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ID/DTD/c14n/dsig mischief?

From: Thomas Maslen <maslen@dstc.edu.au>
Date: Thu, 12 Jul 2001 22:19:56 +1000
Message-Id: <200107121219.f6CCJm427660@piglet.dstc.edu.au>
To: w3c-ietf-xmldsig@w3.org
[ Has this been done to death before?  I looked in the archives and didn't
  see this exactly, but if I missed it can someone steer me toward it? ]

Canonical XML makes use of information that may have started life in a DTD
(e.g. entity declarations and attribute types) but does not directly
include the DTD in the c14n output.

Section 1.3 of the Canonical XML Recommendation discusses cases where this
may lead to (possibly false) negatives, i.e. discarding or changing the DTD 
can produce different c14n results, which when used in dsig will lead to 
digest mismatches.  In particular, the second-last paragraph discusses the
effect on attributes whose type is ID.

I'm wondering about the converse -- cases that can lead to false positives, 
i.e. changing the DTD doesn't change the pile of bits that comes out of c14n 
(so, when used in dsig, the digest will still match and the signature will 
still verify), but the document now means something different.

The example that I have in mind uses attributes whose type is ID, and it
goes something like this...  say I'm a disgruntled screenwriter, and for the 
next episode of the soap that I'm working on I produce a plot outline like
this:

	<!DOCTYPE Soap [
	  <!ELEMENT Character ... >
	  <!ATTLIST Character
	      name     ID    #REQUIRED
	      nickname CDATA #IMPLIED
	  >
	]>

	<Soap>
	  <Characters>
	    <Character name="alice"> ... </Character>
	    <Character name="bob">   ... </Character>
	    ...
	    <Character name="susan" nickname="alice"> ... </Character>
	    <Character name="tim"   nickname="bob"  > ... </Character>
	  </Characters>
	  <Actions>
	    <Kill killer="#alice" victim="#bob" />
            <!-- (Uses URI fragments but could also use IDREF attributes) -->
	  </Actions>
	</Soap>

and get sign-off from the series producer.  (For concreteness, say he signs
the entire document using an enveloped signature, and puts the signature
just before the closing "</Soap>").

Now I modify the ATTLIST declaration, changing the ID attribute from "name"
to "nickname", and end up with

	<!DOCTYPE Soap [
	  <!ELEMENT Character ... >
	  <!ATTLIST Character
	      name     CDATA #REQUIRED
	      nickname ID    #IMPLIED
	  >
	]>

	<Soap>
	  <Characters>
	    <Character name="alice"> ... </Character>
	    <Character name="bob">   ... </Character>
	    ...
	    <Character name="susan" nickname="alice"> ... </Character>
	    <Character name="tim"   nickname="bob"  > ... </Character>
	  </Characters>
	  <Actions>
	    <Kill killer="#alice" victim="#bob" />
            <!-- (Uses URI fragments but could also use IDREF attributes) -->
	  </Actions>
	  <Signature xmlns="..."> ... </Signature>
	</Soap>

This still verifies, but now Tim dies instead of Bob, which isn't the 
outcome that the series producer thought he was signing.

Thomas Maslen
DSTC
Received on Thursday, 12 July 2001 08:20:01 UTC

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