When you create a "paste," the server encrypts the title and content using AES-128 in Cipher Block Chaining (CBC) mode.

Upon entering the challenge, the application claims to use "military-grade 128-bit AES encryption" and asserts that keys are never stored in the database.

This article breaks down the vulnerabilities and step-by-step methods used to capture all four flags in the Encrypted Pastebin challenge. 1. Understanding the Environment

The is one of the most technical "Hard" level challenges in the Hacker101 CTF . Unlike standard web challenges that focus on common bugs like XSS or SQL Injection, this level centers on advanced cryptographic vulnerabilities , specifically targeting the AES-128 CBC mode .

The first flag is often a lesson in paying attention to server responses. By intentionally corrupting the post parameter—such as deleting or modifying a single character—the application may fail to decrypt or unpad the data. Improper error handling.

This flag requires a deep dive into how CBC mode works. Since the server confirms whether padding is valid or invalid, it functions as a "Padding Oracle".

The resulting encrypted string is passed as a post parameter in the URL.

Before decoding, the application replaces standard Base64 characters: ~ for = , ! for / , and - for + . 2. Flag 0: Information Leakage via Error Messages