how to base64 url decode 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

Check out the base64 module.

Answer 2

Apparently you missed the last two characters when copying the original base64-encoded string. Suffix the input string with two is-equal (=) signs and it will be decoded correctly.

Answer 3

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