I can�t decrypt a text encrypted (using RSA) with keys on smartcard.

Question

I use a Cyberflex Access e-gate smartcard and I can encrypt and decrypt any text on the card but if I encrypt a text outside using the exported public key, card is not able to decrypt the message.
On the card side:
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RSAPrivateCrtKey privateKey;
RSAPublicKey publicKey;
Cipher cipherRSA;
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private MyIdentity (byte buffer[], short offset, byte length){
        // initialise PIN
        pin = new OwnerPIN(PinTryLimit, MaxPinSize);
        pin.resetAndUnblock();  
        // Key Pair
        KeyPair kp = new KeyPair(KeyPair.ALG_RSA_CRT, (short)1024);
        kp.genKeyPair();
        privateKey = (RSAPrivateCrtKey) kp.getPrivate();
        publicKey = (RSAPublicKey) kp.getPublic();  
        cipherRSA = Cipher.getInstance(Cipher.ALG_RSA_PKCS1, false);
        if (buffer[offset] == (byte)0) {
            register();
        } else {
            register(buffer, (short)(offset+1) ,(byte)(buffer[offset]));
    private void GetPublicKey (APDU apdu) {
        if (pin.isValidated()){
            byte apduBuffer[] = apdu.getBuffer();
            // short byteRead = (short)(apdu.setIncomingAndReceive());
               short bytesMod = publicKey.getModulus(apduBuffer, (short) 0);
               short bytesExp = publicKey.getExponent(apduBuffer,bytesMod);
               short outbytes = (short) (bytesMod + bytesExp);
                // Send results
             apdu.setOutgoing();
             // indicate the number of bytes in the data field
             apdu.setOutgoingLength((short)outbytes);
             // at offset 0 send 128 byte of data in the buffer
             apdu.sendBytesLong(apduBuffer, (short)APDUDATA, (short)outbytes);
        } else {
            ISOException.throwIt (ISO7816.SW_SECURITY_STATUS_NOT_SATISFIED);
     private void Decrypt (APDU apdu) {
        byte apduBuffer[] = apdu.getBuffer();
         short byteRead = (short)(apdu.setIncomingAndReceive());
        cipherRSA.init(privateKey, Cipher.MODE_DECRYPT);
          cipherRSA.doFinal(apduBuffer,(short)APDUDATA, byteRead, apduBuffer, (short)APDUDATA);
         // Send results
        apdu.setOutgoing();
        // indicate the number of bytes in the data field
        apdu.setOutgoingLength(byteRead);
        // at offset 0 send x byte of data in the buffer
        apdu.sendBytesLong(apduBuffer, (short)APDUDATA, byteRead);
     }Off the card, I have a java client:
public void getPublicKey () {
        int CLA, INS, P1, P2;
        int iArray[] = new int[0];
        short sArray[] = new short[0];
        String ss = new String("");
        String s;
        byte [] sBytes = null;
        byte [] myModulus = new byte[128];
        byte [] myExponent = new byte[3];
        try     {
            CLA = 0x68;
            INS = 0x78;
            P1  = 0;
            P2  = 0;
            sArray = iopCard.SendCardAPDU(CLA,INS,P1,P2,iArray,0x83);
            int iErrorCode = iopCard.GetLastErrorCode();
            if (iErrorCode != 0x9000)     {
                if (iErrorCode == 0x6300) {
                    System.out.println("Wrong PIN");
                } else {
                    s = iopCard.GetErrorMessage();
                    System.out.println("SendCardAPDU: " + s);
            } else {
                System.out.println("Getting Public Key...");
                if (sArray != null)  {
                    sBytes = new byte[sArray.length];
                    for (int i = 0; i < sArray.length; i++)  {
                        sBytes[i] = (byte)sArray;
ss = new String(sBytes);
System.out.println ("------ BEGIN PUBLIC KEY -------------------");
for (int i=0; i < sArray.length; i++){
System.out.print(Integer.toHexString(ss.charAt(i)).toUpperCase());
System.out.println ();
System.out.println ("------ END PUBLIC KEY -------------------");
} else {
System.out.println("Nothing.");
} catch (slbException b) {
s = b.getMessage();
System.out.println("Validate error: " + s);
for (int i=0; i<128; i++){
myModulus[i] = (byte) sArray[i];
for (int i=0; i<3; i++){
myExponent[i] = (byte) sArray[128+i];
BigInteger modulus = new BigInteger (1,myModulus);
BigInteger exponent = new BigInteger ("65537"); // there is a well-known bug in getExponent
RSAPublicKeySpec keySpec = new RSAPublicKeySpec(modulus, exponent);
KeyFactory keyFactory =null;
try {
keyFactory = KeyFactory.getInstance("RSA");
publicKey = keyFactory.generatePublic(keySpec);
} catch (NoSuchAlgorithmException e) {
System.out.println(e.getMessage ());
} catch (InvalidKeySpecException e) {
System.out.println(e.getMessage ());
System.out.println("------------------ BEGIN ------------------");
ss = new String(publicKey.getEncoded());
for (int i=0; i < publicKey.getEncoded().length; i++){
System.out.print(Integer.toHexString(ss.charAt(i)).toUpperCase());
System.out.println ();
System.out.println("------------------ END ------------------");
// to a file
try {
//Store in raw format
FileWriter fw = new FileWriter("public_raw.txt");
for (int i=0; i < publicKey.getEncoded().length; i++){
fw.write(Integer.toHexString(ss.charAt(i)).toUpperCase());
fw.close();
//could also store it as a Public key
System.out.println("Public key saved to file");
} catch(Exception e) {
System.out.println("Error opening and writing Public key to file : "+e.getMessage());
public void encrypt () {
byte cadena[] = {0x01,0x02,0x03,0x04};
byte resultado[] = new byte[256];
// Create Cipher
try {
cipherRSA.init(Cipher.ENCRYPT_MODE, publicKey);
resultado = cipherRSA.doFinal (cadena);
} catch (InvalidKeyException e) {
System.out.println(e.getMessage());
} catch (BadPaddingException e) {
System.out.println(e.getMessage());
} catch (IllegalBlockSizeException e) {
System.out.println(e.getMessage());
String ss = new String (resultado);
System.out.println("------------------ BEGIN 4 ------------------");
for (int i=0; i < resultado.length; i++){
System.out.print(Integer.toHexString(ss.charAt(i)).toUpperCase());
System.out.println ();
System.out.println("------------------ END 4 ------------------");
Another question is that I don�t understand why I get a constant length string when I encrypt a text on the card and variable length string when I encrypt off the card

Answer 1

I thought that exponent was 3 bytes long...
On the card I have the following code:
    private void GetExponent (APDU apdu) {
        if (pin.isValidated()){
            byte apduBuffer[] = apdu.getBuffer();
        short bytesExp = publicKey.getExponent(apduBuffer, (short) 0);
           // Send results
             apdu.setOutgoing();
             // indicate the number of bytes in the data field
             apdu.setOutgoingLength((short)bytesExp);
             // at offset 0 send 128 byte of data in the buffer
             apdu.sendBytesLong(apduBuffer, (short)APDUDATA, (short)bytesExp);
        } else {
            ISOException.throwIt (ISO7816.SW_SECURITY_STATUS_NOT_SATISFIED);
    }And if I don't send an APDU with length expected, I get the exception 6C03 (Correct Expected Length (Le) = 0x6C00) so I send APDU with 03 length and I receive the exponent. The problem is that there is a well know bug in getExponent and it returns 00 00 00... so I set it up to 65537 outside the card.