Mastering Pseudorandom Number Generators for Cybersecurity

Mastering Pseudorandom Number Generators for Cybersecurity

Table of Contents:

1. Introduction
2. Background of the Challenge
3. Understanding the Source Code
4. Reverse Engineering the Program
5. Working Backwards to Retrieve the Original Flag
6. Conclusion

Article:

Introduction

In this article, we will be exploring a cryptographic challenge from the Picot CTF 2017. The challenge is called "So Random" and it involves reversing a program to retrieve the original flag. We will dive into the source code, analyze its workings, and work backwards to uncover the original flag. So let's get started!

Background of the Challenge

The challenge presents us with a running instance of a program called "So Random" on a specific port. The program seems to output a flag, but it randomizes all the characters. Our task is to find a way to retrieve the original flag. The program is written in Python 2, so we have the source code available to analyze.

Understanding the Source Code

To begin our investigation, we download the source code of the program and open it in a text editor. The source code reveals that the program utilizes the random and string modules to create the flag originally. However, as the original flag is not provided, we only know that it should start with the word "flag". The source code also showcases the program's usage of the random number generator, which is seeded with a constant value.

Reverse Engineering the Program

Since we have the source code and the random number generator is seeded with a known constant value, we can reverse engineer the program to retrieve the original flag. The program loops through each character of the original flag and rotates them based on certain conditions. By reversing these operations and subtracting the random number instead of adding it, we can obtain the original flag.

Working Backwards to Retrieve the Original Flag

With the modified code, we execute the program and witness the retrieval of the original flag as expected. It is important to note that since the random number generator is seeded with a constant value, the sequence of random numbers will remain the same, allowing us to obtain the exact original flag.

Conclusion

In this challenge, we successfully reversed a program called "So Random" by analyzing its source code and working backwards to retrieve the original flag. By understanding the workings of the random number generator and exploiting the constant seed value, we were able to bypass the randomization of characters and obtain the flag. This exercise highlights the importance of seed values in pseudo-random number generation and the vulnerabilities associated with constant seeds.

Highlights:

• Reversing a cryptographic challenge from the Picot CTF 2017
• Analyzing the source code of the "So Random" program
• Exploiting the constant seed value in the random number generator
• Working backwards to retrieve the original flag

FAQ Q&A:

Q: What is the Picot CTF 2017? A: The Picot CTF 2017 is a Capture The Flag (CTF) competition that features various challenges in different areas of cybersecurity, including cryptography.

Q: Why is the random number generator seeded with a constant value? A: The constant seed value in the random number generator allows for predictable results, making it easier to reverse engineer and retrieve the original flag in this challenge.

Q: Can this technique be used for other similar challenges? A: Yes, understanding how random number generators work and leveraging the seed value can be helpful in reversing other cryptographic challenges that involve pseudo-randomness.

Q: How can I use this knowledge in real-world scenarios? A: This knowledge can be useful in understanding the vulnerabilities and weaknesses of random number generation in cryptographic systems, potentially leading to improved security measures.