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RE: Default transforms

From: John Boyer <jboyer@PureEdge.com>
Date: Mon, 5 Jun 2000 14:15:38 -0700
To: "Joseph M. Reagle Jr." <reagle@w3.org>, "Petteri Stenius" <Petteri.Stenius@remtec.fi>
Cc: "IETF/W3C XML-DSig WG" <w3c-ietf-xmldsig@w3.org>
Hi Joseph,

The one point I'd make is that our expectations regarding URI="#E" are not
the same as a call to the XPath id("E").  The XPath transform only renders
nodes in the indicated node-set.  The id function returns a node-set
containing the identified element, but all of its descendants are not in the
node-set.  So if E has the following markup:

<elem id="E">I am signed.</elem>

We expect URI="#E" to generate the entire line above, but the XPath
expression id("E") would only generate:


Under our current design, you'd have to tell the XPath to include all of the
descendants.  This was the source of my angst earlier over the meaning of
descendant-or-self::node() because I'd like to say:


but this doesn't give me the attributes and namespace declarations, so the
result would be:

<elem>I am signed.</elem>

Furthermore, it appears to be syntactically invalid to put anything after
the slash that would give me *all* of the nodes.  Therefore, URI="#E" must
be stated as being equal to an XPath transform with the following

(//. | //@ | //namespace::*)[count(ancestor-or-self::* | id("E")) ==

The beginning parenthetic means to consider all nodes in the whole tree in
the node-set, and the square-bracketed expression determines whether E is
the given element node or any of its ancestors.  Only the element E and its
descendants (including namespace and attribute nodes) will pass this test.

By the way, we could change the XPath transform to make this a little
simpler by adding two expressions, <keep> and <omit>.  The idea would be
that any node in the keep set would have itself and all of its descendants
(including namespace and attribute nodes) rendered automatically, except
nodes in the omit set.  A node in the omit set, and all of its descendants
(including namespaces and attributes) would be omitted from rendering,
except those in the keep set, and so on.  Naturally, it should be an error
if the intersection of the two sets is non-empty (equivalently, if the size
of the union is not equal to the sum of the sizes of the two sets).

With this paradigm, URI="#E" would be equal to an Xpath transform with
<keep>id("E")</keep> as a parameter and no <omit> parameter.

Perhaps the chairs should bat this formulation around for a while and talk
to the implementers about it because it might make sense to parameterize
c14n in the same way.

John Boyer
Software Development Manager
PureEdge Solutions Inc. (formerly UWI.Com)
Creating Binding E-Commerce

-----Original Message-----
From: w3c-ietf-xmldsig-request@w3.org
[mailto:w3c-ietf-xmldsig-request@w3.org]On Behalf Of Joseph M. Reagle
Sent: Monday, June 05, 2000 1:41 PM
To: Petteri Stenius
Subject: Re: Default transforms

At 11:26 AM 6/5/00 +0300, Petteri Stenius wrote:
 >Chapter 4.3.3 of [1] reads:
 >"[URI] permits identifiers that specify a fragment identifier via a
 >separating number/pound symbol '#'. (The meaning of the fragment is
 >by the resource's MIME type). XML Signature applications MUST support the
 >XPointer 'bare name' [Xptr] shortcut after '#' so as to identify IDs
 >XML documents. The results are serialized as specified in section
 >6.6.3:XPath Filtering. For example,"
 >My interpretation of 2.1.1 is that there is *no* default serialization or
 >canonicalization algorithm. But reading 4.3.3 would suggest that XPath is
 >used by default.

Hi Petteri. There was a time when I advocated "clean-URIs" such that if
anything beyond merely dereferencing a URI via its protocol scheme was
needed, it had to be explicitly represented in a transform. But there's no
such thing as a 'clean-URI' so we support URIs with fragment identifiers.
When the dereferenced object is XML, that means it's an XPtr expression. We
REQUIRE implementations to support the 'bare name' [1], which is a short
hand of the XPath id() function [2].

So generally speaking there is no default transforms. BUT, if a fragment
identifier is used within a <Reference URI="..."> that identifies an XML
resource, that means XPtr processing is done and the results need to be
serialized according to 6.6.3 [3]. Note, not all XPtr expressions that might
fit in the URI will be supported by Signature applications.

I don't find the fact that people are specifying transforms in the URI to be
very 'clean' but at least it's consistent with the rest of the world (or so
I'm told <smile>). That's why we recommend:

        Regardless, such fragment identification and addressing
        SHOULD be given under Transforms (not as part of the URI)
        so that they can be fully identified and specified. For instance,
        one could reference a fragment of a document that is encoded
        by using the Reference URI to identify the resource, and one
        Transform to specify decoding, and a second to specify an
        XPath selection.

Is this agreeable to your reading? Should we change the text to make
something clearer?

[1] http://www.w3.org/TR/xptr#synth-2.1.2
[2] http://www.w3.org/TR/xpath#section-Node-Set-Functions
[3] http://www.w3.org/TR/xmldsig-core/#sec-XPath

Joseph Reagle Jr.
W3C Policy Analyst                mailto:reagle@w3.org
IETF/W3C XML-Signature Co-Chair   http://www.w3.org/People/Reagle/
Received on Monday, 5 June 2000 17:15:38 UTC

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