Exclusive XML Canonicalization
Version 1.0

Abstract

Canonical XML [XML-C14N] specifies a standard serialization of XML that, when applied to a subdocument, includes the subdocument's ancestor context including all of the namespace declarations and attributes in the "xml:" namespace. However, some applications require a method which, to the extent practical, excludes unused ancestor context from a canonicalized subdocument. For example, one might require a digital signature over an XML payload (subdocument) in an XML message that will not break when that subdocument is removed from its original message and/or inserted into a different context. This requirement is satisfied by Exclusive XML Canonicalization.

Status of this document

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. Security Considertions
6. References
7. Acknowledgements

1. Introduction

The XML Recommendation [XML] specifies the syntax of a class of objects called XML documents. The Names 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 the XPath node set representation of an XML document or subset such that:

1. The node set is minimally affected by any XML context which has been omitted.
2. The canonicalization of a nodeset representing well-balanced XML [XML-Fragment] will be unaltered by further applications of exclusive canonicalization.
3. It can be determined whether two node sets are identical except for transformations considered insignificant by this specification under [XML,XML-NS].

An understanding of the Canonical XML Recommendation [XML-C14N] is required.

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 an element 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; an apex node has no output parent. The output parent of a non-orphan node is the parent of the node. An output ancestor is any ancestor element node in the document subset.

For example in a document tree with three generations under the root node A and where capitalization denotes the node is in the document subset (A,E,G):

A-+-b
  `-c-+-d
      `-E-+-f
          `-G

• A is an apex node, output parent of E, and output ancestor of (E,G);
• E is an orphan node and the output parent of G.

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. 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 non-default namespace declarations made within the element as well as non-default namespace declarations inherited from ancestors of the element. The namespace axis also contains a node representing the default namespace if it is not the empty string, whether the default namespace was declared within the element or by an ancestor 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 InclusiveNamespaces PrefixList 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:

• attributes in the XML namespace, such as xml:lang and xml:space are not imported into orphan nodes of the document subset, and
• namespace nodes that are not on the InclusiveNamespaces PrefixList are expressed only in start tags where they are visible and if they are not in effect from an output ancestor of that tag

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,
   • the XML must be modified so that use of the namespace prefix involved is visible or
   • the namespace declarations must appear and be bound to the same values in every context in which the XML will be interpreted or
   • the prefixes for such namespaces must appear in the InclusiveNamespaces PrefixList.

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="https://b.example">
       content
   </n1:elem1>

this is then enveloped in another document:

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

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="https://a.example"
             xmlns:n1="https://b.example">
       content
   </n1:elem1>

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"
             xmlns:n3="ftp://example.org">
      <n1:elem2 xmlns:n1="https://example.net"
                xml:lang="en">
          <n3:stuff xmlns:n3="ftp://example.org"/>
      </n1:elem2>
   </n0:local>

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

   <n2:pdu xmlns:n1="https://example.com"
           xmlns:n2="https://foo.example"
           xml:lang="fr"
           xml:space="retain">
      <n1:elem2 xmlns:n1="https://example.net"
                xml:lang="en">
          <n3:stuff xmlns:n3="ftp://example.org"/>
      </n1:elem2>
   </n2:pdu>

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"
             xmlns:n1="https://example.net"
             xmlns:n3="ftp://example.org"
             xml:lang="en">
       <n3:stuff></n3:stuff>
   </n1:elem2>

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="https://example.net"
             xmlns:n2="https://foo.example"
             xml:lang="en"
             xml:space="retain">
       <n3:stuff xmlns:n3="ftp://example.org"></n3:stuff>
   </n1:elem2>

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="https://example.net"
             xml:lang="en">
       <n3:stuff xmlns:n3="ftp://example.org"></n3:stuff>
   </n1:elem2>

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 InclusiveNamespace-PrefixList containing a list of namespace prefixes and/or a token indicating the presense of the default namespace. All namespace nodes appearing on this list are handled as provided in Canonical XML [XML-C14N].
3. For every namespace node with a prefix that does not appear in the InclusiveNamespacePrefix List, a namespace declaration is output at every output element where that prefix is visibly utilized and that prefix and its value does not appear in the nearest output ancestor with the same prefix.

One method (non-normative) for implementing the Exclusive XML Canonicalization method is as follows:

1. Recursively process the entire tree (from which the XPath node-set was selected) 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 XPath subset, process its children element nodes recursively.
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 visibly utilized by the immediate parent element or one of its attributes, or is present in InclusiveNamespaces PrefixList, and
      • its prefix and value do not appear in ns_rendered. ns_rendered is obtained by popping the state stack in order to obtain a list of prefixes and their values which have already been rendered by an output ancestor of the namespace node's parent element.
   2. Append the rendered namespace node to the list ns_rendered of namespace nodes rendered by output ancestors. Push ns_rendered on state stack and recurse.
   3. After the recursion returns, pop thestate stack.

   (Note, some XPath implementations do not correctly distinguish namespace nodes from attribute nodes. In [XPath] an element's namespace axis is distinct from attribute nodes and it contains all the namespace declarations from its ancestors. For non-conformant implementations additional processing and stacks are required to separate a namespace node list from the attribute node list and keep a stack of a list of namespace nodes in effect 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].

Identifier:

https://www.w3.org/2001/10/xml-exc-c14n#

https://www.w3.org/2001/10/xml-exc-c14n#WithComments

Just as with [XML-C14N] one may use the "#WithComments" parameter to include the serialization of XML comments. This algorithm also takes an optional explicit parameter of an empty InclusiveNamespaces element with a PrefixList attribute. The value of this attribute, which may be null, is a whitespace delimited list of namespace prefixes, and where #default indicates the default namespace, to be handled as per [XML-C14N]. The list is in NMTOKENS format (a white space separated list). For example:

   <ds:Transform
      Algorithm="https://www.w3.org/2001/10/xml-exc-c14n#">
      <ec:InclusiveNamespaces PrefixList="dsig soap #default"
          xmlns:ec="https://www.w3.org/2001/10/xml-exc-c14n#"/>
   </ds:Transform>

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

   Schema Definition:
   <?xml version="1.0" encoding="utf-8"?>
   <!DOCTYPE schema
     PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "https://www.w3.org/2001/XMLSchema.dtd"
    [
      <!ATTLIST schema 
        xmlns:ec CDATA #FIXED 'https://www.w3.org/2001/10/xml-exc-c14n#'>
      <!ENTITY ec 'https://www.w3.org/2001/10/xml-exc-c14n#'> 
      <!ENTITY % p ''>
      <!ENTITY % s ''>
     ]>
   <schema xmlns="https://www.w3.org/2001/XMLSchema"
           xmlns:ec="https://www.w3.org/2001/10/xml-exc-c14n#"
           targetNamespace="https://www.w3.org/2001/10/xml-exc-c14n#"
           version="0.1" elementFormDefault="qualified">
   <element name="InclusiveNamespaces"
            type="ec:InclusiveNamespaces"/>
   <complexType name="InclusiveNamespaces">
      <attribute name="PrefixList" type="NMTOKENS"/>
   </complexType>

   DTD:
   <!ELEMENT InclusiveNamespaces    EMPTY >
   <!ATTLIST InclusiveNamespaces
      PrefixList    NMTOKENS    #REQUIRED >

5. Security Considerations

This specification is used to serialize an XPath nodeset under certain assumptions given in [XML-C14N] and this specification. For example, implementations of [XML-C14N] do not render a document XML declaration; when implementations of this specification serialize a subset they do not render ancestor attributes from the "xml:" namespace. While we feel such choices are consistent with other XML specifications and satisfy our application requirements it is important that an XML application carefully construct its transforms such that the result is meaningful and unambigous in its application context. The Resolutions of [XML-C14N] and the Security Considerations of [XML-DSig] should be carefully attended to.

6. References

Keywords

RFC 2119. Key words for use in RFCs to Indicate Requirement Levels. Best Current Practice. S. Bradner. March 1997.S. Bradner. March 1997.

https://www.ietf.org/rfc/rfc2119.txt

URI

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

https://www.ietf.org/rfc/rfc2396.txt

XML

Extensible Markup Language (XML) 1.0 (Second Edition). W3C Recommendation. T. Bray, E. Maler, J. Paoli, C. M. Sperberg-McQueen. October 2000.

https://www.w3.org/TR/2000/REC-xml-20001006 .

XML-C14N

Canonical XML. W3C Recommendation. J. Boyer. March 2001.

https://www.w3.org/TR/2001/REC-xml-c14n-20010315

https://www.ietf.org/rfc/rfc3076.txt

XML-DSig

XML-Signature Syntax and Processing. IETF Draft/W3C Proposed Recommendation. D. Eastlake, J. Reagle, and D. Solo. 31 August 2001.

https://www.w3.org/TR/2001/PR-xmldsig-core-20010820/

XML-Fragment

XML Fragment Interchange. W3C Candidate Recommendation. P. Grosso, D. Veillard. February 2001.

https://www.w3.org/TR/2001/CR-xml-fragment-20010212

XML-NS

Namespaces in XML. W3C Recommendation. T. Bray, D. Hollander, and A. Layman. January 1999.

https://www.w3.org/TR/1999/REC-xml-names-19990114/

XML-Enc

XML Encryption Syntax and Processing. D. Eastlake, and J. Reagle. W3C Working Draft. October 2001.

https://www.w3.org/TR/2001/WD-xml-encryption-req-20011018

XML-schema

XML Schema Part 1: Structures W3C Recommendation. D. Beech, M. Maloney, N. Mendelsohn, H. Thompson. May 2001.

https://www.w3.org/TR/2001/REC-xmlschema-1-20010502/
XML Schema Part 2: Datatypes W3C Recommendation. P. Biron, A. Malhotra. May 2001.

https://www.w3.org/TR/2001/REC-xmlschema-2-20010502/

XPath

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

7. Acknowledgements (Informative)

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

• Merlin Hughes, Baltimore
• Thomas Maslen, DSTC
• Paul Denning, MITRE