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

Surprising, but currently accepted answer is not exactly correct. Like some other answers stated, it's something called base64url encoding, and it's a part of RFC7515.

Basically, they replaced '+' and '/' chars by '-' and '_' respectively; and additionally removed any trailing '=' chars, because you can always tell how many chars you're missing, just by looking at the encoded string length.

Here's illustrative example from RFC7515 in C#:

 static string base64urlencode(byte [] arg)
 {
   string s = Convert.ToBase64String(arg); // Regular base64 encoder
   s = s.Split('=')[0]; // Remove any trailing '='s
   s = s.Replace('+', '-'); // 62nd char of encoding
   s = s.Replace('/', '_'); // 63rd char of encoding
   return s;
 }

 static byte [] base64urldecode(string arg)
 {
   string s = arg;
   s = s.Replace('-', '+'); // 62nd char of encoding
   s = s.Replace('_', '/'); // 63rd char of encoding
   switch (s.Length % 4) // Pad with trailing '='s
   {
     case 0: break; // No pad chars in this case
     case 2: s += "=="; break; // Two pad chars
     case 3: s += "="; break; // One pad char
     default: throw new System.Exception(
       "Illegal base64url string!");
   }
   return Convert.FromBase64String(s); // Standard base64 decoder
 }