Function Encrypt with error

string convertToASCII(string letter)
{
    string result = "";
    for (int i = 0; i < letter.length(); i++)
    {
        char x = letter.at(i);
        // fix ASCII code :)
        if (int(x)<10 || int(x)==0)
            result = result + "00";
        if (int(x)<100 && int(x)>9)
            result = result + "0";
        result = concatenate(result, int(x));
    }
    return result;
}

// Função para criptografar a mensagem
string Encrypt(string message, int e, int n)
{
    string Result;
    message = convertToASCII("tes"); // DEBUG
    unsigned long m=0;
    unsigned long message_int = 0;
    istringstream ( message ) >> m;
    message_int = m;
    cout << "\n\n" << message << "\n\n";
    cout << "\n\n-" << m << "-\n\n";
    Result = message;
    long res = 1;

    for(int i =0; i < e-1;++i) res =(res*message_int)% n;

    cout << "\n\n|" << res;
    Result = NumberToString(res);
    return Result;
}