**Tags:** misc

Rating: 5.0

--> [Original post](https://www.hackinprovence.fr/balccon-60-seconds-writeup/) <--

## Basic explanation of the challenge :

In this challenge, the goal is to get rid of all the "1" in the matrix that we get from their netcat server.

To do so we can input coordinates such as "d2" for instance, to invert the value of the number located at those coordinates aswell as its close neighbours (up, right, down and left).

Example of a matrix :

```

a b c d e f g h

1 0 0 0 0 1 0 0 1

2 1 0 1 0 0 0 1 0

3 1 1 0 1 1 0 1 0

4 0 1 1 0 0 0 1 1

5 0 0 1 1 0 1 0 0

6 0 0 0 0 0 1 1 1

7 1 1 0 0 0 0 0 1

8 0 1 1 0 0 1 0 1

```

## How to solve this challenge ?

As I'm not a rocket scientist and I didn't want to think too much, I was like : "Why not try to target every '1' in the matrix and once we're done, repeat until there are no more '1' ?"

So in clear here are the steps that my algorithm will follow :

1 : Find every coordinates for the "1" for the matrix first received, then send them

2 : Once every coordinates are sent, take a snapshot of the new matrix and then repeat the step 1 with the matrix newly received :)

## The code !

### 1] Format the matrix :

The matrix that we receive is a long string that we can't work with, here is my function that returns a usable matrix for python :

```

def input_to_matrice(received_matrix):

received_matrix = str(received_matrix, "utf-8")

temp_matrice = received_matrix.split("")[1:len(received_matrix.split(""))-2]

matrice = []

for i in range(len(temp_matrice)):

matrice.append([])

k = temp_matrice[i].split(" ")

for l in k:

matrice[i].append(int(l))

for i in matrice:

i.pop(0)

return matrice

```

### 2] Get the coordinates

Now that we can work with the matrix, let's get a list of all the coordinates of the "1" :

```

def get_moves_for_ones(matrice):

moves = []

for i in range(len(matrice)):

# matrice[i] will represent the row n°i

for j in range(len(matrice[i])):

# matrice[i][j] will represent the column n°j in the row n°i

if matrice[i][j] == 1:

# If we find a one, add the coordinates to our list of moves to do

move = ""

# The row :

move += 'abcdefgh'[i]

# The column :

move += str(j+1)

moves.append(move)

move = ""

return moves

```

### 3] Put that together :

We can now put those 2 functions together with a nice `while True` and boom, just let it run to get the flag !

Though this script has the advantage to be simple and doesn't require any math, one of its main drawbacks is that it's not "move efficient" because there isn't much logic behind it.

Code :

```

#!/usr/bin/env python3

# Small disclaimer : this program was first written using french words, and also I'm not a python expert, sorry ¯\_(ツ)_/¯

import socket

#Initialization : connect to their server

host = "pwn.institute"

port = 22527

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

s.connect((host, port))

def input_to_matrice(received_matrix):

"""

This functions returns a nicely formatted matrix that we can work with in python

Also this was first written in french, that's why there could be some weir words

"""

received_matrix = str(received_matrix, "utf-8")

temp_matrice = received_matrix.split("

")[1:len(received_matrix.split("

"))-2]

matrice = []

for i in range(len(temp_matrice)):

matrice.append([])

k = temp_matrice[i].split(" ")

for l in k:

matrice[i].append(int(l))

for i in matrice:

i.pop(0)

return matrice

def get_moves_for_ones(matrice):

moves = []

for i in range(len(matrice)):

# matrice[i] will represent the row n°i

for j in range(len(matrice[i])):

# matrice[i][j] will represent the column n°j in the row n°i

if matrice[i][j] == 1:

# If we find a one, add the coordinates to our list of moves to do

move = ""

# The row :

move += 'abcdefgh'[i]

# The column :

move += str(j+1)

moves.append(move)

move = ""

return moves

try:

matrice = s.recv(2048)

s.recv(2024)

print("The received matrix : ")

nice_matrice = input_to_matrice(matrice) #Nice because now it's pretty :)

print(nice_matrice) #Debug

while True:

#The goal here is to follow a simple algorithm :

# 1 : Find every coordinates for the "1", then send them

# 2 : Once every coordinates are sent, repeat :)

# There isn't much thinking behind it but it works ¯\_(ツ)_/¯

moves = get_moves_for_ones(nice_matrice)

print("Moves : ")

print(moves) #Debug

if moves == []:

#Something wrong heppened or we reached the end

break

for i in moves:

s.send(i.encode())

print("Sent : " + str(i))

received_matrice = s.recv(2024)

print("Received first :" + str(received_matrice)) # Those two prints are for debug but also to print out the flag when it arrives

print("Received second :" + str(s.recv(2024)))

#When we sent our wave of coordinates, repeat

nice_matrice = input_to_matrice(received_matrice)

finally:

s.close()

```

In the end we get that delightful message : "You made it in 63 moves and 39.403 seconds! Here is your reward: BCTF{cl0ck_is_tick1ng_but_we_are_fasterrr}" and here is our flag :)

Original writeup (https://www.hackinprovence.fr/balccon-60-seconds-writeup/).