RSA Key Generation Implementation in C#

/*
 *
 * RSA Key Generation Implementation in C#
 *
 *  http://csharpcodewhisperer.blogspot.com
 *
 */
BigInteger P = new BigInteger();
BigInteger Q = new BigInteger();

BigInteger R    = new BigInteger();
BigInteger phiR = new BigInteger();

BigInteger K = new BigInteger();

BigInteger E = new BigInteger();
BigInteger D = new BigInteger();

void CalculateRandPhiR(string LargePrime1,string LargePrime2)
{
    BigInteger.TryParse(LargePrime1,out P);
    BigInteger.TryParse(LargePrime2,out Q);

    R   = BigInteger.Multiply(P,Q);
    phiR    = BigInteger.Multiply(P-1,Q-1);

    textBoxR.Text    = R.ToString();
    textBoxPhiR.Text = phiR.ToString();
}

bool IsCoprime(BigInteger value1, BigInteger value2)
{
    return ( BigInteger.GreatestCommonDivisor(value1,value2) == 1 );
}

// Note: This method for finding candidates for K is extremely weak, and not cryptographically sound.
// The cryptographically secure method would involve picking a random number greater than Phi(R),
// but less than 2*Phi(R), and incrementally checking for co-primeness.
// This would, however, likely take a very long time.
void FindKCandidates()
{
    List<BigInteger> candidates = new List<BigInteger>();

    BigInteger test = new BigInteger();
    for (BigInteger i = 2; i < 30; i++)
    {
        test = BigInteger.Multiply(phiR,i) + 1;
        if(IsCoprime(phiR,test))
        {
            candidates.Add(test);
        }
    }

    if(candidates.Count>0)
    {
        listBoxK.Items.Clear();
        foreach(BigInteger candidate in candidates)
        {
            listBoxK.Items.Add(candidate.ToString());
        }
    }
    else
    {
        listBoxK.Items.Add("No co-primes found!");
    }
}

void ListBoxK_SelectedIndexChanged(object sender, EventArgs e)
{
    if(!BigInteger.TryParse(listBoxK.SelectedItem.ToString(),out K)) {
        return;
    }

    // Factor K
    GetFactorPair(K);

    // Fill E and D
    textBoxE.Text = E.ToString();
    textBoxD.Text = D.ToString();

    // Calculate
    BigInteger remainder = new BigInteger();
    BigInteger.DivRem(BigInteger.Multiply(E,D),R,out remainder);

    checkBoxED1modR.Checked   = ( remainder == new BigInteger(1) );
    checkBoxDRcoprime.Checked = IsCoprime(D,R);
    checkBoxERcoprime.Checked = IsCoprime(E,R);
}

void GetFactorPair(BigInteger numberToFactor)
{
    E = new BigInteger(0);
    D = new BigInteger(0);

    // If numberToFactor is greater than double.MaxValue, our calculation below will fail,
    // As unfortunately BigInteger does not support a Sqrt function. Thus, we are limited
    // to double.MaxValue. Possible improvements include writing own square root function.
    if(numberToFactor>new BigInteger(double.MaxValue)) {
        return;
    }

    BigInteger sqrt = new BigInteger(Math.Ceiling(Math.Sqrt((double)numberToFactor)));

    // If perfect square, return, as this number will not do.
    if (BigInteger.Multiply(sqrt,sqrt) == numberToFactor) {
        return;
    }

    for (BigInteger i = 2; i < sqrt; i++)
    {
        if (numberToFactor % i == 0)
        {
            E = i;
            D = BigInteger.Divide(numberToFactor,i);
            //return;
        }
    }
}