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Define forgiving-base64 #145

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Define forgiving-base64 #145

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Define forgiving-base64
annevk Aug 15, 2017
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annevk Aug 15, 2017
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reuse table from the RFC, nits
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Expand Up @@ -872,6 +872,83 @@ For <a>pairs</a>, a slightly shorter literal syntax can be used, separating the
as 200/`<code>OK</code>`.


<h2 id=forgiving-base64>Forgiving base64</h2>

<p>To <dfn export>forgiving-base64 encode</dfn> given a <a>byte sequence</a> <var>data</var>, apply
the base64 algorithm defined in section 4 of RFC 4648 to <var>data</var> and return the result.
[[!RFC4648]]

<p class="note no-backref">This is named <a>forgiving-base64 encode</a> for symmetry with
<a>forgiving-base64 decode</a>, which is different from the RFC as it defines error handling for
certain inputs.

<p>To <dfn export>forgiving-base64 decode</dfn> given a string <var>data</var>, run these steps:</p>
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I think a note explaining that RFC 4648 actually doesn't contain a decode algorithm would be useful. (Certainly not one with error handling.) Cf. HTML's

  <!-- Note: this function is defined explicitly here because RFC4648 does not specify how to handle
  erroneous input, and no preexisting browser implementation simply throws an exception on all
  erroneous input. -->

but I think having it as an actual note would be nice.

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Isn't that what the note directly above it does?

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I guess so, although by my reading of the RFC, it doesn't actually define any decode algorithm at all.

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It defines an Encoding scheme and some rules around it. The idea is that you infer the decode and encode algorithms from that. Similar to using ABNF and expecting you have a parser that works.

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I guess so. I'm OK with it as-is but I think it'd be nicer for our readers if you frame this as providing the missing decode algorithm. (There is a fairly explicit encode algorithm, in contrast.)


<ol>
<li><p>Remove all <a>ASCII whitespace</a> from <var>data</var>.
<!-- https://lists.w3.org/Archives/Public/public-whatwg-archive/2011May/0207.html -->

<li>
<p>If <var>data</var> contains a <a>code point</a> that is not one of

<ul class="brief">
<li>U+002B (+)
<li>U+002F (/)
<li><a>ASCII alphanumeric</a>
</ul>

<p>then return failure.

<li>
<p>If <var>data</var>'s <a for=string>length</a> divides by 4 leaving no remainder, then:

<ol>
<li><p>If <var>data</var> ends with one or two U+003D (=) <a>code points</a>, then remove them
from <var>data</var>.
</ol>

<li><p>If <var>data</var>'s <a for=string>length</a> divides by 4 leaving a remainder of 1, then
return failure.

<li><p>Let <var>output</var> be an empty <a>byte sequence</a>.

<li><p>Let <var>buffer</var> be an empty buffer that can have bits appended to it.
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Should we just make buffer a list? Then we can use append, and size (instead of "has accumulated"), and empty, and is empty.

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But then the interpretation business gets a lot trickier. I'd rather leave this alone.

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Eh, interpreting 24 bits as three 8-bit big-endian numbers seems to work fine whether those bits are in a buffer or in a list...


<li><p>Let <var>position</var> be a <a>position variable</a> for <var>data</var>, initially
pointing at the start of <var>data</var>.

<li>
<p>While <var>position</var> does not point past the end of <var>data</var>:

<ol>
<li><p>Find the <a>code point</a> pointed to by <var>position</var> in the second column of
Table 1: The Base 64 Alphabet of RFC 4648. Let <var>n</var> be the number given in the first cell
of the same row. [[!RFC4648]]

<li><p>Append the six bits corresponding to <var>number</var>, most significant bit first, to
<var>buffer</var>.

<li><p>If <var>buffer</var> has accumulated 24 bits, interpret them as three 8-bit big-endian
numbers. Append three bytes with values equal to those numbers to <var>output</var>, in the same
order, and then empty <var>buffer</var>.

<li><p>Advance <var>position</var> by 1.
</ol>

<li>
<p>If <var>buffer</var> is not empty, it contains either 12 or 18 bits. If it contains 12 bits,
then discard the last four and interpret the remaining eight as an 8-bit big-endian number. If it
contains 18 bits, then discard the last two and interpret the remaining 16 as two 8-bit big-endian
numbers. Append the one or two bytes with values equal to those one or two numbers to
<var>output</var>, in the same order.</p>

<p class="note">The discarded bits mean that, for instance, "<code>YQ</code>" and
"<code>YR</code>" both return `<code>a</code>`.

<li><p>Return <var>output</var>.
</ol>


<h2 id=namespaces>Namespaces</h2>

<p>The <dfn export>HTML namespace</dfn> is "<code>http://www.w3.org/1999/xhtml</code>".
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