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Re: Updated c14n Spec

From: David Blondeau <blondeau@intalio.com>
Date: Mon, 19 Jun 2000 14:56:37 -0400 (EDT)
Message-ID: <013d01bfda1e$dbb72470$1101a8c0@aquarium>
To: "John Boyer" <jboyer@PureEdge.com>, "Petteri Stenius" <Petteri.Stenius@remtec.fi>, <w3c-ietf-xmldsig@w3.org>
Cc: "XML DSig" <w3c-ietf-xmldsig@w3.org>
Hi John and Petteri,

I just thought to something that annoy me and maybe it was what Petteri was
thinking about.
With the new c14n draft, the canonicalization of the SignedInfo element is
highly context dependant, that is, when generating a signature, you need to
know where the Signature element will be in the final document before doing
the canonicalization because you need to have the namespaces in scope for
the SignedInfo element. This problem is especially visible for enveloped
So you need to put the Signature in the document before doing any

I agree with the need of context when doing subset canonicalization but IMO,
this is really a problem in the context of Signature generation. With that,
a signature cannot be moved without a lot of precaution and creation of
composite document can be tricky.

This was not the case with the 20000119 draft since an element declared only
those namespaces that
appear on the element itself and on the element's attributes and then the
SignedInfo element was canonicalized the same way wherever the Signature
element was in the document..


----- Original Message -----
From: "John Boyer" <jboyer@PureEdge.com>
To: "Petteri Stenius" <Petteri.Stenius@remtec.fi>; "'David Blondeau'"
<blondeau@intalio.com>; <w3c-ietf-xmldsig@w3.org>
Cc: "XML DSig" <w3c-ietf-xmldsig@w3.org>
Sent: Friday, June 16, 2000 2:13 PM
Subject: RE: Updated c14n Spec

> Hi Petteri and David,
> To be honest, my current acceptance of the namespace propagation as done
> XPath was not based on simply accepting default behavior.  I gave it a
> great deal of thought.  Moreover, in accepting the current behavior, I was
> mindful of the desire to perform whole-document serialization in a
> fashion, so I would be quite interested to know why you (Petteri) think
> XPath version of namespace context identification cannot be serialized in
> single pass.  To me, this seems to me to be as easy as following the
> namespace rules in the January c14n spec, which you say can be serialized
> a single pass.  But before I get into that any further...
> I would point out that, in David's statement about the January spec being
> easier because it didn't care about prefixes in attribute values, I
> he means easier than Petteri's suggestion, not easier than the current
> The January spec did not care about prefixes in attribute values and did
> things that sometimes broke them.  The new spec does not break them, but
> also does not really care about them, i.e. attempt to detect their
> existence, which would be quite impossible without an application context.
> In both the January spec and the current specs, namespace prefix
> that appear in attribute values or element character content are simply
> character data to be written out.
> Also, Petteri, in the example you gave below, yes it is true that the expr
> value both is valid XML and does make reference to an undefined namespace
> prefix.  However, an Xpath with an undefined namespace prefix would
> an error within the application attempting to use it.  More to the point,
> though, the example does not seem to be a counterexample of David's point.
> David's point is about namespaces that are in scope, yet there would be no
> way to identify that its declaration is needed.  In other words, the point
> is about attribute values that carry XPath expressions that used to work
> before canonicalization and don't work after canonicalization.
> It is a non-goal of the current c14n to say d1 and d2 are logically
> equivalent if and only if c14n(d1)==c14n(d2).  However, it is the intent
> the current c14n is to say:  if c14n(d1)==c14n(d2), then d1 is logically
> equivalent to d2.  Otherwise, there would be no point to c14n, especially
> for dsig.  Let d1 be a document containing a working XPath that no longer
> works in c14n(d1) because we omitted a namespace declaration that did not
> appear to be used.  Let d2=c14n(d1).  Now, clearly c14n(d1) == c14n(d2),
> we expect that d1 and d2 are logically equivalent, but they aren't because
> the Xpath works in d1 but not d2. This is the same argument against
> namespace prefix rewriting.  I will add a section to the appendix to
> why this change was made, too.
> Despite this problem with your particular proposal, I am quite sympathetic
> to your cause, Petteri, and thought about other alternatives that would
> work.  Let's begin with just doing something that works for a whole
> (sans comments of course).  So, ignore the notion of document subsets for
> moment.  Also, ignore the default namespace declaration for the moment,
> let's focus on actual namespace declarations.  If a given element and its
> parent both have the same namespace declared to be equal to the same URI,
> then the namespace declaration could be omitted from the child.  Since we
> perform a standard depth first descent of the parse tree, this means that
> could retain all relevant namespace declarations, but still only use local
> operations-- we need only consider the namespace context of an element and
> its parent.
> The problem is not much harder when you add the complexity of the default
> namespace declaration.  Whether it's empty or not, the namespace context
> indicates its value in some way, so if the default namespace of an element
> differs from its parent (whether empty or not), then render a default
> namespace declaration for the element.
> The case for document subsets is not much harder, except we would replace
> the notion of parent with the notion of ancestor *in the node set* with
> Finally, when dealing with document subsets, one is certainly using XPath,
> and the problem is not really too hard.  For example, given a namespace
> N in the resultant node-set, we could do the following:
> 1) Find the element E that owns N (even if it is not in the node-set,
> however weird that might be).
> 2) Find the nearest ancestor A of E that is in the node-set.  If A doesn't
> exist, then output N.  If A exists in the node-set and it has a namespace
> node N(A) *that is in the node-set* which declares the same namespace AND
> assigns it to the same URI, then omit N from the output.  Otherwise,
> N.
> So, as you can see, I've been trying to think about getting rid of
> unnecessary namespace declarations.  However, there seems to be enough
> involved in trying to figure out whether to print a namespace node (esp.
> the document subset case) that it did not seem to worthwhile to complicate
> the spec.  In particular, one must still maintain the whole namespace
> context for each element as one passes through a document.  I will
> that this can be done in a one-pass fashion given space linear in the size
> of the namespace context, which should not be a problem even for the most
> rudimentary of devices capable of processing XML.  This is why I've
> the default XPath namespace propagation feature.
> This is not actually a complete account of my thinking on this issue, but
> I'll end this now unless there is an expressed need for me to continue.
> John Boyer
> Software Development Manager
> PureEdge Solutions Inc. (formerly UWI.Com)
> Creating Binding E-Commerce
> jboyer@PureEdge.com
> -----Original Message-----
> From: w3c-ietf-xmldsig-request@w3.org
> [mailto:w3c-ietf-xmldsig-request@w3.org]On Behalf Of Petteri Stenius
> Sent: Thursday, June 15, 2000 11:19 PM
> To: 'David Blondeau'; w3c-ietf-xmldsig@w3.org
> Cc: XML DSig
> Subject: RE: Updated c14n Spec
> You assume that the XML document constructor has properly declared all
> namespaces that appear in XML attribute values. Of course there is nothing
> an XML processor can do to verify this. A short sample:
> <doc>
> <reference expr="foo:bar"/>
> <foo:bar xmlns:foo="uri"/>
> </doc>
> This is completely valid XML and the namespaces axis of the 'reference'
> element is empty even if the attribute value refers to a namespace. One
> not have to declare the namespace at the 'doc' element level, and if this
> document was constructed using DOM then the above XML representation would
> most likely be the result.
> Petteri
> > -----Original Message-----
> > From: David Blondeau [mailto:blondeau@intalio.com]
> > Sent: Thursday, June 15, 2000 12:06 AM
> > To: w3c-ietf-xmldsig@w3.org
> > Cc: XML DSig
> > Subject: Re: Updated c14n Spec
> >
> >
> > > I need to read the attributes of a element anyway, and I
> > also need to sort
> > > them using the attribute name and the namespace uri as sort
> > keys. This is
> > > the minimum requirement in all cases.
> > I just wanted to show that your suggestion was worst than the
> > one in the
> > draft because you have to be carefull about namespace prefixes used in
> > attributes values. Your suggestion was to put  namespaces
> > only when they are
> > used, my question is then: how do you know a prefix is used
> > in an attribute
> > value?
> > For that, you need to know all the prefixes in scope so you
> > need to walk on
> > the tree to get the namespaces prefixes, and then do a really
> > difficult
> > parsing job...
> > No matter how you are doing it, you need all the namespace
> > declarations in
> > scope for each element.
> >
> > The january draft of C14n was easier on this point since it
> > didn't care
> > about prefixes in attribute values.
> >
> > David
> >
> >
Received on Monday, 19 June 2000 15:10:52 UTC

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