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   Canonicalized Schemas

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  • From: "Simon St.Laurent" <simonstl@simonstl.com>
  • To: XML-Dev Mailing list <xml-dev@xml.org>
  • Date: Fri, 10 Mar 2000 09:44:55 -0500

Is it possible to transform an XML Schema into Canonical XML and still be
able to use the Schema?

It seems to me that it isn't, unless the application has some other kind of
access to the namespace prefix-URI mappings that are discarded by
Canonicalization (c14n).  The prefixes will live on in the attribute
values, but the prefix-URI mapping will disappear.

This could suggest a few possibilities:
1) Canonical XML is not intended as a transfer format - the only reason it
exists is to establish a common form that can be used in conjunction with
digital signatures.  While Canonical XML suggests a document syntax, that
document syntax is only metaphorical, and not intended for actual transfer
over the wire.

2) The Schema and c14n groups (and others, including XLink and perhaps
XPath) need to have a discussion about what exactly 'namespace-aware'
processing involves, and the role of prefixes within that processing.  This
could lead to some form of prefix preservation within c14n.

3) While XML document instances may be sent in canonical form, Schemas are
a different type of information and should not be canonicalized.

David Megginson made a convincing argument last week that schemas and style
sheets deserve as much if not more security than documents - see
http://www.megginson.com/ugly/ for the slides.  To me, this suggests that
signatures in particular and c14n as a part of that need to be addressed
directly for schemas.

I'd thought we'd seen the last of namespace implementation controversy, but
I'm afraid it isn't quite over yet.

Simon St.Laurent
XML Elements of Style / XML: A Primer, 2nd Ed.
Building XML Applications
Inside XML DTDs: Scientific and Technical
Cookies / Sharing Bandwidth

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