+++
author = "Maik de Kruif"
title = "Secret Location - Base"
subtitle = "Beginners Quest 4 - Google CTF"
date = 2021-09-24T16:33:25+01:00
description = "A writeup for challenge 4 of the beginners quests of the Google CTF."
cover = "img/writeups/google-ctf/2021/beginners-quest/4/cover.png"
tags = [
    "Google CTF",
    "Beginners Quest",
    "ctf",
    "hacking",
    "writeup",
    "web",
]
categories = [
    "ctf",
    "writeups",
    "hacking",
]
+++

## Story line

You’re taking a stroll in the lab, when Dr. Klostermann is calling your name: "Agent, we’ve discovered the origin of the device. This time you won’t be able to reach your destination by air, but by the new Trans-Sibiriean Railway, as opposed to the old one, which runs along side it at the same time, it is a bit odd. And it goes to Shenzhen. I am sorry agent, but the further you go into this task, the more precautions you will have to take, and remember, the enemy can be anyone. It could be a conductor, the engineer, it could even be our own people that will meet you at the spot you need to be at. Be selective with who you trust. I think you got the point, go now, I got much to do. Agent, much depends on you!."

### Attachment

[attachment.zip](/files/writeups/google-ctf/2021/beginners-quest/4/attachment.zip)

## Recon

When opening the attachment, we can find two files: `chal.c` and `pico.uf2`.

Before this challenge I had never heard of a `uf2` file, but from googling "pico.uf2", I found that it is probably a firmware file for the Paspberry Pi Pico.

While I would have loved to play around with that, I didn't have one at hand. So we'll have to do with the `chal.c` file (which is probably the file running in the firmware).

### `chal.c`

{{< code language="c" title="chal.c" isCollapsed="true" >}}

```c
#include <stdbool.h>

#include "hardware/gpio.h"
#include "hardware/structs/sio.h"
#include "pico/stdlib.h"

int main(void)
{
  for (int i = 0; i < 8; i++) {
    gpio_init(i);
    gpio_set_dir(i, GPIO_OUT);
  }
  gpio_put_all(0);

  for (;;) {
    gpio_set_mask(67);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(20);
    gpio_clr_mask(3);
    sleep_us(100);
    gpio_set_mask(2);
    gpio_clr_mask(16);
    sleep_us(100);
    gpio_set_mask(57);
    gpio_clr_mask(4);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(25);
    sleep_us(100);
    gpio_set_mask(5);
    gpio_clr_mask(2);
    sleep_us(100);
    gpio_set_mask(18);
    gpio_clr_mask(65);
    sleep_us(100);
    gpio_set_mask(1);
    gpio_clr_mask(2);
    sleep_us(100);
    gpio_set_mask(64);
    gpio_clr_mask(17);
    sleep_us(100);
    gpio_set_mask(2);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(1);
    gpio_clr_mask(6);
    sleep_us(100);
    gpio_set_mask(18);
    gpio_clr_mask(65);
    sleep_us(100);
    gpio_set_mask(1);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(4);
    gpio_clr_mask(2);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(64);
    gpio_clr_mask(16);
    sleep_us(100);
    gpio_set_mask(16);
    gpio_clr_mask(64);
    sleep_us(100);
    gpio_set_mask(2);
    gpio_clr_mask(4);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(3);
    sleep_us(100);
    gpio_set_mask(9);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(1);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(8);
    sleep_us(100);
    gpio_set_mask(8);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(65);
    gpio_clr_mask(24);
    sleep_us(100);
    gpio_set_mask(22);
    gpio_clr_mask(64);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(5);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(2);
    sleep_us(100);
    gpio_set_mask(65);
    gpio_clr_mask(16);
    sleep_us(100);
    gpio_set_mask(22);
    gpio_clr_mask(65);
    sleep_us(100);
    gpio_set_mask(1);
    gpio_clr_mask(6);
    sleep_us(100);
    gpio_set_mask(4);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(66);
    gpio_clr_mask(21);
    sleep_us(100);
    gpio_set_mask(1);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(0);
    gpio_clr_mask(2);
    sleep_us(100);
    gpio_set_mask(24);
    gpio_clr_mask(65);
    sleep_us(100);
    gpio_set_mask(67);
    gpio_clr_mask(24);
    sleep_us(100);
    gpio_set_mask(24);
    gpio_clr_mask(67);
    sleep_us(100);
    gpio_set_mask(2);
    gpio_clr_mask(8);
    sleep_us(100);
    gpio_set_mask(65);
    gpio_clr_mask(18);
    sleep_us(100);
    gpio_set_mask(16);
    gpio_clr_mask(64);
    sleep_us(100);
    gpio_set_mask(2);
    gpio_clr_mask(0);
    sleep_us(100);
    gpio_set_mask(68);
    gpio_clr_mask(19);
    sleep_us(100);
    gpio_set_mask(19);
    gpio_clr_mask(64);
    sleep_us(100);
    gpio_set_mask(72);
    gpio_clr_mask(2);
    sleep_us(100);
    gpio_set_mask(2);
    gpio_clr_mask(117);
    sleep_us(100);

    gpio_put_all(0);
    sleep_ms(500);
  }

  return 0;
}
```

{{< /code >}}

In this code, we see a lot of calls to the functions `gpio_set_mask()` and `gpio_clr_mask()`. When looking up what these to, we find [this documentation](https://raspberrypi.github.io/pico-sdk-doxygen/group__hardware__gpio.html):

{{< figure src="/img/writeups/google-ctf/2021/beginners-quest/4/doc-functions.png" title="Functions documentation" >}}

{{< figure src="/img/writeups/google-ctf/2021/beginners-quest/4/doc-set-mask.png" title="gpio_set_mask() documentation" >}}

### Bitmasks

In the `gpio_set_mask()` documentation, we see that it wants a bitmask of GPIO values. So what is a bitmask?

A bitmask is used for logical operations to transform an array of bits using another array. For example, a bitmask can be used with OR:

| Operation | Value                        |
| --------- | ---------------------------- |
|           | `1001 0101`                  |
| OR        | **`1111 0000`** <- _bitmask_ |
| =         | `1111 0101`                  |

## Solving

To get the values at every `sleep()`, we just have to apply the masks one by one.

But before we can do that, we first need the individual values to work with. To get them, I wrote the following python script.

```py
set_mask_str = "gpio_set_mask"
clr_mask_str = "gpio_clr_mask"

set_masks = []
clr_masks = []

with open("chal.c", "r") as file:
    for line in file:
        set_mask_loc = line.find(set_mask_str)
        clr_mask_loc = line.find(clr_mask_str)

        end = line.find(")")

        if set_mask_loc != -1:
            set_masks.append(
                int(line[set_mask_loc + len(set_mask_str) + 1:end]))

        if clr_mask_loc != -1:
            clr_masks.append(
                int(line[clr_mask_loc + len(clr_mask_str) + 1:end]))
```

All this script does, is read the `chal.c` file line by line. If the line contains either the `gpio_set_mask()` or the `gpio_clr_mask()` function, it gets the location of the parameter and appends it to a list.

In the end we get two lists: `set_masks` and `clr_masks`.

### Applying the masks

Now comes the more difficult part, we have to apply these masks.

For setting the masks, we can use the bitwise OR operation (`|`). It works like this:

| State         | Value           |
| ------------- | --------------- |
| Initial value | `1001 0101`     |
| Bitmask       | **`1111 0000`** |
| Result        | `1111 0101`     |

As we can see in this table, it now set the bytes passed to `gpio_set_mask()` to true.

Clearing the masks is a little harder. For this we'll need to first invert the bitmask (`~`), and than use the bistwise AND operation (`&`) with the current value. It would work like this:

| State          | Value           |
| -------------- | --------------- |
| Initial value  | `1111 0101`     |
| Bitmask        | **`1100 0010`** |
| Invert bismask | **`0011 1101`** |
| Result         | `0011 0101`     |

If we turn this into a python script, it would look like this:

```py
current_output = 0
flag = ""

for set_mask, clr_mask in zip(set_masks, clr_masks):
    current_output |= set_mask
    current_output &= ~ clr_mask
    flag += chr(current_output)

print(flag.strip())
```

{{< code language="py" title="Full code" isCollapsed="true" >}}

```py
set_mask_str = "gpio_set_mask"
clr_mask_str = "gpio_clr_mask"

set_masks = []
clr_masks = []

with open("chal.c", "r") as file:
    for line in file:
        set_mask_loc = line.find(set_mask_str)
        clr_mask_loc = line.find(clr_mask_str)

        end = line.find(")")

        if set_mask_loc != -1:
            set_masks.append(
                int(line[set_mask_loc + len(set_mask_str) + 1:end]))

        if clr_mask_loc != -1:
            clr_masks.append(
                int(line[clr_mask_loc + len(clr_mask_str) + 1:end]))

current_output = 0
flag = ""

for set_mask, clr_mask in zip(set_masks, clr_masks):
    current_output |= set_mask
    current_output &= ~ clr_mask
    flag += chr(current_output)

print(flag.strip())
```

{{< /code >}}

## Solution

When executing this code, we get the flag! It's `CTF{be65dfa2355e5309808a7720a615bca8c82a13d7}`.