How to remove padding after RSA decryption??

Question

Hello,
I am testing my host apps ability to read public key that was saved in file after being exported from smart card where it was generated.
I have successfully used the private key on card to encrypt a small piece of data and the cryptogram is returned to the host.
On host side I retrieve public key from file and decrypt ciphertext
The on-card alg for Cipher is declared as follows-
RSAcipher = Cipher.getInstance(Cipher.ALG_RSA_PKCS1, true);and in the host app
Cipher rsaCipher = Cipher.getInstance("RSA","BC");I can see from what is returned from
byte[] decrypted = rsaCipher.doFinal(this.encryptedData);that the correct data is being returned, but is is padded by
01FFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFFFFFFFFFFFFFFFFFFFFFFFFFF00How do I know where the padding ends and the plaintext begins?
Can anyone help me with this?
Thanks in advance,
Ann

Answer 1

Hi!
I have the same problem like you.
But my problem is that i don,t know how to move the key across. I manege to create a pair of keys public and private using RSA Algorithm.
privateKey = KeyBuilder.buildKey(KeyBuilder.TYPE_RSA_PRIVATE,KeyBuilder.LENGTH_RSA_512,false);
        publicKey = KeyBuilder.buildKey(KeyBuilder.TYPE_RSA_PUBLIC,KeyBuilder.LENGTH_RSA_512,true);
                    // KeyPair creation
                    if(keyPair == null){
                        keyPair = new KeyPair(KeyPair.ALG_RSA, (short)publicKey.getSize());
                    // starts key generation process
                    keyPair.genKeyPair();
                  publicKey = keyPair.getPublic();
                    privateKey = keyPair.getPrivate();
                     cipher = Cipher.getInstance(ALG_RSA_NO_PAD, false );then i send the modulus and exponent part of the public key to host
             byte    keyElement = (byte)(apduBuffer[ISO7816.OFFSET_P2] & 0xFF);
                 // check correct type (modulus or exponent)
                 if((keyElement != 0x00) && (keyElement != 0x01))
                     ISOException.throwIt(ISO7816.SW_INCORRECT_P1P2);
                 // check elements request
                 if(keyElement == 0) {
                     // retrieve modulus
                     apduBuffer[0] = (byte)((RSAPublicKey)publicKey).getModulus(apduBuffer, (short)1);
                 } else
                     // retrieve exponent
                    apduBuffer[0] = (byte)((RSAPublicKey)publicKey).getExponent(apduBuffer, (short)1);
                 // send the key element
                 apdu.setOutgoingAndSend((short)0, (short)((apduBuffer[0] & 0xFF) + 1));I decrypt the data as follow in the card
      byte[] apduBuffer = apdu.getBuffer();
      if(!privateKey.isInitialized())
                        ISOException.throwIt(ISO7816.SW_CONDITIONS_NOT_SATISFIED);
      cipher.init(privateKey,Cipher.MODE_DECRYPT);
      apdu.setIncomingAndReceive();
     cipher.doFinal(apduBuffer, (short)ISO7816.OFFSET_CDATA, (short)(apduBuffer[ISO7816.OFFSET_LC] & 0xFF), apduBuffer, (short)0);
    apdu.setOutgoingAndSend((short)0, (short)(KeyBuilder.LENGTH_RSA_512/8));
   in host i encrypt data as follow
// Note modulus and exponent are arrays of bytes from the card
    BigInteger modulus = new BigInteger(1,modulus);
    BigInteger exponent =new BigInteger(1,exponent);
    cipher = Cipher.getInstance("RSA");
RSAPublicKeySpec pubKeySpec = new RSAPublicKeySpec(modulus,exponent);
keyFactory = KeyFactory.getInstance("RSA");
pubKey = keyFactory.generatePublic(pubKeySpec);
cipher.init(Cipher.ENCRYPT_MODE,pubKey);
byte[] p = cipher.doFinal(data);