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Re: [Integrity] Revisit RFC6920 URIs for benefit of servers and Semantic Web applications

From: Devdatta Akhawe <dev.akhawe@gmail.com>
Date: Thu, 12 Mar 2015 17:48:34 -0700
Message-ID: <CAPfop_0YJtpjuAe7fJ9UuBJ0NGbhHt7eCSzZtqhxDwRTrvEVxg@mail.gmail.com>
To: Austin William Wright <aaa@bzfx.net>
Cc: Semantic Web <semantic-web@w3.org>, "public-webappsec@w3.org" <public-webappsec@w3.org>
I don't have a problem with that; I actually like the equals and I think it
makes more sense if we want to support more options.

That said, I am not the one who implemented SRI in the browser. I don't
know what everyone else thinks. Can you submit a pull request to
https://github.com/w3c/webappsec ?


(and yes .. I meant hash-source)

On 12 March 2015 at 15:01, Austin William Wright <aaa@bzfx.net> wrote:

> Alright, thanks.
> I understand the CSP usage is already in the wild, otherwise I would make
> the same submission to CSP. Perhaps future editions can add an option to
> refer to the IANA registry (or even use ni: outright)?
> It's difficult to prove that forward compatibility would be "impossible".
> Looking forward I imagine older user-agents might try to parse a URI as
> an `option-expression`, and so I'd prefer the alternative that looks less
> like a URI, other things being equal. Additionally, use of the equals sign
> has precedent in other contexts, such as the `parameter` production for
> media types (RFC2045), the HSTS `directive` production (RFC6797), the
> HTTP Link header (RFC5988), and more.
> As a maintainer of libraries that parse many of these productions, and
> author of Web robots (that will likely, in the distant future, have to grok
> CSP and SRI), I would find re-using an existing production less complex
> than implementing a new one.
> Since I don't find any option-expression production in CSP, would the WG
> be amenable to using the equals sign character in place of colon, as
> follows, or an existing similar production?
> ; proposed changes
> option-expression  = option-name "=" option-value
> option-name        = 1*option-name-char
> option-value       = *option-value-char
> option-name-char   = ALPHA / DIGIT / "-"
> option-value-char  = ALPHA / DIGIT / "-" / "+" / "." / "/"
> (I noticed the current ABNF only allows a single character, here I've
> allowed multiple characters. As was probably intended before, this
> modification leaves open the possibility of unambigiously expanding the
> range of option-value later to allow a quoted-string or urlencoded value,
> or both.)
> With regards to re-using productions from CSP, I still see a significant
> distinction between CSP and SRI: The former is making a series of
> assertions about an HTTP request, the latter is making an assertion about a
> link relation, and has implications beyond HTTP. If the WG wants to define
> a subset of behavior as it applies to just HTTP, that's good, but I still
> would like to be wary of future re-use and generalization. I don't believe
> importing the CSP syntax satisfies this. Compared to the benefit of being
> able to re-use quoted-string/urlencoded/etc, over defining a new one (as
> the Editor's Draft currently does), I believe the introduced complexity of
> having a different style of token, or even using ni: outright, is minimal.
> Finally, do you refer to the `hash-source` production [1], but without
> the single quotes? I can't find a literal `hash-src` production like you
> refer to, idk if it might be unreleased somewhere, or something like that.
> Cheers,
> Austin.
> [1] <https://w3c.github.io/webappsec/specs/CSP2/#hash_source>
> (Unfortunately, I can't find a canonical URL for this version of the
> editor's draft, but the most recent change appears to be
> commit 7fe5ce1e2e54130702b0a678a40966f39fab1bab)
> On Wed, Mar 11, 2015 at 8:35 PM, Devdatta Akhawe <dev.akhawe@gmail.com>
> wrote:
>> Hi Austin
>> We should definitely fix the whitespace issue. Re the proposed changes:
>> we want to reuse the deployed CSP hash-src format. So, it is unlikely that
>> we can change that format. I don't see that it explicitly makes it
>> impossible to use RFC6920 URIs in the future, if we so deemed necessary? a
>> ni:// URI can just be one without space and we can define its semantics as
>> ignore content type specified earlier if it exists in ni:// URI. While not
>> ideal, there is value to reusing the CSP hash-src format.
>> cheers
>> Dev
>> On 11 March 2015 at 11:38, Austin William Wright <aaa@bzfx.net> wrote:
>>> Understood that it doesn't need to support everything right from the
>>> first version.
>>> The spec, as currently defined, could make minor adjustments to enhance
>>> forward compatibility, that I do not believe would add additional cost to
>>> user agent implementors.
>>> If RFC6920, for whatever reason, is completely off the table, then I
>>> would suggest changing the format of the token so it doesn't look like a
>>> valid RFC3986 URI, by replacing use of the colon and dash with equals and
>>> comma, respectively:
>>> ; proposed changes
>>> option-expression  =  option-name "=" [ option-value ]
>>> hash-expression    = hash-algo "," base64-value
>>> This should be unambiguous, because neither comma nor equal sign will
>>> appear in option-name nor hash-algo. And the lack of a colon prevents
>>> RFC3986 parsing.
>>> This enhancement would also allow the TR to give tokens to hashes
>>> matching those in the IANA registry ("SHA-256" instead of "sha256"),
>>> further supporting forward compatibility.
>>> Aside: The ABNF in the current Editor's Draft seems to allow the
>>> following string:
>>> option-name:option-valuesha256-Cg==
>>> i.e. without a WS between option-expression and hash-expression. Am I
>>> reading this correct? /aside
>>> I don't think RFC6920 needs to be off the table, however; merely define
>>> an ABNF subset compatible with RFC6920 `NI-URI` production. For all intents
>>> and purposes, it would be just a coincidence that it happens to be a valid
>>> NI-URI production (if funny-looking). The only significant difference (not
>>> formatting/parsing related) would seem to be handling of ct= (appearing in
>>> every hash production, instead of just once).
>>> I don't see anything wrong with mixing option-expression and NI-URI
>>> productions, that might be an ideal solution to explore. I'm thinking all
>>> option-expressions must be matched, if any, and at least one
>>> hash-expression must be matched.
>>> Finally, the current option-value production prohibits the expression of
>>> arbitrary media types. I regularly use quoted-strings (containing spaces
>>> and special characters) in media types, including in Accept headers, for
>>> instance with JSON Schema, e.g.:
>>> application/json; charset=utf-8; profile="http://example.com/book"
>>> (Note charset isn't a registered parameter for application/json, utf-8
>>> is by definition, but it's not prohibited, and I've had difficulty getting
>>> some Web browsers to play nicely without it.)
>>> The current option-value production appears to prohibit a media type
>>> like this, whereas the urlencoded ct= named information parameter utilizes
>>> urlencoding.
>>> Thanks,
>>> Austin.
>>> On Wed, Mar 11, 2015 at 9:45 AM, Devdatta Akhawe <dev.akhawe@gmail.com>
>>> wrote:
>>>> Hi Austin
>>>> These are all great points, but we are not really trying to address
>>>> these in the first version of SRI. The goal in version 1 is only to be able
>>>> to check the hashes of scripts and links. That said, I tend to agree with
>>>> you that this should be in our radar; do you think the spec, as currently
>>>> defined, makes it impossible to address these concerns in future
>>>> iterations? For example, the parser is intentionally forgiving of formats
>>>> it is not aware of, so as to allow such changes in the future.
>>>> cheers
>>>> Dev
>>>> On Mar 9, 2015 11:26 PM, "Austin William Wright" <aaa@bzfx.net> wrote:
>>>>> [CCing swig because I know many of you are also using ni: URIs. Let me
>>>>> know if there's anything to add!]
>>>>> I understand that the ni: URI was removed in a recent update to the
>>>>> SRI draft. I would like to ask this be reconsidered.
>>>>> Using the ni: URI for SRI is important to Semantic Web applications as
>>>>> it allows us to treat the assertion as a link relation. This distinction
>>>>> might not be significant to many user-agents (and thus many on this list),
>>>>> but in Semantic Web applications, especially Web servers, this is of great
>>>>> significance, allows us to make useful relationships between resources.
>>>>> It also confers benefits to servers and application designers, as
>>>>> RFC6920 defines a mapping between ni: URIs and a </.well-known/> URL. In a
>>>>> corporate project under development, we're already using ni: URIs to keep a
>>>>> content-addressable database of files, making them accessible through this
>>>>> mapping. I intend to use Subresource Integrity when serving assets from
>>>>> this store.
>>>>> It also provides an intuitive abstraction: If we think of the ni: URI
>>>>> as identifying a resource (the definition of the URI), the integrity=
>>>>> attribute is performing an assertion: "These two URIs must identify the
>>>>> same information resource, otherwise abort!"
>>>>> For additional support for this use case, I'd like to propose making
>>>>> the "integrity" attribute a a link-extension for RFC5988 Web Linking,
>>>>> suitable for use on any declaration of a link.
>>>>> User agents do not need to think of the ni: URI as a URL if they do
>>>>> not need to, they just follow the ABNF defined in the RFC. There's many
>>>>> cases where URIs are used as identifiers in Web applications; in namespaces
>>>>> [1], schemas (e.g. JSON Schema), DTDs, RDFa [2], and in rel= attributes in
>>>>> all sorts of tags and HTTP headers.
>>>>> Additionally, RFC6920 defines a registry of hashes [3], to ensure
>>>>> forward compatibility (e.g. SHA-3, when standardized later this year). I
>>>>> would like to avoid duplication of effort defining a database of hashes.
>>>>> In short, (1) signing was an explicit goal of the ni: URI, along with
>>>>> other features. Not using ni: would mean servers being unable to take
>>>>> advantage of these other features; and, (2) don't forget about the HTTP
>>>>> Link: header.
>>>>> Thanks for your consideration,
>>>>> Austin Wright.
>>>>> [1] http://www.w3.org/TR/html5/infrastructure.html#xml
>>>>> [2] http://www.w3.org/TR/html-rdfa/
>>>>> [3]
>>>>> http://www.iana.org/assignments/named-information/named-information.xhtml
Received on Friday, 13 March 2015 00:49:31 UTC

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