Busting the padding oracle with PadBuster

Secure cryptosystems shouldn’t reveal any plaintext-relevant information about encrypted messages. Oracle attacks are powerful demonstrations of how you don’t need much seemingly meaningless information to end up with a full decrypted message. Our CryptOMG web app provides a challenge that can be defeated by exploiting a padding oracle: a system that gives us information about the validity of padding in a decryption process without revealing the key or message. Let’s start some conversations with our oracle and see what these responses look like.

Interrogating the padding oracle

Let’s load up the CryptOMG main page and select the first challenge (like last time, we’re out to get /etc/passwd). On the test page, there’s nothing of interest in the actual content of the page, so let’s examine the URL: http://127.0.0.1/ctf/challenge1/index.php?cipher=3&encoding=2&c=81c14e504d73a84cc...