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

[Christopher Vance]

Trailing '=' if needed to bring the payload to a multiple of 4
characters are not optional. You could probably add extra '=' after
that if you like.

On 29 December 2011 16:29, Steve Davis <steven.charles.davis@REDACTED> wrote:
> These two refs are interesting in this context:
>
> http://en.wikipedia.org/wiki/Base64#Variants_summary_table
> http://tools.ietf.org/html/rfc4648#section-5
>
> /s
>
> On Dec 28, 10:30 pm, Max Bourinov <bouri...@REDACTED> 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 ofhttp://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 herehttp://tools.ietf.org/html/rfc1421orhttp://tools.ietf.org/html/rfc2045
>>
>> 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.
>>
>> On Wed, Dec 21, 2011 at 7:54 PM, Heinz N. Gies <he...@REDACTED> wrote:
>>
>>
>>
>>
>>
>>
>>
>> > As an addition look closely at the decoding on the webside, it also fails
>> > the last two characters are missing unless you add the missing '=' to the
>> > base64 encoded data.
>>
>> > Regards,
>> > Heinz
>>
>> > --
>> > Heinz N. Gies
>> > he...@REDACTED
>> >http://licenser.net
>>
>> > On Dec 21, 2011, at 16:44, Max Bourinov wrote:
>>
>> > Hi guys,
>>
>> > Does anybody knows why base64:decode fails on strings like this:
>>
>> > eyJhbGdvcml0aG0iOiJITUFDLVNIQTI1NiIsImNyZWRpdHMiOnsib3JkZXJfZGV0YWlscyI6Int cIm9yZGVyX2lkXCI6Mjg1NTAwMDgxNTAxMTU4LFwiYnV5ZXJcIjoxMDI3NDE5NzkzLFwiYXBwXC I6MTY4MzExOTc2NTgwNDUwLFwicmVjZWl2ZXJcIjoxMDI3NDE5NzkzLFwiYW1vdW50XCI6MSxcI nVwZGF0ZV90aW1lXCI6MTMyNDQ3OTA3NixcInRpbWVfcGxhY2VkXCI6MTMyNDQ3OTA3NCxcImRh dGFcIjpcIlwiLFwiaXRlbXNcIjpbe1wiaXRlbV9pZFwiOlwiMFwiLFwidGl0bGVcIjpcIkEgRmF jZWJvb2sgSGF0XCIsXCJkZXNjcmlwdGlvblwiOlwiVGhlIGNvb2xlc3QgaGF0IHlvdSd2ZSBldm VyIHNlZW4uXCIsXCJpbWFnZV91cmxcIjpcImh0dHA6XFxcL1xcXC93d3cuZmFjZWJvb2suY29tX FxcL2ltYWdlc1xcXC9naWZ0c1xcXC83NDAucG5nXCIsXCJwcm9kdWN0X3VybFwiOlwiaHR0cDpc XFwvXFxcL3d3dy5mYWNlYm9vay5jb21cXFwvaW1hZ2VzXFxcL2dpZnRzXFxcLzc0MC5wbmdcIix cInByaWNlXCI6MSxcImRhdGFcIjpcIlwifV0sXCJzdGF0dXNcIjpcInBsYWNlZFwifSIsInN0YX R1cyI6InBsYWNlZCIsIm9yZGVyX2lkIjoyODU1MDAwODE1MDExNTgsInRlc3RfbW9kZSI6MX0sI mV4cGlyZXMiOjEzMjQ0ODMyMDAsImlzc3VlZF9hdCI6MTMyNDQ3OTA3Niwib2F1dGhfdG9rZW4i OiJBQUFDWkFGQ1pCVFZXSUJBQzBhYkhWb3dLRWdTVHVOQ3ZDYjRTR21hWkE4STdYc3Z0bXBZZU1 hTWx3clNia0U5dkxKTHRldUdraWpNakh4aVpBVWY5am9RcG9aQ3ZWYW9YVUN6N21zU3Bjb0FuWG hKRDVRU0lRIiwidXNlciI6eyJjb3VudHJ5IjoicnUiLCJsb2NhbGUiOiJlbl9VUyIsImFnZSI6e yJtaW4iOjIxfX0sInVzZXJfaWQiOiIxMDI3NDE5NzkzIn0
>>
>> > You can easily decode it with online tool like this:
>> >http://www.motobit.com/util/base64-decoder-encoder.asp
>>
>> > But lovely Erlang says:
>>
>> > ** exception error: no function clause matching base64:decode("Jye",
>>
>> > [133,177,157,189,201,165,209,161,180,136,232,137,33,53,5,12,
>>
>> >  181,77,33,4,200,212,216,136,176,137,141|...])
>> >      in function  base64:decode/1
>>
>> > Seems something wrong with pattern matching or what could it be? And most
>> > interesting question - how to find a proper work around?
>>
>> > Best regards,
>> > Max
>>
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-- 
Christopher Vance