Create a CTF Success Strategy

Learning Objectives

After completing this unit, you’ll be able to:

• List the foundational knowledge and skills recommended to effectively participate in CTFs.
  
• Identify the steps involved in learning to capture the flag.
  

Capture the Flag (CTF) challenges can be difficult, even the seemingly simple ones. However, there’s a way to approach CTFs that makes them less intimidating and more enjoyable while maximizing your learning. In this module, we develop a step-by-step process to guide you toward capturing the flag. But first, take a moment to refresh your memory on some key concepts from the previous unit to ensure you’re ready for what’s to come.

Knowledge Check

Ready to review what you’ve learned? The following knowledge check isn’t scored—it’s just an easy way to quiz yourself. To get started let’s review what you’ve learned by matching each CTF domain to the appropriate flag. When you finish matching all the items, click Submit to check your work. If you’d like to start over, click Reset.

Preparing for the Challenge

Consider your current physical and mental fitness. Now, imagine setting a goal to run a 10-mile race four months from now. You wouldn't start running without a plan, would you? You’d create a training schedule and strategy to build endurance and speed. You might even work with a running coach and join a running community for support and motivation. Similarly, successfully navigating Capture the Flag competitions requires goal setting, planning, and preparation.

Begin by choosing a competition. CTFtime is a great platform for finding events, allowing you to filter CTFs by format, location, and restrictions (for example, individual, academic). Alternatively, platforms like HackTheBox and TryHackMe have calendars and announcements for their own CTFs.

The CTF you choose determines the types of challenges you face. If you’re new to CTFs, assess your current knowledge and fill in the gaps to acquire a basic understanding and skill, especially in these key areas.

1. The penetration testing process: Knowing the pentesting process gives you a framework to methodically approach CTF challenges. It can help you understand where to start, what steps to take next, and how to organize your efforts.
   
2. Command line (for example, Powershell, Linux): The command line is your essential tool for most CTF challenges, enabling you to interact directly with systems and execute commands. Where the desktop gives you a user-friendly way to interact with your computer, the command line lets you directly control the inner workings for more advanced tasks, like those needed in CTF challenges.
   
3. Scripting (Python and Bash): Scripting languages like Python and Bash empower you to automate repetitive tasks, craft custom tools, and develop exploit code, giving you advantage in competitions. For example, Python could quickly decrypt a series of ciphertexts or Bash could automate interaction with a remote shell to find a hidden flag.
   
4. Debuggers and disassemblers (GNU Debugger, IDA Pro): These tools allow you to analyze the structure and potential vulnerabilities of a program, website, or network, enabling you to understand how the target system works and identify the path to the flag. Think of them as x-ray glasses for programs and websites that help you spot vulnerabilities.
   
5. Basic networking: Knowledge of network protocols, the TCP/IP stack, packet analysis, and network communication is crucial for understanding how systems communicate and where a connection or form of communication may be vulnerable.
   
6. Web technologies (HTML, CSS, JavaScript): Understanding how web pages and applications are built is essential for navigating web-based challenges.
   
7. Researching: CTFs require more than just basic searches. Hone your skills in OSINT gathering techniques like advanced search operators, image analysis, and using specialized search engines like Shodan. This investigative process is crucial for uncovering information and gaining the knowledge necessary to solve challenges.
   
8. Business skills: Success in CTFs heavily depends on business skills like creativity, critical thinking, teamwork, and problem solving. These skills integrate and amplify the impact of technical and nontechnical skills leading to more effective application within an individual or team.
   

With this foundation, you’ll be well-equipped to tackle CTFs. Remember, it’s about practice, persistence, continuous learning, and pushing your own boundaries! Even seasoned competitors are constantly learning and adapting, so don’t be discouraged if you have minimal knowledge and skill in these areas—every challenge is an opportunity to practice and discover something new.

A Structured Learning Approach

To learn how to capture the flag, we’re going to use a hypothetical, Jeopardy-style, forensics CTF challenge titled Secret Document. This challenge will help you visualize the key steps involved in successfully navigating a challenge. You won't need to download anything or enter information into a command line; images and explanations guide you through each step.

However, if you would like to put this process into practice and follow along with a real CTF challenge, here are two beginner friendly platforms you can start with: picoCTF and CTFLearn.

When engaging in a CTF, especially for skills development, prioritize the process as much as the final result. The process of questioning, researching, and learning is just as valuable as solving the challenge. Invest the time in building your skills and knowledge. This will help ensure you have the confidence and skills needed to succeed in timed competitions and real-world scenarios.

CTF Challenge Details

Secret Document Challenge Scenario

A mysterious file has been discovered on a company server. The file is encrypted, and the only way to decrypt it is by finding a secret key hidden somewhere in the company's network. Your task is to find the secret key, decrypt the file, and retrieve the hidden message (flag). Use the information and tools provided to complete the challenge.

Challenge Files with Content in Each

The challenge files are your tools to find the flag. Some you can choose to open in a text editor or in a command line to find clues, while others are scripts designed to decrypt hidden messages. Typically, in a CTF, you will not see the contents of challenge files upfront, the way they are displayed here. They would be presented as files in a single folder, and you’d need to figure out how they fit together. However, for this demonstration, the contents are revealed to help you understand how each file plays a role in discovering the flag.

1. encrypted_document.txt: This file is your ultimate goal. It holds the encrypted flag (an encrypted string of text that needs to be decrypted). You won’t be able to read its contents directly; you’ll need to decrypt it using the other files.
   

U2FsdGVkX1+T+F3oPpl6zO/G5mPvFQUkV3VJTfY=

2. Network_log.txt: This file is a log of network activity. Hidden within it is a critical clue that will lead to the secret key needed to decrypt encrypted_document.txt.
   

10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /index.html HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “POST /login HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /images/logo.png HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /about.html HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /secret_key.txt HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /contact.html HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “POST /submit-form HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /products.html HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /services.html HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /secret_page.html HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /images/banner.jpg HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “POST /upload HTTP/1.1” 200 –
10.0.0.1 – – [23/Jul/2023:09:00:00 –0700] “GET /downloads/file.zip HTTP/1.1” 200 –

3. decrypt.py: This is a Python script used to decrypt the content of encrypted_document.txt.To use it you will need to find the secret key.
   

from Crypto.Cipher import ATS
import base64
def decrypt(key, ciphertext):
	Cipher - AES.new(key.encode(‘utf-8’), AES.MODE_ECB)
	decoded= base64.b64decode(ciphertext)
	return cipher.decrypt(decoded).strip().decode(‘utf-8’)

	if_name_== “_main_”:
		key = input(“Enter the secret key: “)
		with open(“encrypted_document.text”,”r”) as file:
			encrypted_message = file.read()
		print(“decrypted message:”, decrypt(key, encrypted_message))

Hints

Hints are available in most CTFs and serve as clues to help solve a task without giving away the answer. Here are hints for this CTF.

• Hint 1: The secret key is hidden in the network log.
  
• Hint 2: Look for any unusual or out-of-place entries in the network log.
  
• Hint 3: The secret key is a string of 16 characters.
  

Capture the Flag Success Strategy

Successfully navigating CTF challenges requires a strategic approach. Follow these steps to maximize your chances of success:

1. Set up: Prepare a safe virtual environment with the necessary tools.
   
2. Prepare: Create a note-taking system to document your steps, organize your thoughts, and stay on track.
   
3. Analyze: Carefully examine the challenge details (for example, description, challenge files, hints).
   
4. Research: Seek clarification and guidance from various resources whenever needed.
   
5. Engage: Dive into the challenge, experiment, and try different approaches.
   
6. Reflect: Document your experience highlighting key takeaways and lessons learned.
   

This structured approach will help you to effectively navigate all types of CTF challenges and ultimately capture the flag. Now let's look at each step in more detail.

Step 1: Set Up

Before diving into a CTF challenge, create a safe virtual lab environment by installing a virtual machine (VM). Virtual machines are ideal for CTF challenges because they provide a secure and isolated environment for safely testing exploits and running suspicious scripts without affecting the host system. Kali Linux is often the preferred operating system due to the availability of tools for various challenge types.

For our Secret Document challenge, a virtual lab won’t be necessary since the information is being provided to you and we are simulating a challenge, but feel free to create a lab environment especially if you choose to follow along using an actual CTF challenge. Finally, having more than one monitor makes searching and navigation easier so consider using two or more monitors.

Step 2: Prepare

As previously mentioned, it’s important to take notes as you go along so before you begin the CTF, open a document or note-taking tool to organize and track your progress, questions, research, and solutions. Use this outline as a guide.

1. Title and date: Start each note with the challenge title and the date you began working on it. Since a CTF can take days or longer to complete, document the date each time you resume working on the challenge. This helps keep a clear record of your progress over time.
   
   1. Title: Insert the challenge title
      
   2. Date: Insert date you began working on the challenge
      
2. Challenge description: Copy the challenge description and any important details provided.
   
3. Files provided: List all the files and information provided as part of the challenge.
   
4. Objectives: List the main tasks you need to complete.
   
5. Questions and research: Note any questions that arise and summarize the research you do to answer them.
   
6. Hints and interpretations: Record any hints provided and your interpretation of them.
   
7. Progress tracker and steps taken: Use a checklist format to keep track of your steps and document insights with supporting screenshots.
   
8. Reflect on your experience: After completing the challenge, write a brief reflection on what you learned and any areas for improvement.
   

There are many other ways to organize your notes, so feel free to adapt this example to fit your needs and learning style. Additionally, you can view a walkthrough—a step-by-step guide that explains how to solve a specific CTF challenge–as another example of effective note taking.

Step 3: Analyze

Now you’re ready to analyze the information you’ve been provided. Read the challenge description carefully to gather as much information as possible. If there’s something in the description that you don't understand, take the time to research it. For example, if you’re unsure of what a network log is or what decrypt means, look it up. You can do online research to find resources to help you understand.

Next, break down the problem into smaller, more manageable parts. What’s the main goal? What information are you given? What actions could you potentially take to reach the goal?

Your understanding and research in this step leads you to one or more actions that you can take as a next step. For this challenge, the actions include:

1. Download the challenge files.
   
2. Analyze network_log.txt.
   
3. Use the decrypt.py script with the found key.
   

Step 4: Research

At various points, you will likely feel stuck and unsure of what to do next, so research is a critical skill you need throughout the challenge. While CTFs can be done alone, collaboration and teamwork are essential and can make those uncertain times less discouraging. A fresh perspective can often help break through a roadblock, so call on team members. Or if you are doing it alone, find a walkthrough, YouTube video, or other resource. Additionally, if you get stuck at any time in the challenge, most come with hints that you can use to propel you forward. You can also seek help from forums, Slack channels, or Discord servers dedicated to CTF challenges.

Do you see how these hints can lead you to one or more of the actions in Step 3? It’s also OK to walk away from the challenge for a while or move on to a different challenge if needed. Sometimes stepping away and returning later can provide the clarity and new insights needed to advance to the next step.

Step 5: Engage

Now it’s time to start trying things and engaging with the challenge. Let’s use steps 1 to 4 to find and retrieve the flag, remembering to follow the note taking outline throughout. Using the first hint, “The secret key is hidden in the network log,” you decide to open the network_log.txtfile.

1. Analyze the network log.
   
   • After uploading the network_log.txt file in, you open it using the cat command in a Linux terminal.
     
     • catnetwork-log.txt
   • The log reveals recorded network activity, so you begin scanning for clues such as unusual entries, patterns, or keywords like "secret”. In one log entry you notice a “GET” request to access a file named secret_key.txt at a specific time. This entry stands out from the others because it hints at the existence of a file containing the key you need to decrypt the encrypted_document.txt file.
     
     • 10.0.0.5 – – [23/Jul/2023:10:14:56 -0700] “GET /secret_key.txt HTTP/1.1” 200 -
   • You suspect this secret_key.txtfile holds the key needed for the next step.
     
2. Find the secret key.
   
   • You open the text file using the ‘cat’ command in a Linux terminal
     
   • catsecret_key.txt
   • Notice that the secret key is this string mysecretkey12345 and you look back at your notes to the hint that states, “The key is a string of 16 characters”.
     
   • Now that you’ve found the key, you have to figure out how to use it. The challenge includes a Python script that can decrypt a message with a secret key.
     
3. Decrypt the encrypted file.
   
   • You have another file, encrypted_document.txt, that needs to be decrypted.
     
   • To decrypt this file, you’re going to use the decrypt.py script and the secret key to unlock the content, so in your Linux terminal you run the script providing the secret key as input. The script will prompt you to enter the secret key.
     
     • python decrypt.py
     • Enter the secret key: mysecretkey12345
   • The python script uses the provided key to decrypt the content it reads from the encrypted_document.txt file.
     

If your actions are correct, the script will print the decrypted message (the flag) to your Linux terminal confirming that you've solved the challenge.

“FLAG{congratulations_you_found_it}”!

If your actions are incorrect, you will likely see an error message printed in the Linux terminal. The exact error will vary but you may see something like this: ValueError: Invalid decryption key or UnicodeDecodeError: ‘utf-8’ codec can’t decode byte. If this happens, carefully review your notes and steps, consider alternative approaches, and don't hesitate to consult hints or seek help.

Step 6: Reflect

Here’s an example of a reflection from someone who is describing their own experience with this CTF activity.

“Completing the Secret Document challenge was both rewarding and demanding. There were a few times when I felt quite frustrated, especially at the beginning. But pushing through those moments taught me the importance of sticking with it in cybersecurity.

This challenge was my introduction to forensic analysis. I now see the value that understanding and practicing data recovery and log analysis can bring to my current skill set. Additionally, I gained hands-on experience with using cryptographic techniques to find vulnerabilities and hidden information.

This process clarified the importance of being detail-oriented, recognizing patterns, being curious, and taking detailed notes. It was also a reminder that even when an organization has taken steps to destroy information, it can still be recovered; highlighting the critical importance of following media destruction guidance. Overall this challenge was a valuable learning experience.”

Conclusion

Each CTF challenge you complete will sharpen your cybersecurity skills and give you a better understanding of vulnerabilities, exploitation and effective risk management. So, keep exploring, keep learning, and most importantly, have fun!

In this unit, we examined the ethical considerations of CTF competitions and reviewed foundational knowledge crucial for successful participation. We established a structured process for solving CTF challenges and applied it to uncover a hidden flag within a forensics challenge.

CTFs offer invaluable learning experiences to technical and nontechnical cyber professionals. The knowledge and skills gained are essential for safeguarding systems, networks and for effectively mitigating risks.

By participating in and learning from inclusive CTF events and competitions, both technical and nontechnical professionals can contribute to building a more secure global digital landscape.

Resources

• External Site: Google: Capture the Flag Competition
• External Site: CTF Academy: Cybersecurity Ethics
• External Site: CTF101: Capture the Flag 101
• External Site: CSNP: Capture the Flag for Beginners
• External Site: Blackhat: Not just a game: Why capture the flags matter