bug#10536: 23.3; Make base64-decode more fault tolerant

[Noam Postavsky]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Wolfram Gloger <wmglo@dent.med.uni-muenchen.de> writes:
>
>> -      READ_QUADRUPLET_BYTE (-1);
>> +      /* Be tolerant against missing final padding '='.  */
>> +      READ_QUADRUPLET_BYTE (e-to);
>
> We discussed this a bit last year, but didn't come to any conclusion,
> but I think on the whole, having base64-decode be strict or non-strict
> here doesn't make much practical difference (because it's very uncommon
> to strip the trailing = padding characters), but theoretical paranoia
> about this somehow leading to subsequent errors...  somehow...  may be
> warranted.

We did meanwhile get a base64url-* which has an optional NOPAD argument,
and base64-decode-* functions got an optional BASE64URL argument which
allows decoding unpadded base64, by the way.