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Re: Structured Fields: whitespace in binary content

From: Roy T. Fielding <fielding@gbiv.com>
Date: Wed, 28 Oct 2020 18:34:13 -0700
Cc: HTTP Working Group <ietf-http-wg@w3.org>, Poul-Henning Kamp <phk@varnish-cache.org>
Message-Id: <A76B81BE-8B09-4D2E-9502-9C6244D82501@gbiv.com>
To: Mark Nottingham <mnot@mnot.net>
> On Oct 28, 2020, at 1:23 AM, Mark Nottingham <mnot@mnot.net> wrote:
> 
> Structured Fields is in AUTH48 and we've addressed everything that's come up except for one very late entrant. I know this is very last minute, but I'm becoming convinced that this is something we should consider changing before shipping.
> 
> Background: I've written a script that validates HTTP messages in RFC XML, including Structured Fields. See:
>  https://pypi.org/project/rfc-http-validate/
> 
> Applying this to our current drafts, I encountered a problem; if a header field contains binary data, it's extremely likely that it will need to wrap across multiple lines to fit into the RFC. As a reminder, such folded lines are required by HTTP to be replaced by one or more spaces in <https://httpwg.org/http-core/draft-ietf-httpbis-semantics-latest.html#field-values>.
> 
> For example, here is the PR for the Signature draft:
>  https://github.com/httpwg/http-extensions/pull/1319
> 
> At first I thought this could be addressed by an editorial note explaining that whitespace folding is different in examples. However, things like this make that unworkable:
> 
> ~~~ http-message
> Signature-Input: sig1=(*request-target *created host date
>     cache-control x-empty-header x-example); keyid="test-key-a";
>     alg=hs2019; created=1402170695; expires=1402170995
> Signature: sig1=:K2qGT5srn2OGbOIDzQ6kYT+ruaycnDAAUpKv+ePFfD0RAxn/1BUe
>     Zx/Kdrq32DrfakQ6bPsvB9aqZqognNT6be4olHROIkeV879RrsrObury8L9SCEibe
>    oHyqU/yCjphSmEdd7WD+zrchK57quskKwRefy2iEC5S2uAH0EPyOZKWlvbKmKu5q4
>    CaB8X/I5/+HLZLGvDiezqi6/7p2Gngf5hwZ0lSdy39vyNMaaAT0tKo6nuVw0S1MVg
>    1Q7MpWYZs0soHjttq0uLIA3DIbQfLiIvK6/l0BdWTU7+2uQj7lBkQAsFZHoA96ZZg
>    FquQrXRlmYOh+Hx5D9fJkXcXe5tmAg==:
> ~~~
> 
> As you can see, whitespace in folding is semantically significant in Signature-Input (if it's lost, delimitation will be lost too), whereas it needs to be removed for Signature to contain valid binary content.
> 
> So, the obvious fix is to allow whitespace inside binary content. Delimitation won't be lost, because it's ":" on both ends. The base64 parsers I checked already swallow whitespace in input (not surprising since the motivating use case for base64 was line-wrapped MIME).

That seems unnecessary to me. Just make the example very long lines
and change the spec rendering tools to render a wrap that fits
within the current view. The tools can provide a notation of
their own to indicate that the folding isn't part of the value.

....Roy
Received on Thursday, 29 October 2020 01:34:32 UTC

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