How to decode base64 url in python?

Question

For Facebook fbml Apps Facebook is sending in a signed_request parameter explained here:

http://developers.facebook.com/docs/authentication/canvas

They have given the php version of decoding this signed request:

http://pastie.org/1054154

How to do the same in python?

I tried base64 module but I am getting Incorrect padding error:

>>> base64.urlsafe_b64decode("eyJhbGdvcml0aG0iOiJITUFDLVNIQTI1NiIsImV4cGlyZXMiOjEyNzk3NDYwMDAsIm9hdXRoX3Rva2VuIjoiMjk1NjY2Njk1MDY0fDIuRXpwem5IRVhZWkJVZmhGQ2l4ZzYzUV9fLjM2MDAuMTI3OTc0NjAwMC0xMDAwMDA0ODMyNzI5MjN8LXJ6U1pnRVBJTktaYnJnX1VNUUNhRzlNdEY4LiIsInVzZXJfaWQiOiIxMDAwMDA0ODMyNzI5MjMifQ") Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/base64.py", line 112, in urlsafe_b64decode     return b64decode(s, '-_')   File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/base64.py", line 76, in b64decode     raise TypeError(msg) TypeError: Incorrect padding 

Answer 1

try

s = 'iEPX-SQWIR3p67lj_0zigSWTKHg' base64.urlsafe_b64decode(s + '=' * (4 - len(s) % 4)) 

as it is written here

Answer 2

I have shared a code snippet for parsing signed_request parameter in a python based facebook canvas application at http://sunilarora.org/parsing-signedrequest-parameter-in-python-bas:

import base64 import hashlib import hmac import simplejson as json  def base64_url_decode(inp):     padding_factor = (4 - len(inp) % 4) % 4     inp += "="*padding_factor      return base64.b64decode(unicode(inp).translate(dict(zip(map(ord, u'-_'), u'+/'))))  def parse_signed_request(signed_request, secret):      l = signed_request.split('.', 2)     encoded_sig = l[0]     payload = l[1]      sig = base64_url_decode(encoded_sig)     data = json.loads(base64_url_decode(payload))      if data.get('algorithm').upper() != 'HMAC-SHA256':         log.error('Unknown algorithm')         return None     else:         expected_sig = hmac.new(secret, msg=payload, digestmod=hashlib.sha256).digest()      if sig != expected_sig:         return None     else:         log.debug('valid signed request received..') return data