bati06 bati06 - 4 years ago 81
C# Question

How to solve the task called 'Fast and Furious' from 'Break In 2017' challenge in C#?

I try to solve the task called Fast and Furious from Break In 2017 challenge.

The task is simple. Need to HTTP post back the result of a math expression contained in a HTML page. It can be achieved by this python script:

import requests
import re

url = 'https://felicity.iiit.ac.in/contest/extra/fastandfurious/'
s = requests.Session()
r = s.get(url)
print(r.text)
m = re.search('\(.*\)', r.text)
while m:
ans = eval(m[0])
print(m[0] + ' -> ' + str(ans))
r = s.post(url, data={'ques_ans' : ans})
print(r.text)
if ('the_flag_is_' in r.text): break
m = re.search('\(.*\)', r.text)


I want to do the same in C#. I tried like this:

using System;
using System.Data;
using System.IO;
using System.Net;
using System.Text;
using System.Text.RegularExpressions;

class Program
{
static CookieContainer cookies = new CookieContainer();

static HttpWebRequest Create(string url)
{
var request = (HttpWebRequest)WebRequest.Create(url);
request.CookieContainer = cookies;
request.Accept = "*/*";
request.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
request.UserAgent = "dummy";
return request;
}

static string Get(string url)
{
var request = Create(url);
request.Method = "GET";

using (var response = request.GetResponse())
using (var reader = new StreamReader(response.GetResponseStream()))
{
return reader.ReadToEnd();
}
}

static string Post(string url, string postData)
{
var request = Create(url);
request.Method = "POST";

var data = Encoding.UTF8.GetBytes(postData);
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = data.Length;

using (var stream = request.GetRequestStream())
{
stream.Write(data, 0, data.Length);
}

using (var response = request.GetResponse())
using (var reader = new StreamReader(response.GetResponseStream()))
{
return reader.ReadToEnd();
}
}

static string Eval(string expression)
{
DataTable dt = new DataTable();
return dt.Compute(expression, "").ToString();
}

static void Main()
{
string url = "https://felicity.iiit.ac.in/contest/extra/fastandfurious";
string text = Get(url);
Console.WriteLine(text);
var m = Regex.Match(text, @"\(.*\)");

while (m.Success)
{
var ans = Eval(m.Value);
Console.WriteLine(m.Value + " -> " + ans);
text = Post(url, "ques_ans=" + ans);
Console.WriteLine(text);
if (text.Contains("the_flag_is_")) break;
m = Regex.Match(text, @"\(.*\)");
}
}
}


But it does not work, because I always get back the 'Level 1' question. I used HttpWebRequest.CookieContainer property to reuse the cookies across different requests to keep up a session.

I don't know what is the problem. Maybe the session doesn't work.
Or perhaps HttpWebRequest is too slow to post back the result in time.

How to solve this automation task in C#?

Answer Source

Your code isn't handling being redirected. Watching the traffic we can see that this particular server would like your requests to /fastandfurious to end with a trailing slash. so change '../fastandfurious' to '../fastandfurious/' and that will fix it.

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