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RFC 3653 - XML-Signature XPath Filter 2.0


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Network Working Group                                           J. Boyer
Request for Comments: 3653                            PureEdge Solutions
Category: Informational                                        M. Hughes
                                                         Betrusted, Inc.
                                                               J. Reagle
                                                                     W3C
                                                           December 2003

                     XML-Signature XPath Filter 2.0

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   XML Signature recommends a standard means for specifying information
   content to be digitally signed and for representing the resulting
   digital signatures in XML.  Some applications require the ability to
   specify a subset of a given XML document as the information content
   to be signed.  The XML Signature specification meets this requirement
   with the XPath transform.  However, this transform can be difficult
   to implement efficiently with existing technologies.  This
   specification defines a new XML Signature transform to facilitate the
   development of efficient document subsetting implementations that
   interoperate under similar performance profiles.

   This document is the W3C XML Signature XPath-Filter 2.0
   Recommendation.  This document has been reviewed by W3C Members and
   other interested parties and has been endorsed by the Director as a
   W3C Recommendation.  It is a stable document and may be used as
   reference material or cited as a normative reference from another
   document.  W3C's role in making the Recommendation is to draw
   attention to the specification and to promote its widespread
   deployment.  This enhances the functionality and interoperability of
   the Web.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  Acknowledgements (Informative) . . . . . . . . . . . .  4
       1.2.  W3C Status . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology. . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Specification of Signature Filter Transform. . . . . . . . .  5
       3.1.  Algorithm Identifier . . . . . . . . . . . . . . . . .  5
       3.2.  Syntax of Signature Filter Transform . . . . . . . . .  5
       3.3.  Input and Evaluation Context of Signature Filter
             Transform. . . . . . . . . . . . . . . . . . . . . . .  7
       3.4.  Processing Model of Signature Filter Transform . . . .  7
   4.  Examples of Signature Filter Transform . . . . . . . . . . .  9
   5.  Normative References . . . . . . . . . . . . . . . . . . . . 13
   6.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
   7.  Full Copyright Statement . . . . . . . . . . . . . . . . . . 15

1.  Introduction

   The XML Recommendation [XML] specifies the syntax of a class of
   objects called XML documents.  The Namespaces in XML Recommendation
   [XML-NS] specifies additional syntax and semantics for XML documents.
   The XML Signature Recommendation [XML-DSig] defines standard means
   for specifying information content to be digitally signed, including
   the ability to select a portion of an XML document to be signed using
   an XPath transform.

   This specification describes a new signature filter transform that,
   like the XPath transform [XML-DSig, section 6.6.3], provides a method
   for computing a portion of a document to be signed.  In the interest
   of simplifying the creation of efficient implementations, the
   architecture of this transform is not based on evaluating an [XPath]
   expression for every node of the XML parse tree (as defined by the
   [XPath] data model).  Instead, a sequence of XPath expressions is
   used to select the roots of document subtrees -- location sets, in
   the language of [XPointer] -- which are combined using set
   intersection, subtraction and union, and then used to filter the
   input node-set.  The principal differences from the XPath transform
   are:

      *  A sequence of XPath operations can be executed in a single
         transform, allowing complex filters to be more easily expressed
         and optimized.
      *  The XPath expressions are evaluated against the input document
         resulting in a set of nodes, instead of being used as a boolean
         test against each node of the input node-set.

      *  To increase efficiency, the expansion of a given node to
         include all nodes having the given node as an ancestor is now
         implicit so it can be performed by faster means than the
         evaluation of an XPath expression for each document node.
      *  The resulting node-sets can be combined using the three
         fundamental set operations (intersection, subtraction, and
         union), and then applied as a filter against the input node-
         set, allowing operations such as signing an entire document
         except for a specified subset, to be expressed more clearly and
         efficiently.

   As with the original XPath transform, the primary purpose of this
   transform is to ensure that only specifically defined changes to the
   input XML document are permitted after the signature is affixed.
   This can be done by excluding precisely those nodes that are allowed
   to change once the signature is affixed, and including all other
   input nodes in the output.  It is the responsibility of the signature
   filter transform author to ensure that nodes are not excluded which
   could affect the interpretation of the transform output in the
   application context.

   Consider the motivating scenario where an application wishes to affix
   two enveloped signatures to the document; any other change to the
   document must cause the signatures to be invalid.  When the
   application creates the first signature that signature is
   automatically omitted from its own digest calculations.  However, it
   will also be necessary to exclude the subsequent (second) signature
   element from the digest calculations of the first signature.  This
   specification can be used to efficiently satisfy this requirement
   using the set subtraction operation.

   This transform also supports the ability to specify a set of nodes
   that will be included in a signature, with all non-specified nodes
   being excluded.  This formulation is useful for isolating a portion
   of a document, such as a chapter of a document, or a payload in a
   protocol message, and can be expressed using the set intersection
   operation.

   Complete familiarity with the first XML Signature XPath Transform
   [XML-DSig, section 6.6.3] is required.

   NOTE: Since XPath Filter 2.0 depends on details of XPath, be sure to
   take into account the XPath Errata at
   <http://www.w3.org/1999/11/REC-xpath-19991116-errata>.

1.1.  Acknowledgements (Informative)

   The following people provided valuable feedback that improved the
   quality of this specification:

      *  Christian Geuer-Pollmann, Universitat Siegen
      *  Donald Eastlake, 3rd, Motorola
      *  Gregor Karlinger, IAK TU Graz
      *  Aleksey Sanin

1.2.  W3C Status

   The World Wide Web Consortium Recommendation corresponding to this
   RFC is at:

      http://www.w3.org/TR/xmldsig-filter2/

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [Keywords].

   The XPath 1.0 Recommendation [XPath] defines the term node-set as
   "(an unordered collection of nodes without duplicates)" and specifies
   a data model for representing an input XML document as a set of nodes
   of various types (element, attribute, namespace, text, comment,
   processing instruction, and root).

   An input document is the document that contains all the nodes
   available to processing by this transform.  A document subset is a
   portion of an XML document indicated by an XPath node-set, which may
   not include all of the nodes in the document.  For example, the input
   node-set is a collection of XPath nodes from the input document that
   is passed as a parameter to this transform.  A subtree rooted by a
   given node is a document subset containing the given node and every
   node having the given node as an ancestor.  Subtree expansion is the
   process of expanding a node-set to include all subtrees rooted at any
   node in the node-set.  For example, the subtree expansion of a node-
   set consisting of just a single element node would be a node-set
   containing that element, its attribute nodes, namespace nodes, and
   all its descendants including their attribute nodes and namespaces
   nodes.

   The XML Signature Recommendation [XML-DSig] defines a reference as a
   sequence of steps performed to obtain an octet stream to be digitally
   signed.  A transform is an identified algorithm to be used as a step

   in the reference processing model.  A transform takes an octet stream
   or XPath node-set as input, and it produces an octet stream or XPath
   node-set as output (the reference processing model automatically
   converts the final output to an octet stream if it is an XPath node-
   set).

3.  Specification of Signature Filter Transform

   The transform operates by computing a node-set that is used to filter
   the input node-set: The output node-set consists of only those nodes
   in both the input node-set and the filter node-set.  In other words,
   the output node-set is the intersection of the input node-set and the
   computed filter node-set.

   The filter node-set is computed by evaluating a sequence of XPath
   expressions and combining their results.  A node-set is initially
   computed containing the entire input document.  In sequence, each
   XPath expression is then evaluated, subtree-expanded, and then used
   to transform the filter node-set according to a specified set
   operation; intersection, subtraction, or union.  After all XPaths
   have been applied, the resulting node-set is used as the filter
   node-set.

3.1.  Algorithm Identifier

   The XML Signature Recommendation [XML-DSig] uses a [URI] to identify
   each algorithm to be performed when creating or validating a
   signature.  The signature filter transform is identified as follows:

   Algorithm Identifier
        http://www.w3.org/2002/06/xmldsig-filter2

3.2.  Syntax of Signature Filter Transform

   The signature filter transform shall be represented by a sequence of
   one or more elements named XPath.  The content of XPath is character
   data containing an XPath expression.  The XPath has an attribute
   named Filter whose possible values are intersect, subtract, and
   union.  The Filter attribute indicates the set operation that is
   performed with the resulting node-set when computing the filter
   node-set.  The following is an example of markup for a signature
   filter that signs the entire input node-set except for elements with
   identifier foo and bar (and all nodes with one of those elements as
   an ancestor):

 <XPath Filter="subtract"
  xmlns="http://www.w3.org/2002/06/xmldsig-filter2">
    id("foo bar")

 </XPath>

 Schema Definition:

 <?xml version="1.0" encoding="utf-8"?>
 <!DOCTYPE schema

  PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
         "http://www.w3.org/2001/XMLSchema.dtd"
 [
   <!ATTLIST schema
     xmlns:xf CDATA #FIXED 'http://www.w3.org/2002/06/xmldsig-filter2'>
   <!ENTITY xf 'http://www.w3.org/2002/06/xmldsig-filter2'>
   <!ENTITY % p ''>
   <!ENTITY % s ''>
  ]>

 <schema xmlns="http://www.w3.org/2001/XMLSchema"
         xmlns:xf="http://www.w3.org/2002/06/xmldsig-filter2"
         targetNamespace="http://www.w3.org/2002/06/xmldsig-filter2"
         version="0.1" elementFormDefault="qualified">

 <element name="XPath"
          type="xf:XPathType"/>

 <complexType name="XPathType">
  <simpleContent>
    <extension base="string">
      <attribute name="Filter">
         <simpleType>
           <restriction base="string">
             <enumeration value="intersect"/>
             <enumeration value="subtract"/>
             <enumeration value="union"/>
           </restriction>
         </simpleType>
      </attribute>
    </extension>
  </simpleContent>
 </complexType>

 </schema>

 DTD:

 <!ELEMENT XPath    (#PCDATA) >
 <!ATTLIST XPath
    Filter         (intersect|subtract|union) #REQUIRED >

3.3.  Input and Evaluation Context of Signature Filter Transform

   The input required by this transform is an XPath node-set over the
   input document.  If the input document is an octet stream, then the
   application MUST convert the octet stream to an XPath node-set that
   contains all of the document nodes (including comment nodes).  The
   evaluation context for the XPath expressions in the filter transform
   will be:

      *  A context node equal to the root node of the document whose
         node-set was provided as input to this transform.  The root
         node is the parent of the document element and any comment and
         processing instruction nodes outside of the document element.
      *  A context position, initialized to 1.
      *  A context size, initialized to 1.
      *  A library of functions equal to the function set defined in
         [XPath] plus a function named here().
      *  A set of variable bindings.  No means for initializing these is
         defined.  Thus, the set of variable bindings used when
         evaluating the XPath expression is empty, and use of a variable
         reference in the XPath expression results in an error.
      *  The set of namespace declarations in scope for the XPath
         element.

   The function here() is defined as follows:

   Function: node-set here()

      The here() function returns a node-set containing the attribute or
      processing instruction node or the parent element of the text node
      that directly bears the XPath expression.  In this transform, this
      will be the XPath element.  This expression results in an error if
      the containing XPath expression does not appear in the same XML
      document against which the XPath expression is being evaluated.

3.4.  Processing Model of Signature Filter Transform

   Using the aforementioned evaluation context, the signature filter
   transform evaluates the XPath expressions appearing in the character
   content of the XPath elements and uses these to compute a filter
   node-set F, which is then used to filter the input node-set I
   resulting in an output node-set O:

      *  Initialize the filter node-set F to consist of all nodes in the
         input document.
      *  Iterate through each XPath expression, X, in sequence, and
         update the filter node-set F as follows:

         o  Evaluate the XPath expression X.  The result is a node-set
            S.
         o  Compute the set S' consisting of all nodes in the input
            document that are either present in S or that have an
            ancestor in S.  This is equal to the union of all the
            document subtrees rooted by a node in S.
         o  If the Filter attribute value is intersect, then compute the
            intersection of the selected subtrees, S', with the filter
            node-set F.  The result will include only those nodes that
            are in both the filter node-set and the selected subtrees:
            F' = F INTERSECT S'.
         o  If the Filter attribute value is subtract, then compute the
            subtraction of the selected subtrees, S', from the filter
            node-set F.  The result will include only those nodes that
            are in the filter node-set, but not the selected subtrees:
            F' = F - S'.
         o  Otherwise, if the Filter attribute value is union, then
            compute the union the selected subtrees, S', with the filter
            node-set F.  The result will include all those nodes that
            are in either the filter node-set, the selected subtrees, or
            both: F' = F UNION S'.
         o  Update the filter node-set F to be the new node-set F'.
      *  Finally, after applying all the XPath expressions, compute the
         output node-set O to be the intersection of the computed filter
         node-set, F, with the input node-set, I.  The result will
         include all nodes from the input node-set that are also in the
         filter node-set: O = I INTERSECT F.
      *  An empty input node-set will always result in an empty output
         node-set.

   In this processing model, the conversion from a subtree
   interpretation of the XPath expressions to a node-set containing all
   nodes that must be used during the set operation, along with actual
   performance of the set operation, is described explicitly.
   Implementors SHOULD observe that it is possible to compute the
   effective result of this operation in a single pass through the input
   document without performing subtree expansion or any set operations:

      *  For each XPath expression X, in sequence, evaluate the
         expression and store the resulting node-set, S, along with the
         associated set operation.
      *  Prepend a node-set consisting of just the document node, along
         with the operation union.
      *  Create a new, empty filter node-set.
      *  Process each node in the input node-set document, adding each
         node to the output node-set F if a flag Z is true.  The flag is
         computed as follows:

         o  Z is true if and only if the node is present in any
            subtree-expanded union node-set and all subsequent subtree-
            expanded intersect node-sets but no subsequent subtree-
            expanded subtract node-sets, or false otherwise.  If there
            are no subsequent intersect or subtract node-sets, then that
            part of the test is automatically passed.
         o  Presence in a subtree-expanded node-set can be efficiently
            determined without actually expanding the node-set, by
            simply maintaining a stack or count that identifies whether
            any nodes from that node-set are an ancestor of the node
            being processed.

   Implementers MAY further observe that, if this transform is followed
   by a canonicalization operation (e.g., [XML-C14N]), the described
   filter computation can be efficiently commingled with the document-
   order canonicalization processing.

4.  Examples of Signature Filter Transform

   The example below illustrates one way to create an enveloped
   signature with the signature filter transform.  The function here()
   identifies the XPath element, and the subsequent location path
   obtains the nearest ancestor Signature element.  Due to the subtract
   value of the Filter attribute, the output of the signature filter
   transform is a node-set containing every node from the input node-set
   except the nodes in the subtree rooted by the Signature element
   containing the example signature filter transform below.

   <XPath Filter="subtract"
    xmlns="http://www.w3.org/2002/06/xmldsig-filter2"
    xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">
      here()/ancestor::dsig:Signature[1] </XPath>

   A suitable signature reference URI to use with this subtract filter
   would be URI="" (the entire signature document, without comments),
   URI="#xpointer(/)" (the entire signature document, with comments) or
   any same-document reference that includes the signature itself.

   An example of an intersect filter is a signature that co-signs
   another signature.  In this example, a Signature element identified
   by PrimaryBorrowSig must be signed.  The XPath expression obtains the
   element node, and the transform expands the output node-set to
   contain all nodes from the input node-set that are also in the
   subtree rooted by the element node.

   <XPath Filter="intersect"
    xmlns="http://www.w3.org/2002/06/xmldsig-filter2">
      id("PrimaryBorrowerSig") </XPath>

   This type of intersect filter is useful for efficiently signing
   subsets of a document, whether this is the same document as the
   signature or an external document.  For example, if the signature
   reference URI is URI="document.xml", then this document will be
   automatically parsed and just the identified element and its
   descendants will be signed.

   Union filters, by themselves are of no particular use: The initial
   filter node-set consists of the entire input document; any union with
   this will have no effect, so the output of the transform will be
   identical to the input.  The union operation is intended to follow a
   subtract operation, to allow a subtree to be removed, with the
   exception of a lower subtree which is still included in the output.

   Consider the following document which contains a same-document
   enveloped signature reference with an XPath filter containing three

   XPath operations:

   <Document>
     <ToBeSigned>
       <!-- comment -->
       <Data />
       <NotToBeSigned>
         <ReallyToBeSigned>
           <!-- comment -->
           <Data />
         </ReallyToBeSigned>
       </NotToBeSigned>
     </ToBeSigned>
     <ToBeSigned>
       <Data />
       <NotToBeSigned>
         <Data />
       </NotToBeSigned>
     </ToBeSigned>
     <dsig:Signature
      xmlns:dsig="http://www.w3.org/2000/09/xmldsig#"
      xmlns:dsig-xpath="http://www.w3.org/2002/06/xmldsig-filter2">
       <dsig:SignedInfo>
         ...
         <dsig:Reference URI="">
           <dsig:Transforms>
             <dsig:Transform
              Algorithm="http://www.w3.org/2002/06/xmldsig-filter2">
               <dsig-xpath:XPath
                Filter="intersect"> //ToBeSigned </dsig-xpath:XPath>
               <dsig-xpath:XPath

                Filter="subtract"> //NotToBeSigned </dsig-xpath:XPath>
               <dsig-xpath:XPath
                Filter="union"> //ReallyToBeSigned </dsig-xpath:XPath>
             </dsig:Transform>
           </dsig:Transforms>
           ...
         </dsig:Reference>
       </dsig:SignedInfo>
       ...
     </dsig:Signature> </Document>

   The intersect operation computes the intersection of the XPath-
   selected subtrees with the filter node-set.  In this case, the filter
   node-set initially contains the entire input document, and the XPath
   expression evaluates to the two ToBeSigned elements; these are
   expanded to include all their descendents and intersected with the
   filter node-set, resulting in the following:

     <ToBeSigned>
       <!-- comment -->
       <Data />
       <NotToBeSigned>
         <ReallyToBeSigned>
           <!-- comment -->
           <Data />
         </ReallyToBeSigned>
       </NotToBeSigned>
     </ToBeSigned><ToBeSigned>
       <Data />
       <NotToBeSigned>
         <Data />
       </NotToBeSigned>
     </ToBeSigned>

   The subtract filter computes the subtraction of the XPath-selected
   subtrees from the filter node-set.  In this case, the XPath
   expression evaluates to the two NotToBeSigned elements; these are
   expanded to include all their descendents and subtracted from the
   filter node-set:

   <ToBeSigned>
       <!-- comment -->
       <Data />

     </ToBeSigned><ToBeSigned>
       <Data />

   </ToBeSigned>

   Next, the union filter computes the union of the XPath-selected
   subtrees with the filter node-set.  In this case, the XPath
   expression evaluates to the ReallyToBeSigned element; this is
   expanded to include all its descendents and added to the filter
   node-set:

   <ToBeSigned>
       <!-- comment -->
       <Data />
       <ReallyToBeSigned>
           <!-- comment -->
           <Data />
         </ReallyToBeSigned>
     </ToBeSigned><ToBeSigned>
       <Data />

   </ToBeSigned>

   Finally, this resulting filter node-set is used to transform the
   input node-set.  In this example, the input node-set is the entire
   document, with comments removed.  The transformed node-set will thus
   be all those nodes from the input document, less comments, that are
   also in the filter node-set:

   <ToBeSigned>

       <Data />
       <ReallyToBeSigned>

           <Data />
         </ReallyToBeSigned>
     </ToBeSigned><ToBeSigned>
       <Data />

   </ToBeSigned>

   Note that the result contains no nodes that were not in the input
   node-set.  Although the filter node-set included comments, these were
   not present in the input node-set so they are not present in the
   output node-set.

   This signature filter does not provide any increased capability over
   the original XPath transform.  For example, this reference could be
   replicated using the XPath transform as follows.

   <dsig:Reference URI="">
     <dsig:Transforms>
       <dsig:Transform

        Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
         <dsig:XPath>
           (ancestor-or-self::ToBeSigned and
            not (ancestor-or-self::NotToBeSigned))
           or ancestor-or-self::ReallyToBeSigned
         </dsig:XPath>
       </dsig:Transform>
     </dsig:Transforms>
     ...  </dsig:Reference>

   The advantage of the signature filter transform over the XPath
   transform is that the latter requires evaluation of a potentially-
   complex expression against every node in the input set, which has
   proved costly in practice for many useful operations.  This
   specification's filter requires evaluation of simple XPath
   expressions and then the execution of some basic set operations or
   their equivalent, which can be implemented significantly more
   efficiently.

5.  Normative References

   [Keywords]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [URI]       Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
               Resource Identifiers (URI): Generic Syntax", RFC 2396,
               August 1998.

   [XML]       "Extensible Markup Language (XML) 1.0 (Second Edition)",
               T.  Bray, E.  Maler, J. Paoli, and C. M. Sperberg-
               McQueen.  W3C Recommendation, October 2000.  Available at
               <http://www.w3.org/TR/2000/REC-xml-20001006>.

   [XML-C14N]  Boyer, J., "Canonical XML", RFC 3076, March 2001.  Also a
               W3C Recommendation available at
               <http://www.w3.org/TR/2001/REC-xml-c14n-20010315>.

   [XML-DSig]  Eastlake, J., Reagle, J. and D. Solo, "XML-Signature
               Syntax and Processing", RFC 3275, March 2002.  Also a W3C
               Recommendation available at
               <http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/>.

   [XML-NS]    "Namespaces in XML", T. Bray, D. Hollander, and A.
               Layman.  W3C Recommendation, January 1999.  Available at
               <http://www.w3.org/TR/1999/REC-xml-names-19990114/>.

   [XPath]     "XML Path Language (XPath) Version 1.0", J. Clark and S.
               DeRose.  W3C Recommendation, November 1999.  Available at
               <http://www.w3.org/TR/1999/REC-xpath-19991116>. (Note
               also XPath Errata at <http://www.w3.org/1999/11/REC-
               xpath-19991116-errata>.)

   [XPointer]  "XML Pointer Language (XPointer)", S. DeRose, R. Daniel,
               and E. Maler.  W3C Candidate Recommendation, January
               2001.  Available at <http://www.w3.org/TR/2001/CR-xptr-
               20010911/>.

6.  Authors' Addresses

   John Boyer
   PureEdge Solutions Inc.
   4396 West Saanich Rd.
   Victoria, BC, Canada V8Z 3E9

   Phone: +1-888-517-2675
   EMail: jboyer@PureEdge.com

   Merlin Hughes
   Betrusted, Inc.
   11000 Broken Land Parkway Suite 900
   Columbia, MD 21044

   Phone: +1-443-367-7000
   EMail: Merlin.Hughes@betrusted.com

   Joseph M. Reagle Jr., W3C
   Massachusetts Institute of Technology
   Laboratory for Computer Science
   NE43-350, 545 Technology Square
   Cambridge, MA 02139

   Phone: +1.617.258.7621
   EMail: reagle@mit.edu

7.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

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