using System;
using Org.BouncyCastle.Math;

public BigInteger CalculateRSA(BigInteger p, BigInteger q, BigInteger e)
{
      // n = p*q - for illustration
      BigInteger n = p.Multiply(q);

      // phi / r = (p-1)*(q-1)
      BigInteger phi = p.Subtract(BigInteger.One).Multiply(q.Subtract(BigInteger.One));

      // d = e^−1 (mod λ(n))
      BigInteger d = e.ModInverse(phi);

      // Return d
      return d;
}

public void Main(string[] args){
      // Example usage - primes would have to be factored from n of public key
      string pText = "CC9EEF4A31452B93A2C3658566841BA7DFCD10B847D6749A91A32271DAAC6EFB5A45150AE684D632EDF8EAC03AA3F65EAA2648738E1B86137F66BE238B96D4AB",
            qText = "C364D7C501E30FC2076E5CFA57DFD4D8970AF695D754DD41A5CA13E760E2C5A1A447A1B7B8985225C25D0966C8CD8DB198860C37F3CE6CB190FB2B1FCDE11E61",
            eText = "65537";

      // Load p and q as hex, e as decimal
      BigInteger p = new BigInteger(pText, 16),
            q = new BigInteger(qText, 16),
            e = new BigInteger(eText, 10);

      // Calculate d - the private key
      BigInteger d = CalculateRSA(p, q, e);

      // Output as hex
      Console.WriteLine(d.ToString(16));
}