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RE: Re: Re: XPath Expression

From: Dournaee, Blake <bdournaee@rsasecurity.com>
Date: Thu, 2 Aug 2001 15:47:41 -0700
Message-ID: <E7B6CB80230AD31185AD0008C7EBC4D2DAF011@exrsa01.rsa.com>
To: w3c-ietf-xmldsig@w3.org
Cc: "'reagle@w3.org'" <reagle@w3.org>
Hello All,
 
I certainly didn't expect I would receive the flurry of discussion that
transpired on this issue! I wan to thank everyone for their input on this.
 
The reason why I asked was because I am focusing on the usability side of
our XML Dsig toolkit - our customers (developers) must be able to understand
XPath well-enough to be able to make useful, *well-understood*,
transformations for the power of XPath to be fully realized.
 
One problem right now is that it is difficult, if not impossible, to see
what you have actually signed if an XPath expression is employed. That is,
right now, you can "set" an expression and then make a signature - but if
your expression is wrong or improper, there is no feedback in the final
<Signature> structure that gives you a hint that you goofed. The problem
gets compounded further if this signature is sent off and it verifies
improperly. This sends developers on a huge bug-hunt for the problem.
 
Further, because XPath is used as a boolean filter, and not a selection
language (for XML Dsig),  it operates slightly differently than what is
expected. Expressions like "/a/b/c" (to select the "c" child element of "b"
and of "a") don't work as expected, (this would evaluate to "True" for all
nodes and every node would get included, which is not the intended
behavior).
 
To alleviate the confusion in this area, I am working on a sample
application that takes an arbitrary input XML file and an XPath expression,
and then performs (and then canonicalizes) the output and spits it back out.
This type of mechanism allows customers to "see" what they are actually
signing after an XPath expression occurs.
 
I have built the tool using Apache's Xalan 2.01 and it seems to work for the
simpler expressions, but when I tried John's suggestion (removed the dsig
namespace for now):
 
XPath Expression: (//. | //@* | //namespace::*)[not (self::attribute and
(name()=URI) and parent::Reference)]
 
My transformed node-set was the same as the input node set. My input file
was decidedly simple:
 
<?xml version="1.0" encoding="UTF-8"?>
<Reference URI="foo"> Hello People! </Reference>
 
I was expecting to get:
 
<Reference> Hello People! </Reference>
 
But instead got:
 
<Reference URI="foo"> Hello People! </Reference>
 
I am perfectly willing to accept that I have a few bugs in my tool, or in
Xalan for that matter, but I wanted ask others if they have actually tried
this expression and met with success. 
 
Joseph: What is the possibility of drafting a W3C Note that contains a set
of "cooked" XPath expressions for performing common tasks? I am guessing
this would be a godsend to developers who are struggling with this issue.
 
Kind Regards,
 
 
Blake Dournaee
Toolkit Applications Engineer
RSA Security
 
"The only thing I know is that I know nothing" - Socrates
 
 

-----Original Message-----
From: John Boyer [mailto:JBoyer@PureEdge.com]
Sent: Thursday, August 02, 2001 10:26 AM
To: edsimon@xmlsec.com; merlin
Cc: reagle@w3.org; Dournaee, Blake; w3c-ietf-xmldsig@w3.org
Subject: RE: Re: Re: XPath Expression 



Hi Ed, 

Part of the problem with the exercise is that it really depends on how the
<Reference> element was obtained in the first place.  Do we want to omit the
URI attribute of every Reference element, or a particular Reference element?


Moreover, in the context of dsig, it is already known that you can't apply
an XPath transform to the SignedInfo anyway.  The question Blake originally
asked sounds exactly like the type of thing I've been wanting to do for some
time now, which is to have the ability to vary where signed information
comes from without breaking the signature.  Having the ability to apply
XPath to SignedInfo (with some caveats) is one clean way of doing that.  The
mail archives have lots of comments from me on this issue, along with
repeated statements from the WG that the method is too scary.  So, the short
answer is that 'it can't be done'.

From the purely academic standpoint, i.e. supposing it could be done AND
supposing that XPath transforms returned a node-set result from a whole tree
rather than returning a boolean for each node AND supposing that you wanted
to get rid of every URI attribute from every Reference, the following should
do the trick:

(//. | //@* | //namespace::*)[not (self::attribute and (name()="URI") and
parent::dsig:Reference)] 

This is similar to your prior expression with the following exceptions.  

1) I replaced node() with (//. | //@* | //namespace::*) because we want to
descend to all element nodes (// is for descendant-or-self::node) plus all
attribute and namespace nodes (//@* means all attributes of all nodes
obtained by //., and //namespace::* means all namespace nodes of all
elements).

2) I replaced NodeType(attribute) with self::attribute.  NodeType is a
non-terminal symbol in the XPath grammar, not a function that can be called.
However, it is the right idea to make sure the node we want to reject is
actually an attribute.  The prior expressions would've also thrown out child
elements of Reference if they were named URI.

If you want to get rid of a URI for a specific Reference element, then it is
necessary to figure out how to identify that Reference element, then replace
parent::dsig:Reference in the expression above.  For example, if you
identified the reference with an ID attribute having the value 'foo', then
the replacement would be parent::dsig:Reference[id('foo')].

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>  <
http://www.pureedge.com/ <http://www.pureedge.com/> >     
        


-----Original Message----- 
From: edsimon@xmlsec.com [ mailto:edsimon@xmlsec.com
<mailto:edsimon@xmlsec.com> ] 
Sent: Thursday, August 02, 2001 9:43 AM 
To: merlin 
Cc: reagle@w3.org; bdournaee@rsasecurity.com; w3c-ietf-xmldsig@w3.org 
Subject: Re: Re: Re: XPath Expression 


I think if XPath could be used to return a nodeset that excludes certain 
attribute nodes, then what you have suggested would fit in.  I remain
curious, 
as an academic exercise, as to whether there exists such a single XPath 
expression.  Perhaps something like 

"node()[not (NodeType(attribute) and (name()="URI") and
parent::dsig:Reference)]" 

is getting closer. 

Ed 

-- Original Message -- 

> 
>Hi Ed, 
> 
>r/edsimon@xmlsec.com/2001.08.02/10:57:14 
>>I'd certainly be keen to see the full working XPath expression if someone 
>>wants to take up the challenge.  Merlin's suggestion, by itself, is not 
>>the whole solution but only part as it returns a boolean, not nodes. 

> 
>Blake asked for something to put in an XPath transform; 
>our XPath transform does a boolean test of each node in 
>the node set against its expression. 
> 
>So, you are right; what I suggested is not a general 
>XPath expression for selecting all of a document but 
>its URI attributes; XSLT is the tool for that job. 
>But, it is a suitable test expression for our XPath 
>transform to perform this task. 
> 
>I think. (sometimes) 
> 
>Merlin 
> 
>>In general, XPath is good at selecting nodes but was not really intended 
>>for removing subnodes.  With current tools, I'd say removing subnodes 
is 
>>best done with XSLT. 
>> 
>>BTW, the XSLT I've presented below does actually do the trick. 
>> 
>>Ed 
>>-- Original Message -- 
>> 
>>> 
>>>I'd guess that something like not ((name() =3D "URI") and parent::dsig:R=

>>eference) 
>>>might work. 
>>> 
>>>merlin 
>>> 
>>>r/edsimon@xmlsec.com/2001.08.02/09:37:38 
>>>>>>This would allow me to change the URI without altering the validity 
>>of 
>>>>the 
>>>>>>signature. 
>>>>> 
>>>>>If it's being used in the Canonical XML context, remove the '//@*' 
fro= 
>>m: 
>>>>>         (//. | //@* | //namespace::*) 
>>>> 
>>>> 
>>>>Correct me if I'm wrong but wouldn't the above remove ALL attributes, 
>>not 
>>>>just the one Blake wants removed. 
>>>> 
>>>>I'm not sure that there is an XPath solution but the solution in XSLT 
>>would 
>>>>look like this: 
>>>> 
>>>> 
>>>><?xml version=3D"1.0" encoding=3D"UTF-8"?> 
>>>><xsl:stylesheet version=3D"1.0" 
>>>>  xmlns:xsl =3D" http://www.w3.org/1999/XSL/Transform
<http://www.w3.org/1999/XSL/Transform> " 
>>>>  xmlns:xmlsig=3D" http://www.w3.org/2000/09/xmldsig
<http://www.w3.org/2000/09/xmldsig> #"> 
>>>> 
>>>><!-- Identity transform --> 
>>>><xsl:template match=3D"@*|node()"> 
>>>>  <xsl:copy> 
>>>>    <xsl:apply-templates select=3D"@*|node()"/> 
>>>>  </xsl:copy> 
>>>></xsl:template> 
>>>> 
>>>> 
>>>><!-- Skip the Reference/@URI attribute --> 
>>>><xsl:template match=3D"xmlsig:Reference/@URI" priority=3D"2"/> 
>>>> 
>>>> 
>>>></xsl:stylesheet> 
>>>> 
>>>> 
>>>>-------------------------------------------------- 
>> 
>> 
>>-------------------------------------------------- 
>>Ed Simon 
>>XMLsec Inc. 
>> 
>>Interested in XML Security Training and Consulting services?  Visit "www.=

>>xmlsec.com". 
>> 
>> 
> 
> 
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------------------- 
Ed Simon 
XMLsec Inc. 

Interested in XML Security Training and Consulting services?  Visit
"www.xmlsec.com". 
Received on Thursday, 2 August 2001 18:44:12 UTC

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