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Category: Crypto Difficulty: Medium/Hard 1. Challenge Overview The challenge provides a network service that accepts hex-encoded ciphertexts. Upon connection, the server presents a large RSA modulus $N$ and an initial encrypted blob. This blob follows a specific structure: The server’s behavior is simple: it tries to decrypt the RSA key, uses it to decrypt the…

Category: Cryptography Difficulty: Medium 1. Challenge Overview The challenge provides a Python script and a remote service. Upon connecting, the server encrypts a hidden flag using a custom scheme and then acts as an oracle, allowing us to encrypt any hex-encoded message of our choice. The core of the encryption lies in a linear transformation…

Category: Crypto / Reverse Difficulty: Easy/Medium 1. Challenge Overview Upon connecting to the server, I was greeted with two integers. Looking at the provided source code, chall.py, I saw that the program takes a secret flag, converts it into a large integer, and then passes it through a function called reduce(a, f) using a random…

Category: Misc (DNS / Recon) Difficulty: Medium 1. Challenge Overview The challenge provided a DNS server at 52.59.124.14:5054 for the domain flag.ctf.nullcon.net. The author hinted that the flag was hidden and challenged me to “show that I know all about DNS.” 2. Vulnerability Analysis Step 1: Initial Reconnaissance I started by checking for the flag…

Category: Web / Misc Difficulty: Easy/Medium 1. Challenge Overview The challenge presented a “BillSplitter Lite” web application. The application allows users to input names and expense amounts, which it then calculates to settle debts. The prompt mentioned an “administrative fee” of 0.01 that was somehow hidden in the “extremely advanced math” of the calculation. 2.…

Category: Misc Difficulty: Easy 1. Challenge Overview The challenge provides a single README.md file containing what appears to be a solitary emoji: 💯. However, when looking at the file metadata or simply clicking and dragging over the text, it becomes clear that there is a significant amount of “invisible” data trailing the emoji. We are…

Category: DNS / Network Security Difficulty: Medium 1. Challenge Overview The challenge provided a DNS server at 52.59.124.14:5053. The hint stated that “latest and greatest DNS features” now allow internal networks to resolve names without VPNs or Firewalls. This was a clear pointer toward EDNS Client Subnet (ECS) or DNS over QUIC (DoQ). 2. Vulnerability…

Category: DNS / Misc Difficulty: Medium 1. Challenge Overview The challenge provided a custom DNS server (52.59.124.14:5052) with the hint: “Some flag escaped its enclosure. Now it is mixed up with the herd(dinos.nullcon.net).” The goal was to locate the flag hidden within a large number of DNS records. 2. Vulnerability Analysis Step 1: Initial Connectivity…

Category: Web Difficulty: Easy 1. Challenge Overview The challenge presents a web application designed to convert WordPress XML export files into static websites. The interface is simple: The goal is to read the /flag.txt file stored on the server. 2. Vulnerability Analysis My first step was to explore how the “Generate” feature works. I intercepted…

Category: Web Difficulty: Medium 1. Challenge Overview The challenge presents a web service called Virus Analyzer. The UI is sleek and professional, mimicking a security tool. It invites users to upload a .zip archive, which it promises to extract and analyze for malicious content. Upon visiting the site, I noticed there was no source code…