Simple Socket server With Fibonacci sequence

FibSocket Challenge

hello everyone my here is my solution for the FibSocket Challenge

Category

Reaverse Engineering

Level:

hard

Points:

200

File:

FibSocket.exe

Solution

first thing get into my mind when i read the challenge name Sockets i said it’s about server and client and it really was

at the competition i didn’t think alot on this challenge and this was a big mistake :(

anyway when i open the file in IDA i got this main function and this Code:

which tells us this that listen port is 7777 in the htons()

so i created a client wrote in c# to connect to this server through localhost and port 7777 which they told us as a Hint

now let’s see the code of this server:

and it’s:

-accepting the client and sending a buffer to it

-then it’s going into a loop and sends another buffers with length 4

-v27 and v26 are random values

-note that the dword_EB4024 is a fibonacci sequence starting from 3

if you don’t know what is fibonacci numbers you can read this wiki Fibonacci

then receives an input and compare it with a random number from the squence if it’s not equal .. it sends msg and disconnect

so i wrote a code which take what the server sends which is the random from dword_EB4020 and then search in the fibonacci nums to get it’s index and resend it to the server to make the comparison returns true

 for (int i = 0; i < 1000; i++)
                    {
                        try
                        {

                            byte[] inStream = new byte[4];
                            Int32 bytes = s.Receive(inStream, inStream.Length, 0);
                            string st = "";
                            st = Encoding.UTF8.GetString(inStream, 0, bytes);
                            Console.Write(st);
                            if (i == 13)
                            {
                                var test = BitConverter.ToInt16(inStream, 0);
                                Console.WriteLine(test.ToString());
                                int indexOfNum = 0;

                                //just for debugging :)
                                foreach (char c in st)
                                    Console.WriteLine("{}==> " + ((int)c).ToString());
                                for (int t = 0; t < fibos.Length; t++)
                                {
                                    if (fibos[t] == (int)st[0])
                                    {
                                        indexOfNum = t;
                                        break;
                                    }
                                }

                                //first loop
                                int response = fibos[indexOfNum + 1];
                                var rawBytes = BitConverter.GetBytes(response);
                                //   uint send =  BitConverter.ToUInt32(rawBytes , 0);
                                // Console.WriteLine("{}==> " + send.ToString());
                                s.Send(rawBytes, rawBytes.Length, 0);
}

now we got to the second part

after this the server send’s us a msg that we sent the write num and it’s need the right byte:

to be honest i didn’t got this code but all what got into my head that it needs a right byte…..so the byte only take 255

so i bruteforced it and the right one was 233

and here is the code i wrote for it :

and i got the flag :)

iam really not so good with RE ….yet and if there is any wrong in my saying just tell me i will be so grateful

## Flag TM{5702887,9227465}