W3C

XML-Signature XPath Filter 2.0

W3C Recommendation 08 November 2002

This version:
http://www.w3.org/TR/2002/REC-xmldsig-filter2-20021108/
Latest version:
http://www.w3.org/TR/xmldsig-filter2/
Previous version:
http://www.w3.org/TR/2002/PR-xmldsig-filter2-20020827/
Authors/Editors:
John Boyer, PureEdge Solutions Inc., <[email protected]>
Merlin Hughes, Baltimore Technologies Ltd., <[email protected]>
Joseph Reagle, W3C, <[email protected]>

Please see the errata for this document, which may include some normative corrections. See also translations.


Abstract

XML Signature [XML-DSig] 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.

Status of this document

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.

This specification was produced by the IETF/W3C XML Signature Working Group (W3C Activity Statement) which believes the specification is sufficient for the creation of independent interoperable implementations as demonstrated in the Interoperability Report.

Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page, in conformance with W3C policy, and the IETF Page of Intellectual Property Rights Notices, in conformance with IETF policy. At the time of publication, there are no declarations specific to this document.

Please report errors in this document to [email protected] (archive).

The list of known errors in this specification is available at http://www.w3.org/2002/11/xpath-filter2-errata.

The English version of this specification is the only normative version. Information about translations of this document (if any) is available http://www.w3.org/Signature/2002/02/xmldsig-translations

A list of current W3C Technical Reports can be found at http://www.w3.org/TR/.

Table of Contents

  1. Introduction
    1. Acknowledgements
  2. Terminology
  3. Specification of Signature Filter Transform
    1. Algorithm Identifier
    2. Syntax of Signature Filter Transform
    3. Input and Evaluation Context of Signature Filter Transform
    4. Processing Model of Signature Filter Transform
  4. Examples of Signature Filter Transform
  5. References

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:

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.

1.1. Acknowledgements (Informative)

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

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 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:

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:

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:

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 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. References

Keywords
RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. Best Current Practice, March 1997.
http://www.ietf.org/rfc/rfc2119.txt
URI
RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax. T. Berners-Lee, R. Fielding, and L. Masinter. Standards Track, August 1998.
http://www.ietf.org/rfc/rfc2396.txt
XML
Extensible Markup Language (XML) 1.0 (Second Edition). T. Bray, E. Maler, J. Paoli, and C. M. Sperberg-McQueen. W3C Recommendation, October 2000.
http://www.w3.org/TR/2000/REC-xml-20001006 .
XML-C14N
Canonical XML. J. Boyer. W3C Recommendation, March 2001.
http://www.w3.org/TR/2001/REC-xml-c14n-20010315
http://www.ietf.org/rfc/rfc3076.txt
XML DSig
XML-Signature Syntax and Processing. D. Eastlake, J. Reagle, and D. Solo. W3C Recommendation, February 2002.
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.
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.
http://www.w3.org/TR/1999/REC-xpath-19991116
XPointer
XML Pointer Language (XPointer). S. DeRose, R. Daniel, and E. Maler. W3C Candidate Recommendation, January 2001.
http://www.w3.org/TR/2001/CR-xptr-20010911/