Code to decrypt Rails 4.0+ cookies in python

rails_cookie_decryptor.py

import ujson
import urllib
import base64
from Crypto.Cipher import AES
from Crypto.Protocol.KDF import PBKDF2

unpad = lambda s : s[:-ord(s[len(s)-1:])]

class RailsCookieDecryptor(object):
  def __init__(self, secret_key_base, salt="encrypted cookie", keylen=64, iterations=1000):
    self.secret = PBKDF2(secret_key_base, salt, keylen, iterations)

  def get_cookie_data(self, cookie):
    cookie = base64.b64decode(urllib.unquote(cookie).split('--')[0])
    encrypted_data, iv = map(base64.b64decode, cookie.split('--'))
    cipher = AES.new(self.secret[:32], AES.MODE_CBC, iv)
    plaintext = unpad(cipher.decrypt(encrypted_data))
    return ujson.loads(plaintext)