Encrypted Pastebin Repack - Hacker101

In the walkthrough published in the repository, the authors note that calling the endpoint with /?post= (no content) generates an error response containing a flag.

This binary feedback (valid vs. invalid) allows an attacker to brute-force the intermediate state of the decryption process. 2. Understand CBC Decryption

In the Hacker101 CTF (Capture the Flag), there is a common challenge called "Pastebin Clone." The vulnerability is often that the developer tried to implement encryption but did it server-side.

: You need to craft a valid encrypted string that decrypts to a different command or ID (e.g., changing "id": "123" to "id": "1" ). hacker101 encrypted pastebin

[Hacker101 CTF] Encrypted Pastebin – [Vulnerability Type]

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

: Use the -plaintext flag in PadBuster to "encrypt" a custom string of your choice. Use code with caution. Copied to clipboard In the walkthrough published in the repository, the

The presence of a distinct "invalid padding" response confirms the server is acting as a . 2. Analyze the Cipher

If the padding of a decrypted block is incorrect, the server often throws a specific error (e.g., "Padding Error" or a generic 500 status).

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. focusing on cryptography flaws

Symmetric encryption algorithms process data in fixed-size blocks (usually 16 bytes). Because data is rarely exactly 16 bytes, the application must use a mode of operation:

(If you want, I can adapt this post for Twitter/X, LinkedIn, or a Hacker News-style submission.)

The process involves iteratively modifying ciphertext blocks and observing the server's response. For each byte position, the attacker sends crafted ciphertext to the server and, based on whether a padding error occurs, deduces the corresponding plaintext byte.

One of the most direct ways is to send a GET request with an empty post parameter: https://[challenge-url].ctf.hacker101.com/?post= . The server's error page will contain ^FLAG^...$FLAG$ , giving you your first easy capture. This flag is a lesson in information disclosure: always test how an application handles unexpected or malformed input.

This challenge is a fantastic exercise for beginners and intermediate security enthusiasts, focusing on cryptography flaws, cookie manipulation, and understanding how data is encrypted and decrypted in web applications. What is the Hacker101 Encrypted Pastebin?