RSA implementation basics ...

Question

Hi,
Iam totally new to the Java Card programming. I want to find out how is RSA implemented. Now if I need to get some information from card (eg. serial number) and check the same. How do I implement the same using the Host and Smart Card.
Any light on the same would be appreciated. Also, if anyone has example of implementation of RSA between Host and Smart card would be appreciated (in Delphi and Java) ...
Thanks

Answer 1

Hi,
I have written a code based on a sample. The program has a client which accepts a string at frontend and sends the information to be encrypted at card, then writes the encrypted information to the card. To decrypt the same, there is an option at the frontend to read the string from card, so the program, gets the string from the card (in encrypted form), then sends the string to card decrypt the same. Iam getting an Techincal error (error 38) while decrypting. Can you please help? I need a solution immediately. I been trying to work on the same for last few days.
I have pasted the code below for reference. Appreciate if some one could respond quickly.
package rsa_encrypt_decrypt;
import javacard.framework.*;
import javacard.security.*;
import javacardx.crypto.Cipher;
Host Call:
iopCard.SendCardAPDU(0x00,0xAA,0x02,P2,iArray,iArray.length);
Card Applet:
public class RSAEncryptDecrypt extends javacard.framework.Applet
     // This applet is designed to respond to the following
     // class of instructions.
     final static byte GETSET_CLA = (byte) 0x85;
     final static byte CRYPT_CLA = (byte) 0x00;
     // Instruction set for SimpleString
     final static byte SET = (byte)0x10;
     final static byte GET = (byte)0x20;
     final static byte SELECT = (byte) 0xA4;
// This buffer contains the string data on the card
     byte TheBuffer[];     
     //globals
     RSAPrivateCrtKey rsa_PrivateCrtKey;
     RSAPublicKey rsa_PublicKey;
     KeyPair rsa_KeyPair;
     Cipher cipherRSA;
     final short dataOffset = (short) ISO7816.OFFSET_CDATA;
     //constructor
     private HandsonRSAEncryptDecrypt(byte bArray[], short bOffset, byte bLength)
     TheBuffer = new byte[100];
     //generate own rsa_keypair
rsa_KeyPair = new KeyPair( KeyPair.ALG_RSA_CRT, KeyBuilder.LENGTH_RSA_1024 );
rsa_KeyPair.genKeyPair();
          rsa_PublicKey = (RSAPublicKey) rsa_KeyPair.getPublic();
          rsa_PrivateCrtKey = (RSAPrivateCrtKey) rsa_KeyPair.getPrivate();
          cipherRSA = Cipher.getInstance(Cipher.ALG_RSA_PKCS1, false);
          register(bArray, (short) (bOffset + 1), bArray[bOffset]);
     //install
     public static void install(byte bArray[], short bOffset, byte bLength)
          new HandsonRSAEncryptDecrypt(bArray, bOffset, bLength);
     public void process(APDU apdu)
          if (selectingApplet())
               return;
          byte[] buf = apdu.getBuffer();
          byte cla = buf[ISO7816.OFFSET_CLA];
          byte ins = buf[ISO7816.OFFSET_INS];
          if ((buf[ISO7816.OFFSET_CLA] != 0) && (buf[ISO7816.OFFSET_CLA] != GETSET_CLA)) ISOException.throwIt (ISO7816.SW_CLA_NOT_SUPPORTED);
          if ((buf[ISO7816.OFFSET_INS] != (byte) (0xAA)) && (buf[ISO7816.OFFSET_INS] != (byte) (0x10)) && (buf[ISO7816.OFFSET_INS] != (byte) (0x20))) ISOException.throwIt (ISO7816.SW_INS_NOT_SUPPORTED);
          switch (cla)
               case GETSET_CLA:
                    switch (ins)
                         case SET:
                              SetString(apdu);
                              break;
                         case GET:
                              GetString(apdu);
                              break;
               case CRYPT_CLA:
                    switch (buf[ISO7816.OFFSET_P1])
                         case (byte) 0x01:
                              encryptRSA(apdu);
                              return;
                         case (byte) 0x02:
                              decryptRSA(apdu);
                              return;
     private void encryptRSA(APDU apdu)
          byte a[] = apdu.getBuffer();
          short byteRead = (short) (apdu.setIncomingAndReceive());
          cipherRSA.init(rsa_PrivateCrtKey, Cipher.MODE_ENCRYPT);
          short cyphertext = cipherRSA.doFinal(a, (short) dataOffset, byteRead, a, (short) dataOffset);
          // Send results
          apdu.setOutgoing();
          apdu.setOutgoingLength((short) cyphertext);
          apdu.sendBytesLong(a, (short) dataOffset, (short) cyphertext);
          //SetString(apdu);
     private void decryptRSA(APDU apdu)
          byte a[] = apdu.getBuffer();
          short byteRead = (short) (apdu.setIncomingAndReceive());
          cipherRSA.init(rsa_PublicKey, Cipher.MODE_DECRYPT);
          cipherRSA.doFinal(a, (short) dataOffset, byteRead, a, (short) dataOffset);
          // Send results
          apdu.setOutgoing();
          apdu.setOutgoingLength((short) 24);
          apdu.sendBytesLong(a, (short) dataOffset, (short) 24);
     // SetString stores the string on the card.
     private void SetString(APDU apdu) {
          byte buffer[] = apdu.getBuffer();
          byte size = (byte)(apdu.setIncomingAndReceive());
          byte index;
          // Store the length of the string and the string itself
          TheBuffer[0] = size;
          for (index = 0; index < size; index++)
               TheBuffer[(byte)(index + 1)] = buffer[(byte)(ISO7816.OFFSET_CDATA + index)];
          return;
     //     1. Client sends a GetString APDU with a length of 0
     //     2. Card responds with a Status Word of 0x62YY, where YY is the length
     //          of the string (in hex).
     //     3. The client sends its GetString APDU again, but this time with the
     //          correct length.
     private void GetString(APDU apdu) {
          byte buffer[] = apdu.getBuffer();
          byte numBytes = buffer[ISO7816.OFFSET_LC];
          if (numBytes == (byte)0) {
               ISOException.throwIt((short)(0x6200 + TheBuffer[0]));
          apdu.setOutgoing();
          apdu.setOutgoingLength(numBytes);
          byte index;
          for (index = 0; index <= numBytes; index++)
               buffer[index] = TheBuffer[(byte)(index + 1)];
          apdu.sendBytes((short)0,(short)numBytes);
          return;
}