Decrypter la sign request de facebook

 # separate the signature from the data
try:
  l = signed_request.split('.', 2)
  encoded_sig = str(l[0])
  payload = str(l[1])
except IndexError:
  raise ParsingError("'signed_request' malformed")


# pad them with "=" to make them valid base64 data if they are too short

b64_padding = lambda x: (x + "=" * ((4 - len(x) % 4) % 4))
# decode them from base 64 to a string
# TODO: check encoding


sig = base64.urlsafe_b64decode(b64_padding(encoded_sig))
data = base64.urlsafe_b64decode(b64_padding(payload))



# decode the JSON string into a dictionary
try:
    data = json.loads(data)
except ValueError:
    ParsingError("JSON malformed")
    

# check if the data is from facebook
if data['algorithm'].upper() != 'HMAC-SHA256':
  raise ParsingError("'signed_request' is using an unknown algorithm")
else:
  expected_sig = hmac.new(secret_key, msg=payload, 
                          digestmod=hashlib.sha256).digest()
if sig != expected_sig:
  raise ParsingError("'signed_request' signature mismatch")


return data