Exclusive XML Canonicalization
Version 1.0

W3C Working Draft 20 November 2001

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John Boyer, PureEdge Solutions Inc., jboyer@PureEdge.com
Donald E. Eastlake 3rd, Motorola, Donald.Eastlake@Motorola.com
Joseph Reagle, W3C, reagle@w3.org


Canonical XML [XML-C14N] recommends a standard means of serializing XML that, when applied to a subdocument, includes its namespace and some other XML context. However, for many applications, it is desirable to have a method which, to the extent practical, excludes such context from a canonicalized subdocument. In particular, where a digital signature over an XML subdocument is needed which will not break when that subdocument is removed from its original document and/or inserted into a different context. The Exclusive XML Canonicalization method described herein provides such a method.

Status of this document

This is the first official publication and Last Call for the "Exclusive XML Canonicalization" Working Draft from the IETF/W3C XML Signature Working Group (Activity Statement). This version satisfies the Working Groups requirement to exclude ancestor context from the canonicalized form of a subset of an XML document. This specification is an alternative to behaviour of the Canonical XML Recommendation; it is based on an implementation; the Working Group expects to move through last call and towards Recommendation quickly. The last call period is in 3 weeks, ending on 11 December 2001.

Publication of this document does not imply endorsement by the W3C membership. This is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to cite a W3C Working Draft as anything other than a "work in progress." Please send comments to the editors and cc: the list w3c-ietf-xmldsig@w3.org (archives).

Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page in conformance with W3C.

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

Table of Contents

  1. Introduction
    1. Terminology
    2. Applications
    3. Limitations
  2. The Need for Exclusive XML Canonicalization
    1. A Simple Example
    2. General Problems with Enveloping and de-Enveloping
  3. Specification of Exclusive XML Canonicalization
  4. Use in XML Security
  5. References
  6. Acknowledgements

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. It is normal for XML documents and subdocuments which are equivalent for the purposes of many applications to differ in their physical representation. For example, they may differ in their entity structure, attribute ordering, and character encoding. The goal of this specification is to establish a method for serializing an XPath node set representing a subdocument such that this method has the following properties:

  1. It is minimally affected by the XML context of the subdocument.
  2. If the input represents a well-formed XML document, then the output will be a well-formed XML document which will be unaltered by further applications of exclusive canonicalization.
  3. So far as practical, it can be determined whether two subdocuments are identical, or whether an application has not changed a subdocument, except for insignificant transformations permitted by XML 1.0 and Namespaces in XML, by comparing their exclusive canonicalization.

Complete familiarity with the Canonical XML Recommendation [XML-C14N] is assumed.

1.1 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 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). The nodes are included in or excluded from a node-set based on the evaluation of an expression. Within this specification and [XML-C14N], a node-set is used to directly indicate whether or not each node should be rendered in the canonical form (in this sense, it is used as a formal mathematical set). A node that is excluded from the set is not rendered in the canonical form being generated, even if its parent node is included in the node-set. However, an omitted node may still impact the rendering of its descendants (e.g. by affecting the namespace context of the descendants).

A document subset is a portion of an XML document indicated by an XPath node-set that may not include all of the nodes in the document. An apex node is an element node in a document subset having no element node ancestor in the document subset. An orphan node is a node whose parent element node is not in the document subset. The output parent of an orphan node that is not an apex node is the nearest ancestor element of the orphan node that is in the document subset. The output parent of a non-orphan node is the parent of the node.

An element E in a document subset visibly utilizes a namespace declaration, i.e. a namespace prefix P and bound value V, if E or an attribute node in the document subset with parent E has a qualified name in which P is the namespace prefix and V is the namespace name. A similar definition applies for an element E in a document subset that visibly utilizes the default namespace declaration, which occurs if E has no namespace prefix.

The namespace axis of an element contains nodes for all namespace declarations made within the element as well as namespace declarations inherited from ancestors of the element. Any subset of the nodes in a namespace axis can be included in a document subset.

The method of canonicalization described in this specification receives an InclusiveNamespacePrefix List parameter, which lists namespace prefixes that are handled in the manner described by the Canonical XML Recommendation [XML-C14N].

The exclusive canonical form of a document subset is a physical representation of the XPath node-set, that is an octet sequence, produced by the method described in this specification. It is as defined in the Canonical XML Recommendation [XML-C14N] except for the changes summarized as follows:

The term exclusive canonical XML refers to XML that is in exclusive canonical form. The exclusive XML canonicalization method is the algorithm defined by this specification that generates the exclusive canonical form of a given XML document subset. The term exclusive XML canonicalization refers to the process of applying the exclusive XML canonicalization method to an XML document subset.

1.2 Applications

The applications of Exclusive XML Canonicalization are very similar to those for Canonical XML [XML-C14N]. However, exclusive canonicalization, or equivalent means of excluding most XML context, is necessary for signature applications where the XML context of signed XML will change. This sort of change is typical of many protocol applications.

Note that in the case of the SignedInfo element of [XML-DSIG], the specification of an appropriate canonicalization method is the only technique available to protect the signature from insignificant changes in physical form and changes in XML context.

1.3 Limitations

Exclusive XML Canonicalization has the limitations of Canonical XML [XML-C14N] plus two additional limitations as follows:

  1. The XML being canonicalized may depend on the effect of xml namespace attributes, such as xml:lang and xml:space, appearing in ancestor nodes. To avoid problems due to the non-importation of such attributes into an enveloped document subset, either they must be explicitly given in the apex nodes of the XML document subset being canonicalized or they must always be declared with an equivalent value in every context in which the XML document subset will be interpreted.
  2. The XML being canonicalized may depend on the effect of XML namespace declarations where the namespace prefix being bound is not visibly utilized. An example would be an attribute whose value is an XPath expression and whose evaluation therefore depends upon namespace prefixes referenced in the expression. To avoid problems with such namespace declarations,

2. The Need for Exclusive XML Canonicalization

In some cases, particularly for signed XML in protocol applications, there is a need to canonicalize a subdocument in such a way that it is substantially independent of its XML context. This is because, in protocol applications, it is common to envelope XML in various layers of message or transport elements, to strip off such enveloping, and to construct new protocol messages, parts of which were extracted from different messages previously received. If the pieces of XML in question are signed, they need to be canonicalized in a way such that these operations do not break the signature but the signature still provides as much security as can be practically obtained.

2.1 A Simple Example

As a simple example of the type of problem that changes in XML context can cause for signatures, consider the following document:

   <n1:elem1 xmlns:n1="http://b.example">

this is then enveloped in another document:

   <n0:pdu xmlns:n0="http://a.example">
      <n1:elem1 xmlns:n1="http://b.example">

The first document above is in canonical form. But assume that document is enveloped as in the second case. The subdocument with elem1 as its apex node can be extracted from this second case with an XPath expression such as

 (//. | //@* | //namespace::*)[ancestor-or-self::n1:elem1]

The result of applying Canonical XML to the resulting XPath node set is the following (except for line wrapping to fit this document):

   <n1:elem1 xmlns:n0="http://a.example"

Note that the n0 namespace has been included by Canonical XML because it includes namespace context. This change which would break a signature over elem1 based on the first version.

2.2 General Problems with re-Enveloping

As a more complete example of the changes in canonical form that can occur when the enveloping context of a document subset is changed, consider the following document:

   <n0:local xmlns:n0="foo:bar"
      <n1:elem2 xmlns:n1="http://example.net"
          <n3:stuff xmlns:n3="ftp://example.org"/>

And the following which has been produced by changing the enveloping of elem2:

   <n2:pdu xmlns:n1="http://example.com"
      <n1:elem2 xmlns:n1="http://example.net"
          <n3:stuff xmlns:n3="ftp://example.org"/>

Assume an XPath node set produced from each case by applying the following XPath expression

 (//. | //@* | //namespace::*)[ancestor-or-self::n1:elem2]

Applying Canonical XML to the node set produced from the first document yields the following serialization (except for line wrapping to fit in this document):

   <n1:elem2 xmlns:n0="foo:bar"

However, although elem2 is represented by the same octet sequence in both pieces of external XML above, the Canonical XML version of elem2 from the second case would be (except for line wrapping so it will fit into this document) as follows:

   <n1:elem2 xmlns:n1="http://example.net"
       <n3:stuff xmlns:n3="ftp://example.org"></n3:stuff>

Note that the change in context has resulted in lots of changes in the subdocument as serialized by the inclusive Canonical XML [XML-C14N]. In the first example, n0 had been included from the context and the presence of an identical n3 namespace declaration in the context had elevated that declaration to the apex of the canonicalized form. In the second example, n0 has gone away but n2 has appeared, n3 is no longer elevated, and an xml:space declaration has appeared, due to changes in context. But not all context changes have effect. In the second example, the presence at ancestor nodes of an xml:lang and n1 prefix namespace declaration have no effect because of existing declarations at the elem2 node.

On the other hand, using Exclusive XML Canonicalization as specified herein, the physical form of elem2 as extracted by the XPath expression above is (except for line wrapping so it will fit into this document) as follows:

   <n1:elem2 xmlns:n1="http://example.net"
       <n3:stuff xmlns:n3="ftp://example.org"></n3:stuff>

in both cases.

3. Specification of Exclusive XML Canonicalization

The data model, processing, input parameters, and output data for Exclusive XML Canonicalization are the same as for Canonical XML [XML-C14N] with the following exceptions:

  1. Canonical XML applied to a document subset requires the search of the ancestor nodes of each orphan element node for attributes in the xml namespace, such as xml:lang and xml:space. These are copied into the element node except if a declaration of the same attribute is already in the attribute axis of the element (whether or not it is included in the document subset). This search and copying are omitted from the Exclusive XML Canonicalization method.
  2. The Exclusive XML Canonicalization method may receive an additional, possibly null, parameter InclusiveNamespacePrefix List containing a white space separated list of namespace prefixes. All namespace nodes appearing on this list are handled as provided in Canonical XML [XML-C14N]
  3. For namespace prefixes not appearing on the InclusiveNamespacePrefix List, a namespace declaration is output at every output element where that prefix is visibly utilized and an equivalent declartion is not made in an output parent.

One method for implementing the Exclusive XML Canonicalization method is as follows:

  1. Recursively process the entire XPath node-set in document order starting with the root. (The operation of copying ancestor xml: namespace attributes into output apex element nodes is not done.)
  2. If the node is not in the subset, process its children element nodes recusively.
  3. If the element node is in the XPath subset then output the node in accordance with Canonical XML except for namespace nodes which are rendered as follows:
    1. Render each namespace node iff it is
    2. Append the rendered namespace node to ns_rendered. Place ns_rendered on the stack and recurse.

    (Note, many XPath implementations do not distinguish namespace nodes from attribute nodes. Consequently, additional processing and stacks are required to seperate a namespace node list from the attribute node list and keep a stack of a list of namespace nodes in affect for one's parent.)

4. Use in XML Security

Exclusive Canonicalization may be used as a Transform or CanonicalizationMethod algorithm in XML Digital Signature [XML-DSig] and XML Encryption [XML-Enc].


This algorithm takes an optional explicit parameter of an empty InclusiveNamespacePrefix element with a List attribute. The value of this attribute, which may be null, is the list of namespace prefixes to be handled as per Canonical XML. The list is in NMTOKENS format (a white space separated list). For example:

      <InclusiveNamespacePrefix List="foo bar"/>

indicates the exclusive canonicalization transform, but that namespaces with prefix "foo" or "bar" should be processed according to [XML-C14N].

   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:dec CDATA #FIXED 'http://www.w3.org/2001/10/xml-exc-c14n#'>
      <!ENTITY dec 'http://www.w3.org/2001/10/xml-exc-c14n#'> 
      <!ENTITY % p ''>
      <!ENTITY % s ''>

   <schema xmlns="http://www.w3.org/2001/XMLSchema"
           version="0.1" elementFormDefault="qualified">

   <element name="InclusiveNamespaces"
   <complexType name="InclusiveNamespaces">
      <attribute name="PrefixList" type="NMTOKENS"/>
   <!ELEMENT InclusiveNamespaces    EMPTY >
   <!ATTLIST InclusiveNamespaces
      PrefixList    NMTOKENS    #REQUIRED >

5. References

RFC 2119. Key words for use in RFCs to Indicate Requirement Levels. Best Current Practice. S. Bradner. March 1997.S. Bradner. March 1997.
RFC 2396 . Uniform Resource Identifiers (URI): Generic Syntax. T. Berners-Lee, R. Fielding, L. Masinter. August 1998.
Extensible Markup Language (XML) 1.0 (Second Edition). W3C Recommendation. T. Bray, E. Maler, J. Paoli, C. M. Sperberg-McQueen. October 2000.
http://www.w3.org/TR/2000/REC-xml-20001006 .
Canonical XML. W3C Recommendation. J. Boyer. March 2001.
XML-Signature Syntax and Processing. IETF Draft/W3C Proposed Recommendation. D. Eastlake, J. Reagle, and D. Solo. 31 August 2001.
Namespaces in XML. Recommendation. T. Bray, D. Hollander, and A. Layman. Janaury 1999.
XML Encryption Syntax and Processing. D. Eastlake, and J. Reagle. W3C Working Draft. October 2001.


XML Path Language (XPath) Version 1.0 , W3C Recommendation. eds. James Clark and Steven DeRose. 16 November 1999. http://www.w3.org/TR/1999/REC-xpath-19991116.

5. Acknowledgements (Informative)

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