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Provokant proposal on Exclusive C14n

From: Christian Geuer-Pollmann <geuer-pollmann@nue.et-inf.uni-siegen.de>
Date: Wed, 05 Jun 2002 09:46:50 +0200
To: Joseph Reagle <reagle@w3.org>, John Boyer <jboyer@PureEdge.com>, "Donald E. Eastlake 3rd" <Donald.Eastlake@Motorola.com>
Cc: w3c-ietf-xmldsig@w3.org
Message-ID: <3492271.1023270410@clouseau>

Hi all,

first a big thank you to Merlin who made the very cool edge-cases for c14n 
and exclC14n to understand how these standards handle the namespace stuff. 
Till a few weeks ago, I did not understood that a properly choosen document 
subset (in c14n) can exclude namespaces from the documents subset. For me, 
namespaces were not 'regular' nodes but they were inseparable twisted with 
the document.

For "Canonical XML", I see that the possibility to include only particular 
namespaces to a document subset is really cool if a transfroms author wants 
to create context-independent document subsets.

For "Exclusive Canonical XML", I don't see why we have to inherit the 
(complicated) namespace handling from "Canonical XML".

Provokant proposal: If the PR-Status of exclC14n allows this (substantial) 
change, I want to propagate to canonicalize document subsets as follows:

  "If a document subset is to be canonicalized using 'Exclusive C14n',
   all namespace nodes in the original document are included in the
   document subset prior the serialization process; this inclusion is
   done regardless whether a namespace node is already in the subset
   or if it's excluded from the subset."

After that 'pre-processing', the exclusive c14n process is started with the 
following change: All passages in the text which refer to namespace nodes 
which are not in the document subset can be omitted.

Why do I suggest that: For standard c14n, it was necessary to be able to 
omit namespace nodes from the document subset. For exclusive c14n, we have 
(1) the mechanism of the "InclusiveNamespaces PrefixList" and (2) the 
visibly-utilizes mechanism. I think that such a change will make exclusive 
c14n reliable and consistent (not consistent to the c14n REC but consistent 
to what c14n should really do).

I think canonicalization should serve two purposes:

 (1) create a bit-accurate representation of a document
     or document subset for use in cryptographic algorithms
     like a message digest

 (2) allow the verifier of a signature to take these signed
     octets and re-parse the octets to get back a
     "trusted" XML structure which can be reliably used in
     the application. This goes to "process-what-is-signed".
     But with the current processing model where namespaces
     can be excluded from the document subset, it's possible
     that a "reparse signed contents" step does encounter
     'illegal' XML.

I had no better word as 'illegal'. I know that it's possible that the 
signed contents are not well-formed, e.g. like this:

      <A /><B />

or like this

      foo text <A />

but these are problems which can be handled easily by "wrapping" the octets 
into a dummy root element. But if a namespace is used e.g. by an element 
but the namespace decl does not appear, this can't be handled in any way, 
and from the semantics point, it's even completely meaningless:

<foo:A>
   <foo:B xmlns:foo="http://foo" />
</foo:A>

In this case, the namespace is (maybe accidently?) omitted from the foo:A 
element, but what happens if we have such an input document:

<foo:Contract  xmlns:foo="http://companyA.com">
   <foo:Detail xmlns:foo="http://companyB.com" />
</foo:Contract>

and I choose a rogue document subset which results in

<foo:Contract  xmlns:foo="http://companyA.com">
   <foo:Detail />
</foo:Contract>

That's so bad; I think that the above proposal will stop that kind of 
cheating: foo:Detail visibly utilizes foo and so 
xmlns:foo="http://companyB.com" is output in the exclusive canonical form, 
regardless whether the XPath transform author did include it or not.



Kind regards,
hope that you all don't eat me alive for this ;-)

Christian
Received on Wednesday, 5 June 2002 03:54:09 GMT

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