public sealed class CryptoHelper
{
private static int GetIntegerSize(BinaryReader binr)
{
    byte bt = 0;
    byte lowbyte = 0x00;
    byte highbyte = 0x00;
    int count = 0;
    bt = binr.ReadByte();
    if (bt != 0x02)        //expect integer
        return 0;
    bt = binr.ReadByte();


    if (bt == 0x81)
        count = binr.ReadByte();    // data size in next byte
    else
        if (bt == 0x82)
    {
        highbyte = binr.ReadByte();    // data size in next 2 bytes
        lowbyte = binr.ReadByte();
        byte[] modint = { lowbyte, highbyte, 0x00, 0x00 };
        count = BitConverter.ToInt32(modint, 0);
    }
    else
    {
        count = bt;        // we already have the data size
    }


    while (binr.ReadByte() == 0x00)
    {    //remove high order zeros in data
        count -= 1;
    }
    binr.BaseStream.Seek(-1, SeekOrigin.Current);        //last ReadByte wasn't a removed zero, so back up a byte
    return count;
}


public static RSACryptoServiceProvider DecodeRSAPrivateKey(string priKey)
{
    var privkey = Convert.FromBase64String(priKey);
    byte[] MODULUS, E, D, P, Q, DP, DQ, IQ;

    MemoryStream m = new MemoryStream(privkey);
    BinaryReader binr = new BinaryReader(m);    
    byte bt = 0;
    ushort twobytes = 0;
    int elems = 0;
    try
    {
        twobytes = binr.ReadUInt16();
        if (twobytes == 0x8130)
            binr.ReadByte();        //advance 1 byte
        else if (twobytes == 0x8230)
            binr.ReadInt16();       //advance 2 bytes
        else
            return null;


        twobytes = binr.ReadUInt16();
        if (twobytes != 0x0102) //version number
            return null;
        bt = binr.ReadByte();
        if (bt != 0x00)
            return null;

        elems = GetIntegerSize(binr);
        MODULUS = binr.ReadBytes(elems);


        elems = GetIntegerSize(binr);
        E = binr.ReadBytes(elems);


        elems = GetIntegerSize(binr);
        D = binr.ReadBytes(elems);


        elems = GetIntegerSize(binr);
        P = binr.ReadBytes(elems);


        elems = GetIntegerSize(binr);
        Q = binr.ReadBytes(elems);


        elems = GetIntegerSize(binr);
        DP = binr.ReadBytes(elems);


        elems = GetIntegerSize(binr);
        DQ = binr.ReadBytes(elems);


        elems = GetIntegerSize(binr);
        IQ = binr.ReadBytes(elems);


        // ------- create RSACryptoServiceProvider instance and initialize with public key -----
        RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
        RSAParameters RSAparams = new RSAParameters();
        RSAparams.Modulus = MODULUS;
        RSAparams.Exponent = E;
        RSAparams.D = D;
        RSAparams.P = P;
        RSAparams.Q = Q;
        RSAparams.DP = DP;
        RSAparams.DQ = DQ;
        RSAparams.InverseQ = IQ;
        RSA.ImportParameters(RSAparams);

        return RSA;
    }
    catch (Exception e)
    {
        Console.WriteLine(e.Message + e.StackTrace);
        return null;
    }
    finally
    {
        binr.Close();
    }
  }

}