[erlang-patches] Improve pad character handling in base64 MIME decoding functions

Niclas Axelsson <>
Wed Dec 8 10:06:44 CET 2010

On 12/07/2010 03:40 AM, Thomas O'Dowd wrote:
> Hi all,
> This is my first patch to OTP. Hopefully, I've ticked all the boxes.
> Here is the git fetch request:
>    git fetch git://github.com/tpodowd/otp.git base64_mime_decoding
> This patch fixes some of the problems we've run into when decoding
> base64 encoded text generated by various email clients. Most of the
> issues are related to to many padding characters at the end of the text.
> A typical example is "abcd====" where the trailing padding is all
> extraneous.
> After reading RFC4648, I decided that we should implement the MAY
> clauses for MIME base64 decoding.
> This patch breaks one existing unit test.
> - %% One pad, followed by ignored text
> -<<"Hello World">>  = base64:mime_decode(<<"SGVsb)(G8gV29ybGQ=apa">>),
> + %% One pad to ignore, followed by more text
> +<<"Hello World!!">>  = base64:mime_decode(<<"SGVsb)(G8gV29ybGQ=h IQ= =">>),
> In the old test the trailing "apa" text is ignored as it follows a pad
> character. RFC4648 states that we MAY ignore pad characters that are followed
> by further text by treating them the same as illegal characters. As such, I
> have left the pad as-is and followed it up with the same number of valid
> base64 characters but which decode to something a bit more friendlier and
> added the proper "==" padding to correctly terminate the encoded stream.
> Given the new test, the old implementation would still return<<"Hello World">>
> without the trailing "!!".
> I took the liberty of adding more tests and also added tests for the two
> different implementations for mime_decode/1 and mime_decode_to_string/1. All
> base64 tests pass currently.
> Best regards,
> Tom.
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Thank you, Tom. Will include your patch into 'pu'. But since it breaks the
behaviour (and might not be backwards compatible), I can not guarantee 
it will
be merged into 'dev'.

Niclas Axelsson, Erlang/OTP

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