdu);
                        break;
                return;
            default:
                ISOException.throwIt(ISO7816.SW_INS_NOT_SUPPORTED);
    private void setServerKeyMod(APDU apdu) {
        byte[] buffer = apdu.getBuffer();
        byte byteRead = (byte) (apdu.setIncomingAndReceive());
        try {
            publicKey.setModulus(buffer, ISO7816.OFFSET_CDATA, byteRead);
        } catch (ArrayIndexOutOfBoundsException ex) {
            ISOException.throwIt((short) 0x6889);
        } catch (TransactionException ex) {
            ISOException.throwIt((short) (0x6803 + ex.getReason()));
    private void setServerKeyExp(APDU apdu) {
        byte[] buffer = apdu.getBuffer();
        byte byteRead = (byte) (apdu.setIncomingAndReceive());
        try {
           publicKey.setExponent(buffer, ISO7816.OFFSET_CDATA, byteRead);
        } catch (ArrayIndexOutOfBoundsException ex) {
            ISOException.throwIt((short) 0x6801);
        } catch (NullPointerException ex) {
            ISOException.throwIt((short) 0x6800);
        } catch (TransactionException ex) {
            ISOException.throwIt((short) 0x6803);
    private void rsaEncrypt(APDU apdu) {
        byte[] buffer = apdu.getBuffer();
        byte byteRead = (byte) (apdu.setIncomingAndReceive());
        cipher.init(publicKey, Cipher.MODE_ENCRYPT);
        short length = cipher.doFinal(buffer, (short) ISO7816.OFFSET_CDATA, byteRead, buffer, (short) ISO7816.OFFSET_CDATA);
        apdu.setOutgoing();
        apdu.setOutgoingLength((short) length);
        apdu.sendBytesLong(buffer, (short) ISO7816.OFFSET_CDATA, (short) length);
}Best regards,
Thorsten