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RE: XML Encryption Data/Processing Model

From: John Boyer <JBoyer@PureEdge.com>
Date: Fri, 30 Mar 2001 10:33:17 -0800
Message-ID: <7874BFCCD289A645B5CE3935769F0B520C337F@tigger.PureEdge.com>
To: "Hiroshi Maruyama" <MARUYAMA@jp.ibm.com>, <xml-encryption@w3.org>, <w3c-ietf-xmldsig@w3.org>
Cc: "Andy_Clark/Santa_Teresa/IBM" <Andy_Clark/Santa_Teresa/IBM@jp.ibm.com>
Hi Hiroshi,

Summary: 

Your problem has little to do with a 'consistency' issue on C14N, and is
trivially solvable.  The solution you are considering is fraught with
problems.  Details below.

John Boyer
Senior Product Architect, Software Development
Internet Commerce System (ICS) Team
PureEdge Solutions Inc. 
Trusted Digital Relationships
v: 250-708-8047  f: 250-708-8010
1-888-517-2675   http://www.PureEdge.com <http://www.pureedge.com/>  	
 	


<hiroshi>
One thing that we noticed as problematic when looking at
signature/encryption
is the treatment of xml:lang and xml:space in
C14N, if we use C14N for serialization.  C14N requires you to "push" the
nearest
xml:lang and xml:space declarations in the outer context into the C14Ned
form.

For example, canonicalizing the <bar/> element in

<foo xml:lang='JA">
   <bar />
</foo>

will yield

<bar xml:lang="JA"/>.

</hiroshi>

<john>
It would actually be <bar xml:lang="JA"></bar>.
This is a desired behavior, esp. according to I18N types.
</john>

<hiroshi>
Now, consider a sign-and-then-encrypt scenario for the following
document.

<el2sign xml:lang="JA">
   <el2enc>secret</el2enc>
</el2sign>

where <el2sign> is signed and then <el2enc> is encrypted.  If we use
C14N
before encrypting <el2enc>, it will be converted to

<el2enc xml:lang="JA">secret</el2enc>

and consequently, after decryption the signature verifier will see

<el2sign xml:lang="JA">
   <el2enc xml:lang="JA">secret</el2enc>
</el2sign>

Unfortunately, C14Ning this will not remove the duplicate xml:lang
declaration
in <el2enc>.  Therefore, the signature does not verify.

</hiroshi>

<john>
Firstly, if you change something that is signed after it is signed, the
signature is supposed to break.  This is a good thing.

As to your practical problem, it does not appear to have a very
difficult solution.  The fact that it is a sign-then-encrypt scenario
does not mean you are restricted to two black box operations to be done
in precisely that sequence.  If you know it is a sign then encrypt, then
you should simply apply the C14N to the block to be encrypted, then
affix the signature, then perform the encryption.
</john>

***

We rather think this is due to lack of consistency in C14N.  If C14N
treats "xml:lang in effect" rather than "xml:lang declarations" as
significant, it should remove redundant xml:lang declarations.  If C14N
thinks "xml:lang declarations" significant, it should not copy
outer declaration into C14Ned element.

<john>
The fact that C14N doesn't remove unnecessary xml:* attributes seems to
be orthogonal to the problem you are having.  The problem you are having
is that attributes are being added when you don't want them, not that
C14N is failing to remove unnecessary ones when you don't want them.

When you C14N the <el2enc> element, you are canonicalizing it outside of
the context of its parent, so transferring the xml:lang into the element
is necessary.  To wit, you may have an application scenario where you
send only the encrypted blob (containing the scrambled version of the
<el2enc> element) to the holder of the appropriate private key.  The
absence of xml:lang in such a case would be detrimental.
</john>

Incidentally, C14N is consistent in treatment of namespace declarations
-
it always treats "in-scope namespaces" only and redundant
namespace declarations are removed.

<john>
Yes, this is what it does for namespaces, but you only receive an
xml:lang that is in-scope, and removing redundant ones does not solve
your problem, as mentioned above.  As an exercise, do this problem again
with a namespace declaration.  You will see that you have the same
trouble.  Please see Section 2.3 of C14N for details on the handling of
namespace nodes.
</john>

<hiroshi>
Because C14N is already fixed, I do not think we can use C14N
as serialization of encrypted data.  We are trying to define a
serialization
that would preserve the declarations as they appear in the original
document.
</hiroshi>

<john>
This would be an unfortunate choice given the simplicity with which your
problem can be solved, esp. given the above counterexample under which
your technical choice would result in application limitations.
</john>

Hiroshi




--
Hiroshi Maruyama
Manager, Internet Technology, Tokyo Research Laboratory
+81-46-215-4576
maruyama@jp.ibm.com



From: "Joseph M. Reagle Jr." <reagle@w3.org>@w3.org on 2001/03/30 04:15

Please respond to "Joseph M. Reagle Jr." <reagle@w3.org>

Sent by:  xml-encryption-request@w3.org


To:   Takeshi Imamura/Japan/IBM@IBMJP
cc:   "XML Encryption WG " <xml-encryption@w3.org>, Hiroshi
      Maruyama/Japan/IBM@IBMJP
Subject:  Re: XML Encryption Data/Processing Model



At 19:33 3/29/2001 +0900, Takeshi Imamura wrote:
>The serialization is not so sensitive if only encryption is performed.
>However, that may not be true if both encryption and signature are
>performed on a document.  For example, suppose that a user signs and
then
>encrypts a document, where he defines and uses his own serialization
which
>does not preserve information set.  Another user will fail in verifying
the
>signature because the original document cannot be retrieved.  For such
>reason, I think we should define at least a serialization for
information
>set.

Very good point, I added that to the text in that section. However, I
don't
think the Infoset or DOM data models lack anything required by Canonical
XML. (The part of XPath data model used by Canonical XML is a subset of
both
DOM and Infoset, right?) Even if this is the case (doesn't necessitate
the
use of Infoset or DOM), do you have a preference between the two?

__
Joseph Reagle Jr.                 http://www.w3.org/People/Reagle/
W3C Policy Analyst                mailto:reagle@w3.org
IETF/W3C XML-Signature Co-Chair   http://www.w3.org/Signature
W3C XML Encryption Chair          http://www.w3.org/Encryption/2001/
Received on Friday, 30 March 2001 13:34:03 GMT

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