If you are currently trying to solve it, here are the key concepts you'll need to master: Automation is Key
Once PadBuster or a custom script is running, it will start determining the plaintext of the encrypted paste. This will reveal the structure of the data the server is decrypting, leading to the first flag. 4. Bypassing Further Security (Flag 1 and Flag 2)
A "padding oracle" is a server behavior that leaks information about whether the padding of a decrypted ciphertext block is correct.
The Base64 encrypted string taken from the URL/cookie. BlockSize: 16 (for AES). hacker101 encrypted pastebin
Decrypting the entire post ciphertext yields a JSON payload. Hidden in this JSON is the second flag. Decrypting this data yourself, or using a ready-made script, is a rite of passage for this challenge. As the application boasts it doesn't store the key, the flag is hidden in the encrypted data itself.
The challenge in Hacker101 CTF is a classic exercise in identifying and exploiting a Padding Oracle Attack . In this scenario, the application uses Cipher Block Chaining (CBC) mode for encryption but leaks information through its error responses, allowing an attacker to decrypt data without the key. Technical Overview
To understand this vulnerability, we must look at how block ciphers handle data: If you are currently trying to solve it,
The Hacker101 Encrypted Pastebin challenge is an excellent lesson in modern cryptography, showing how subtle design choices (like error messages) can lead to complete security breakdowns. By understanding the padding oracle attack, we can effectively turn the application's decryption mechanism against itself.
Before decrypting anything, the server must verify the HMAC. If the ciphertext has been modified, the HMAC check fails, and the server rejects the request immediately. 3. Sanitize Server Errors
While you can write a custom Python script using requests , standard penetration testing tools like or custom automated scripts are highly effective for this specific challenge. To map the vulnerability out with a script, you need: The target URL. The sample encrypted parameter string. Bypassing Further Security (Flag 1 and Flag 2)
const encryptedText, keyHash = pastes[id]; res.send( encryptedText, keyHash ); );
This challenge is a favorite among learners because it moves beyond basic web vulnerabilities (like simple XSS) into the world of cryptographic attacks Padding Oracle Attack