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

Answer 1

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 2

try

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

as it is written here

Answer 3

Alternative to @dae.eklen's solution, you can append === to it:

s = 'iEPX-SQWIR3p67lj_0zigSWTKHg'
base64.urlsafe_b64decode(s + '===')

This works because Python only complains about missing padding, but not extra padding.

Answer 4

If you are sending your base64 string from .net as a param it seems that chars that have special meaning in the URI ie + or / are replaced with " " spaces.

So before you send your string in .net you should probably do something like this

base64img.Replace("+", "-").Replace("/", "_"))

Then in python decode the string (also add '=' until the length is divisible by 4)

def decode_base64(data):
    data += '=' * (len(data) % 4)
    return base64.urlsafe_b64decode(data)

Further if you want to use the image in openCV

def get_cv2_img_from_base64(base_64_string):
    data = decode_base64(base_64_string)
    np_data = np.frombuffer(data, dtype=np.uint8)
    return cv2.imdecode(np_data, cv2.IMREAD_UNCHANGED)

Answer 5

just

base64.urlsafe_b64decode(s)

Answer 6

import base64
import simplejson as json

def parse_signed_request( signed_request ):
    encoded_sig, payload = signed_request.split('.',2)
    data = json.loads(base64.b64decode( payload.replace('-_', '+/') ))
    return data