[erlang-questions] base64:decode fails on facebook base64 encoded strings

[Max Bourinov]

That is great! Thank you!

Sent from my iPhone

On 29.12.2011, at 17:55, Michael Santos <michael.santos@REDACTED> wrote:

> On Thu, Dec 29, 2011 at 08:30:09AM +0400, Max Bourinov wrote:
>> So Erlangers,
>>
>> I wrote about this little bug in Facebook and I got an answer!
>>
>>>> I believe the trailing = is an optional padding in base64 your code needs
>> to accept base64 with and without trailing '=' characters. See last part of
>> http://email.about.com/cs/standards/a/base64_encoding.htm
>>
>> This is what they said. That is very interesting, I personally would not
>> omit data from protocol specification, but it appears that if it possible.
>> Anyway I didn't see anything about omitting here
>> http://tools.ietf.org/html/rfc1421 or http://tools.ietf.org/html/rfc2045
>
> There is something about dropping the padding in RFC 4648:
>
> 5.  Base 64 Encoding with URL and Filename Safe Alphabet
>
> The pad character "=" is typically percent-encoded when used in an URI
> [9], but if the data length is known implicitly, this can be avoided by
> skipping the padding; see section 3.2.
>
>> So, if you ever face such a loose implementation of base64 here is a
>> snippet that might help you to deal with it:
>>
>> -spec fb_decode_base64(Base64 :: list()) -> binary() | error.
>> fb_decode_base64(Base64) when is_list(Base64) ->
>>    try base64:decode(Base64)
>>    catch
>>        error:_ -> % could be missing =
>>            try base64:decode(Base64 ++ "=")
>>            catch
>>                error:_ -> % could be missing ==
>>                    try base64:decode(Base64 ++ "==")
>>                    catch
>>                        error:_ -> % base64 is really wrong. we cannot fix it
>>                            error
>>                    end
>>            end
>>
>>    end.
>
> The length can be used to figure out the padding (not exactly the same
> as your code, throws an exception on bad data rather than returning
> an error):
>
> -module(tt).
> -export([fb_decode_base64/1]).
>
> fb_decode_base64(Base64) when is_list(Base64) ->
>    decode(list_to_binary(Base64)).
>
> decode(Base64) when byte_size(Base64) rem 4 == 3 ->
>    base64:decode(<<Base64/bytes, "=">>);
> decode(Base64) when byte_size(Base64) rem 4 == 2 ->
>    base64:decode(<<Base64/bytes, "==">>);
> decode(Base64) ->
>    base64:decode(Base64).