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Re: Interop and c14n(x) != c14n(c14n(x))

From: merlin <merlin@baltimore.ie>
Date: Fri, 24 May 2002 18:47:57 +0100
To: Christian Geuer-Pollmann <geuer-pollmann@nue.et-inf.uni-siegen.de>
Cc: reagle@w3.org, Gregor Karlinger <gregor.karlinger@iaik.at>, w3c-ietf-xmldsig@w3.org
Message-Id: <20020524174757.AFA0D4432D@yog-sothoth.ie.baltimore.com>

Hi Christian,

There are at least two problems I see with your c14n:
  ...
  <ds:Reference URI="#object1">
  <ds:Transforms>
  <ds:Transform Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
  <ds:XPath>self::node()[local-name()='included']</ds:XPath>
  </ds:Transform>
  </ds:Transforms>
  ...
  <ds:Object Id="object1">
  <included xml:lang="de">
  <notIncluded>
  <notIncluded xml:lang="uk">
  <included>
  </included>
  </notIncluded>
  </notIncluded>
  </included>
  </ds:Object>
  ...

Your c14n is:

<included xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xml:lang="de"><included xml:lang="uk"></included></included>

1) Your XPath does not include any nodes from the namespace
axis, so xmlns:ds="http://www.w3.org/2000/09/xmldsig#" should
not be emitted. You probably want an XPath of this form (I
haven't thought much about it):
  count(ancestor-or-self::included) >
    count(ancestor-or-self::notIncluded[1]/ancestor::included)
This will include xml:lang="de" which you may or may not want;
tweak as necessary.

If you want a detailed discussion of the disconnect between
the XPath info set concept of the namespace axis and DOM,
look back at some of the threads from one or two years ago.
I know I contributed my confusion to some of them.

2) xml:lang="de" is not defined in the ancestor axis of the
first included element, and is not included in the node set,
so should not be emitted.

While I'm not convinced that there is a requirement that
c14n(X) == c14n(c14n(X)) - c14n produces the canonical form of
a node set, and the output of c14n is not the same node set
as the input, so there is not a claim that the outputs should
necessarily be the same - I think in fact that property may
hold true.

Can you send a clearer counterexample in-line, even just
in pseudo XML?

Merlin

r/geuer-pollmann@nue.et-inf.uni-siegen.de/2002.05.24/18:29:44
>> Yes, *and* when you do copy all the xml:* from the ancestors because of a
>> missing parent element you go *all* the way up to the root, not just up
>> to  the next ancestor element in the subset. Counter-intuitive but the
>> spec is  clear once you get over your intuitions -- I had the same
>> problem! [1]
>
>First, here is my example on c14n and exclC14n. Maybe others can validate? 
>Thanks. (secret key for hmac is six octets of "secret").
>
>
>This weird behaviour of c14n of document subsets leads to a very strange 
>result. Till today, my understanding of c14n was that given some input 'X', 
>the c14n algorithm can be applied multiple times without changing the 
>result. But now, I see that
>
>1.) Given a document subset 'X' with a form that the above rule comes to 
>play.
>
>2.) Then we see that
>
>        c14n(X) != c14n(c14n(x))
>
>I included a sample for this (the last three references) in the interop.
>
>
>
>Regards,
>Christian


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Received on Friday, 24 May 2002 13:48:14 GMT

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