Hey guys! Today, we're diving deep into a classic picoCTF challenge: "What Lies Within." This challenge is fantastic for beginners because it introduces fundamental concepts of file analysis and data extraction. So, buckle up, and let's get started!

Challenge Overview

The challenge presents us with a file, often without any specific instructions beyond the challenge's title. The core task is to examine the file, identify hidden information, and extract a flag. These flags are the keys to solving picoCTF challenges, and they usually follow a specific format (picoCTF{}).

Initial File Examination

First things first, you'll need to download the file provided in the challenge. Once you've got it, the initial step is to determine the file type. Now, you might be tempted to rely solely on the file extension (like .txt or .jpg), but that can be misleading. A file might be intentionally mislabeled to throw you off track. The *nix command file is your best friend here. Open your terminal, navigate to the directory where you saved the file, and run:

file <filename>

Replace <filename> with the actual name of the downloaded file. The file command analyzes the file's contents and tells you what type of file it actually is. This is crucial because it informs you how to proceed with the analysis. For example, it could be a zip archive, a text file, an image, or something else entirely. This initial step can save you a lot of time and prevent you from going down the wrong path.

For this specific challenge, let's assume the file command reveals that the file is a zip archive. This means we can move on to the next stage: extracting the contents.

Extracting the Contents

Since the file is a zip archive, we need to extract its contents. In your terminal, use the following command:

unzip <filename>

Again, replace <filename> with the name of the file. This command will extract all the files and directories contained within the zip archive into the current directory. It's good practice to create a separate directory for each challenge to keep your files organized.

After extracting, you'll likely find one or more files. These could be text files, images, or even other archives. Now, it's time to examine these extracted files individually.

Analyzing Extracted Files

This is where the real detective work begins. The approach you take depends on the types of files you extracted.

Text Files

If you find text files, open them using a text editor or the cat command in your terminal:

cat <filename>

Carefully read through the contents. Look for anything that seems out of place, like unusual strings, encoded data (base64, hexadecimal), or fragments of text that resemble the flag format (picoCTF{}). Don't dismiss anything immediately! Sometimes the flag is hidden in plain sight, or it might be slightly obfuscated.

Image Files

Image files can hide information in several ways. One common technique is steganography, where data is concealed within the image's pixels. You can use tools like StegSolve or online steganography decoders to analyze images. Another approach is to use the strings command on the image file:

strings <filename>

This command extracts all printable strings from the image file. Sometimes, metadata or comments embedded in the image contain clues or even the flag itself. Examine the output of the strings command carefully.

Other Files

If you encounter other types of files, like executables or data files, you might need more specialized tools to analyze them. For example, you can use a hex editor to examine the raw bytes of a file, or a disassembler to analyze an executable's code. However, for the "What Lies Within" challenge, it's more likely that you'll find the flag within text or image files.

Decoding and Extracting the Flag

Once you've identified a potential flag or a piece of encoded data, you'll need to decode it. Common encoding schemes include:

• Base64: Base64 is a widely used encoding scheme that represents binary data in an ASCII string format. You can use the base64 command in your terminal to decode Base64 encoded strings:

  base64 -d <filename>
  

  or

  echo "<encoded_string>" | base64 -d
  

• Hexadecimal: Hexadecimal represents data using base-16 numbers. You can use online hex decoders or scripting languages like Python to convert hexadecimal strings to ASCII.

• URL Encoding: URL encoding is used to represent special characters in URLs. Online URL decoders can easily decode these strings.

• Caesar Cipher: This is a simple substitution cipher where each letter is shifted by a certain number of positions in the alphabet. You can try different shift values to see if you can decipher the message.

Keep trying different decoding methods until you get something that looks like the flag format: picoCTF{...}. Remember that the flag might be split across multiple files or encoded in multiple layers, so be persistent!

Example Scenario

Let's walk through a possible scenario. Suppose you extract a file named hidden.txt, and its contents look like this:

U29tZVRleHQKClVuaXRlZFRleHQKClVuaXRlZFRleHQKcGlrb0NURnt0aGlzX2lzX2Zha2VfZmxhZ30K

At first glance, it might seem like random text. However, notice the last line looks suspiciously like a Base64 encoded string. Let's try decoding it:

echo