public static BigInteger operator &(BigInteger bi1, BigInteger bi2)
{
    BigInteger result = new BigInteger();
    int len = (bi1.dataLength > bi2.dataLength) ? bi1.dataLength : bi2.dataLength;
    for (int i = 0; i < len; i++)
    {
        result.data[i] = bi1.data[i] & bi2.data[i];
    }
    result.dataLength = 70;
    while ((result.dataLength > 1) && (result.data[result.dataLength - 1] == 0))
    {
        result.dataLength--;
    }
    return result;
}