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RE: Enveloped signatures

From: John Boyer <jboyer@PureEdge.com>
Date: Thu, 11 May 2000 16:37:15 -0700
To: "Joseph M. Reagle Jr." <reagle@w3.org>, "Mariano P. Consens" <mconsens@uwaterloo.ca>
Cc: <w3c-ietf-xmldsig@w3.org>
Hi Joseph and Mariano,

Whether the XPath transform adds a here() function or a $here variable is
not nearly as important as whether it will actually solve the problems it is
intended to solve.

Any conformant XPath implementation MUST offer a way to add to the function
library, so there is no problem doing it as a function.

The larger issue, though, is whether here() does the trick.  Mariano, when
we spoke at the FTF, you said you'd post an example regarding the problem
you actually want to solve with here(), which was some way to get around the
XML global id problem.  I would appreciate it if you would work that out and
show it.

As for enveloped signatures, here() did not at first appear to be enough.
The problem is not to simply to identify the ancestor signature element
containing the XPath expression.  Instead, the problem is to exclude that
element and all of its descendants from a digest calculation.  A transform
containing the expression you wrote would *include* only the signature and
its descendants, which is the opposite of what we want.  At first I did not
think it could be done, but then I got a little creative with XPath, and the
following expression appears to suffice:

  count(ancestor::Signature | here()/ancestor::Signature[1]) >

The /descendant-or-self::node() means that we examine all nodes in the tree,
accepting only those for which the square-bracketed predicate expression is

Since the XPath equality operator converts node sets to string values before
comparison, we must instead use the 'union' operator, which is |. The size
of the union of the two node sets will be greater than the size of the
node-set containing the Signature element ancestors of the context node if
and only if the latter does not include the first signature element ancestor
of here().  Thus, the predicate expression is true for every node that is
not a descendant of the immediate Signature ancestor element of the XPath

Therefore, if we add here(), then we can provide an XPath transform
definition for an enveloped signature.

John Boyer
Software Development Manager
PureEdge Solutions Inc. (formerly UWI.Com)
Creating Binding E-Commerce

-----Original Message-----
From: Joseph M. Reagle Jr. [mailto:reagle@w3.org]
Sent: Thursday, May 11, 2000 12:35 PM
To: Mariano P. Consens
Cc: w3c-ietf-xmldsig@w3.org; John Boyer
Subject: RE: Enveloped signatures

Good point! But we have decided not to require XPtr... However, it still
might provide enough 'elegance' in defining the behaviour, but I'm not sure
if we could also reproduce that in XPath for those that plan on doing XPath
solution. I'll defer to John on that.

At 11:54 2000-05-10 -0400, Mariano P. Consens wrote:
 >Joseph (and Donald and John, who were also part of the discussion after
 >I've found the following function defined in XPointer (Working Draft 6
 >December 1999):
 >FUNCTION: location-set here( )
 >The here() function returns a location-set with a single member. That
 >location is of type element, and locates the element that directly
 >(as text) or bears (as an attribute value) the XPointer. This expression
 >results in a syntax error if the containing XPointer does not appear in an
 >XML document.
 >Using it we can find the ancestor Signature element (without defining
 >variable bindings) as:
 >  --------------------------------------------------------------------
 >  Mariano P. Consens                    Department of Computer Science
 >  e-mail: mconsens@uwaterloo.ca         University of Waterloo
 >  phone: (519) 888-4567 x-3023          Waterloo, Ontario, N2L 3G1
 >  fax: (519) 885-1208                   CANADA
 >  --------------------------------------------------------------------
 >> -----Original Message-----
 >> From: w3c-ietf-xmldsig-request@w3.org
 >> [mailto:w3c-ietf-xmldsig-request@w3.org]On Behalf Of Joseph M. Reagle
 >> Jr.
 >> Sent: May 10, 2000 8:29 AM
 >> To: Petteri Stenius
 >> Cc: IETF/W3C XML-DSig WG (E-mail)
 >> Subject: Re: Enveloped signatures
 >> At 11:35 2000-05-10 +0300, Petteri Stenius wrote:
 >>  >6.6.5 Signature Exclusion
 >>  >
 >>  >Identifier:
 >>  >    http://www.w3.org/2000/02/xmldsig#exclude-signature
 >>  >
 >>  >The transform excludes the ancestor Signature element of the
 >>  >element that is being calculated. The intention is to support
 >>  >signatures where the DigestValue calculation would overlap with
 >> Petteri, the idea was that it would be somewhat elegant if we
 >> specified the
 >> identifier and defined its behavior using an XPath expression (the
 >> equivalent of your natural language text above). That way, people
 >> that don't
 >> expect to implement XPath can implement the feature rather simply; those
 >> that already have XPath don't have to do something special, and both
 >> interoperate!
 >> The challenge then is to find the XPath equivalent of your description
 >> above. This seemed to be potentially tricky in that the context
 >> node in your
 >> description above is "the ancestor Signature element of the DigestValue
 >> element that is being calculated" and if you go the XPath/XML route, it
 >> isn't going to know this (what is being calculated), you just hand it
 >> Then the XPath processor sets the context node at the root root, and
 >> you have to specify an expression that finds and excludes the right
 >> Signature (if they are nested.) There might be a clever way to do this
 >> though, and I think Boyer might've mentioned you might be able to
 >> this by passing arguments from a function ... ? But we concluded by
 >> let's give it some thought and discuss it on the list.
 >> At the meeting, we had agreement to include this feature in the
 >> REC / Proposed Draft, I'm waiting/hoping/thinking about if we can
 >> define it
 >> as an XPath expression as well.
 >> _________________________________________________________
 >> Joseph Reagle Jr.
 >> W3C Policy Analyst                mailto:reagle@w3.org
 >> IETF/W3C XML-Signature Co-Chair   http://www.w3.org/People/Reagle/

Joseph Reagle Jr.
W3C Policy Analyst                mailto:reagle@w3.org
IETF/W3C XML-Signature Co-Chair   http://www.w3.org/People/Reagle/
Received on Thursday, 11 May 2000 19:37:20 UTC

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