What Is Wrong With This Picture??

Question

Hi All,
I have been working on an RSA encryption program that allows the user to enter a message 'M'.
This message undergoes a series of permutations based on two prime numbers that have been randomly generated in order to encrypt the message.
The message is then decrypted using similar calculations.
The problem is it only returns the correct value some of the time! I'm sure it has to do with the final calculations, because if I use a calculator to do the functions I get the expected result 100% of the time.
Can anyone solve this problem?
// Perform calculation on M to encrypt the message
     double cipherText;
     cipherText = (double) Math.pow (encryption[0], e) % n;     
     System.out.println("CIPHERTEXT = " + cipherText );     
// Decrypt the message
     double decryption [] = new double [1];
     double plainText;
     decryption [0] = cipherText ++;          
     plainText = (double) Math.pow (decryption[0], d) % n;     
     System.out.println("PLAINTEXT = " + plainText );

Answer 1

You don't need to use the BigInteger everywhere... you can create some convenience methods to replace the spots where the numbers get as large as you say and hide the BigInteger's existence from the world...Let's say like this:
public static int getCipherText(int codedChar, int exponent, int modulo)
  BigInteger theBigOne = new BigInteger(""+codedChar).pow(exponent).mod(new BigInteger(""+modulo));
  return theBigOne.intValue();
}and just use the metod everywhere...
HTH,
Fredrik