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Canonical XML
Version 1.0

W3C Candidate Recommendation 12 December 2000

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John Boyer, PureEdge Solutions Inc., jboyer@PureEdge.com


Any XML document is part of a set of XML documents that are logically equivalent within an application context, but which vary in physical representation based on syntactic changes permitted by XML 1.0 [XML] and Namespaces in XML [Names]. This specification describes a method for generating a physical representation, the canonical form, of an XML document that accounts for the permissible changes. Except for limitations regarding a few unusual cases, if two documents have the same canonical form, then the two documents are logically equivalent within the given application context. Note that two documents may have differing canonical forms yet still be equivalent in a given context based on application-specific equivalence rules for which no generalized XML specification could account.

Status of this document

This is a revised Candidate Recommendation of the IETF/W3C XML Signature Working Group (W3C Activity Statement). This version includes three clarifications resulting from comments (1,2,3) made during the four week call for implementation (which formally ended November 24th). The XML Signature Working Group believes this specification incorporates the resolution of all last call and call for implementation issues; furthermore it considers the specification to be very stable (as demonstrated by its interoperability report). We hope to refer this document to the W3C Director for consideration as Proposed Recommendation in early January.

Please send comments to the editors and cc: the list <w3c-ietf-xmldsig@w3.org>.

There have been no declarations regarding patents related to this specification within the XML Signature Working Group.

A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR.

Table of Contents

  1. Introduction
    1. Terminology
    2. Applications
    3. Limitations
  2. XML Canonicalization
    1. Data Model
    2. Document Order
    3. Processing Model
    4. Document Subsets
  3. Examples of XML Canonicalization
    1. PIs, Comments, and Outside of Document Element
    2. Whitespace in Document Content
    3. Start and End Tags
    4. Character Modifications and Character References
    5. Entity References
    6. UTF-8 Encoding
    7. Document Subsets
  4. Resolutions
    1. No XML Declaration
    2. No Character Model Normalization
    3. Handling of Whitespace Outside Document Element
    4. No Namespace Prefix Rewriting
    5. Order of Namespace Declarations and Attributes
    6. Superfluous Namespace Declarations
    7. Propagation of Default Namespace Declaration in Document Subsets
    8. Sorting Attributes by Namespace URI
  5. References
  6. Acknowledgements

1 Introduction

The XML 1.0 Recommendation [XML] specifies the syntax of a class of resources called XML documents. The Namespaces in XML Recommendation [Names] specifies additional syntax and semantics for XML documents. It is possible for XML documents which are equivalent for the purposes of many applications to differ in physical representation. For example, they may differ in their entity structure, attribute ordering, and character encoding. It is the goal of this specification to establish a method for determining whether two documents are identical, or whether an application has not changed a document, except for transformations permitted by XML 1.0 and Namespaces in XML.

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].

See [Names] for the definition of QName.

A document subset is a portion of an XML document indicated by a node-set that may not include all of the nodes in the document.

The canonical form of an XML document is physical representation of the document produced by the method described in this specification. The changes are summarized in the following list:

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

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, 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 augmenting the namespace context of the descendants).

1.2 Applications

Since the XML 1.0 Recommendation [XML] and the Namespaces in XML Recommendation [Names] define multiple syntactic methods for expressing the same information, XML applications tend to take liberties with changes that have no impact on the information content of the document. XML canonicalization is designed to be useful to applications that require the ability to test whether the information content of a document or document subset has been changed. This is done by comparing the canonical form of the original document before application processing with the canonical form of the document result of the application processing.

For example, a digital signature over the canonical form of an XML document or document subset would allow the signature digest calculations to be oblivious to changes in the original document's physical representation, provided that the changes are defined to be logically equivalent by the XML 1.0 or Namespaces in XML. During signature generation, the digest is computed over the canonical form of the document. The document is then transferred to the relying party, which validates the signature by reading the document and computing a digest of the canonical form of the received document. The equivalence of the digests computed by the signing and relying parties (and hence the equivalence of the canonical forms over which they were computed) ensures that the information content of the document has not been altered since it was signed.

1.3 Limitations

Two XML documents may have differing information content that is nonetheless logically equivalent within a given application context. Although two XML documents are equivalent (aside from limitations given in this section) if their canonical forms are identical, it is not a goal of this work to establish a method such that two XML documents are equivalent if and only if their canonical forms are identical. Such a method is unachievable, in part due to application-specific rules such as those governing unimportant whitespace and equivalent data (e.g. <color>black</color> versus <color>rgb(0,0,0)</color>). There are also equivalencies established by other W3C Recommendations and Working Drafts. Accounting for these additional equivalence rules is beyond the scope of this work. They can be applied by the application or become the subject of future specifications.

The canonical form of an XML document may not be completely operational within the application context, though the circumstances under which this occurs are unusual. This problem may be of concern in certain applications since the canonical form of a document and the canonical form of the canonical form of the document are equivalent. For example, in a digital signature application, it cannot be established whether the operational original document or the non-operational canonical form was signed because the canonical form can be substituted for the original document without changing the digest calculation. However, the security risk only occurs in the unusual circumstances described below, which can all be resolved or at least detected prior to digital signature generation.

The difficulties arise due to the loss of the following information not available in the data model:

  1. base URI, especially in content derived from the replacement text of external general parsed entity references
  2. notations and external unparsed entity references
  3. attribute types in the document type declaration

In the first case, note that a document containing a relative URI [URI] is only operational when accessed from a specific URI that provides the proper base URI. In addition, if the document contains external general parsed entity references to content containing relative URIs, then the relative URIs will not be operational in the canonical form, which replaces the entity reference with internal content (thereby implicitly changing the default base URI of that content). Both of these problems can typically be solved by adding support for the xml:base attribute [XBase] to the application, then adding appropriate xml:base attributes to document element and all top-level elements in external entities. In addition, applications often have an opportunity to resolve relative URIs prior to the need for a canonical form. For example, in a digital signature application, a document is often retrieved and processed prior to signature generation. The processing SHOULD create a new document in which relative URIs have been converted to absolute URIs, thereby mitigating any security risk for the new document.

In the second case, the loss of external unparsed entity references and the notations that bind them to applications means that canonical forms cannot properly distinguish among XML documents that incorporate unparsed data via this mechanism. This is an unusual case precisely because most XML processors currently discard the document type declaration, which discards the notation, the entity's binding to a URI, and the attribute type that binds the attribute value to an entity name. For documents that must be subjected to more than one XML processor, the XML design typically indicates a reference to unparsed data using a URI in the attribute value.

In the third case, the loss of attribute types can affect the canonical form in different ways depending on the type. Attributes of type ID cease to be ID attributes. Hence, any XPath expressions that refer to the canonical form using the id() function cease to operate. The attribute types ENTITY and ENTITIES are not part of this case; they are covered in the second case above. Attributes of enumerated type and of type ID, IDREF, IDREFS, NMTOKEN, NMTOKENS, and NOTATION fail to be appropriately constrained during future attempts to change the attribute value if the canonical form replaces the original document during application processing. Applications can avoid the difficulties of this case by ensuring that an appropriate document type declaration is prepended prior to using the canonical form in further XML processing. This is likely to be an easy task since attribute lists are usually acquired from a standard external DTD subset, and any entity and notation declarations not also in the external DTD subset are typically constructed from application configuration information and added to the internal DTD subset.

While these limitations are not severe, it would be possible to resolve them in a future version of XML canonicalization if, for example, a new version of XPath were created based on the XML Information Set [Infoset] currently under development at the W3C.

2 XML Canonicalization

2.1 Data Model

The data model defined in the XPath 1.0 Recommendation [XPath] is used to represent the input XML document or document subset. Implementations SHOULD but need not be based on an XPath implementation. XML canonicalization is defined in terms of the XPath definition of a node-set, and implementations MUST produce equivalent results.

The first parameter of input to the XML canonicalization method is either an XPath node-set or an octet stream containing a well-formed XML document. Implementations MUST support the octet stream input and SHOULD also support the document subset feature via node-set input. For the purpose of describing canonicalization in terms of an XPath node-set, this section describes how an octet stream is converted to an XPath node-set.

The second parameter of input to the XML canonicalization method is a boolean flag indicating whether or not comments should be included in the canonical form output by the XML canonicalization method. If a canonical form contains comments corresponding to the comment nodes in the input node-set, the result is called canonical XML with comments. Note that the XPath data model does not create comment nodes for comments appearing within the document type declaration (DTD). Implementations are REQUIRED to be capable of producing canonical XML excluding all comments that may have appeared in the input document or document subset. Support for canonical XML with comments is RECOMMENDED.

If an XML document must be converted to a node-set, XPath REQUIRES that an XML processor be used to create the nodes of its data model to fully represent the document. The XML processor performs the following tasks in order:

  1. normalize line feeds
  2. normalize attribute values
  3. replace CDATA sections with their character content
  4. resolve character and parsed entity references

The input octet stream MUST contain a well-formed XML document, but the input need not be validated. However, the attribute value normalization and entity reference resolution MUST be performed in accordance with the behaviors of a validating XML processor. As well, nodes for default attributes (declared in the ATTLIST with an AttValue but not specified) are created in each element. Thus, the declarations in the document type declaration are used to help create the canonical form, even though the document type declaration is not retained in the canonical form.

The XPath data model represents data using UCS characters. Implementations MUST use XML processors that support UTF-8 and UTF-16 and translate to the UCS character domain. For UTF-16, the leading byte order mark is treated as an artifact of encoding and stripped from the UCS character data (subsequent zero width non-breaking spaces appearing within the UTF-16 data are not removed) [UTF-16, Section 3.2]. Support for ISO-8859-1 encoding is RECOMMENDED, and all other character encodings are OPTIONAL.

All whitespace within the root document element MUST be preserved (except for any #xD characters deleted by line delimiter normalization). This includes all whitespace in external entities. Whitespace outside of the root document element MUST be discarded.

In the XPath data model, there exist the following node types: root, element, comment, processing instruction, text, attribute and namespace. There exists a single root node whose children are processing instruction nodes and comment nodes to represent information outside of the document element (and outside of the document type declaration). The root node also has a single element node representing the top-level document element. Each element node can have child nodes of type element, text, processing instruction, and comment. The attributes and namespaces associated with an element are not considered to be child nodes of the element, but they are associated with the element by inclusion in the element's attribute and namespace axes. Note that attribute and namespace axes may not directly correspond to the text appearing in the element's start tag in the original document.

Note: An element has attribute nodes to represent the non-namespace attribute declarations appearing in its start tag as well as nodes to represent the default attributes.

By virtue of the XPath data model, XML canonicalization is namespace-aware [Names]. However, it cannot and therefore does not account for namespace equivalencies using namespace prefix rewriting (see explanation in Section 4). In the XPath data model, each element and attribute has a name returned by the function name() which can, at the discretion of the application, be the QName appearing in the original document. XML canonicalization REQUIRES that the XML processor retain sufficient information such that the QName of the element as it appeared in the original document can be provided.

Note: An element E has namespace nodes that represent its namespace declarations as well as any namespace declarations made by its ancestors that have not been overridden in E's declarations, the default namespace if it is non-empty, and the declaration of the prefix xml.

Note: This specification supports the recent XML plenary decision to deprecate relative namespace URIs as follows: implementations of XML canonicalization MUST report an operation failure on documents containing relative namespace URIs. XML canonicalization MUST NOT be implemented with an XML parser that converts relative URIs to absolute URIs.

Character content is represented in the XPath data model with text nodes. All consecutive characters are placed into a single text node. Furthermore, the text node's characters are represented in the UCS character domain. The XML canonicalization method does not perform character model normalization (see explanation in Section 4). However, the XML processor used to prepare the XPath data model input is REQUIRED to use Normalization Form C [NFC] when converting an XML document to the UCS character domain from any encoding that is not UCS-based (currently, UCS-based encodings include UTF-8, UTF-16, UTF-16BE, and UTF-16LE, UCS-2, and UCS-4).

Since XML canonicalization converts an XPath node-set into a canonical form, the first parameter MUST either be an XPath node-set or it must be converted from an octet stream to a node-set by performing the XML processing necessary to create the XPath nodes described above, then setting an initial XPath evaluation context of:

and evaluating the following default expression:

Comment Parameter Value Default XPath Expression
Without (false) (//. | //@* | //namespace::*)[not(self::comment())]
With (true) (//. | //@* | //namespace::*)

The expressions in this table generate a node-set containing every node of the XML document (except the comments if the comment parameter value is false).

If the input is an XPath node-set, then the node-set must explicitly contain every node to be rendered to the canonical form. For example, the result of the XPath expression id("E") is a node-set containing only the node corresponding to the element with an ID attribute value of "E". Since none of its descendant nodes, attribute nodes and namespace nodes are in the set, the canonical form would consist solely of the element's start and end tags, less the attribute and namespace declarations, with no internal content. Section 3.7 exemplifies how to serialize an identified element along with its internal content, attributes and namespace declarations.

2.2 Document Order

Although an XPath node-set is defined to be unordered, the XPath 1.0 Recommendation [XPath] defines the term document order to be the order in which the first character of the XML representation of each node occurs in the XML representation of the document after expansion of general entities, except for namespace and attribute nodes whose document order is application-dependent.

The XML canonicalization method processes a node-set by imposing the following additional document order rules on the namespace and attribute nodes of each element:

Lexicographic comparison, which orders strings from least to greatest alphabetically, is based on the UCS codepoint values, which is equivalent to lexicographic ordering based on UTF-8.

2.3 Processing Model

The XPath node-set is converted into an octet stream, the canonical form, by generating the representative UCS characters for each node in the node-set in ascending document order, then encoding the result in UTF-8 (without a leading byte order mark). No node is processed more than once. Note that processing an element node E includes the processing of all members of the node-set for which E is an ancestor. Therefore, directly after the representative text for E is generated, E and all nodes for which E is an ancestor are removed from the node-set (or some logically equivalent operation occurs such that the node-set's next node in document order has not been processed). Note, however, that an element node is not removed from the node-set until after its children are processed.

The result of processing a node depends on its type and on whether or not it is in the node-set. If a node is not in the node-set, then no text is generated for the node except for the result of processing its namespace and attribute axes (elements only) and its children (elements and the root node). If the node is in the node-set, then text is generated to represent the node in the canonical form in addition to the text generated by processing the node's namespace and attribute axes and child nodes.

NOTE: The node-set is treated as a set of nodes, not a list of subtrees. To canonicalize an element including its namespaces, attributes, and content, the node-set must actually contain all of the nodes corresponding to these parts of the document, not just the element node.

The text generated for a node is dependent on the node type and given in the following list:

The QName of a node is either the local name if the namespace prefix string is empty or the namespace prefix, a colon, then the local name of the element. The namespace prefix used in the QName MUST be the same one which appeared in the input document.

2.4 Document Subsets

Some applications require the ability to create a physical representation for an XML document subset (other than the one generated by default, which can be a proper subset of the document if the comments are omitted). Implementations of XML canonicalization that are based on XPath can provide this functionality with little additional overhead by accepting a node-set as input rather than an octet stream.

The processing of an element node E MUST be modified slightly when an XPath node-set is given as input and the element's parent is omitted from the node-set. The method for processing the attribute axis of an element E in the node-set is enhanced. All element nodes along E's ancestor axis are examined for nearest occurrences of attributes in the xml namespace, such as xml:lang and xml:space (whether or not they are in the node-set). From this list of attributes, remove any that are in E's attribute axis (whether or not they are in the node-set). Then, lexicographically merge this attribute list with the nodes of E's attribute axis that are in the node-set. The result of visiting the attribute axis is computed by processing the attribute nodes in this merged attribute list.

NOTE: XML entities can derive application-specific meaning from anywhere in the XML markup as well as by rules not expressed in XML 1.0 and the Namespaces in XML Recommendations. Clearly, these rules cannot be specified in this document, so the creator of the input node-set must be responsible for preserving the information necessary to capture the full semantics of the members of the resulting node-set.

The canonical XML generated for an entire XML document is well-formed. The canonical form of an XML document subset may not be well-formed XML. However, since the canonical form may be subjected to further XML processing, most XPath node-sets provided for canonicalization will be designed to produce a canonical form that is a well-formed XML document or external general parsed entity. Whether from a full document or a document subset, if the canonical form is well-formed XML, then subsequent applications of the same XML canonicalization method to the canonical form make no changes.

3 Examples of XML Canonicalization

The examples in this section assume a non-validating processor, primarily so that a document type declaration can be used to declare entities as well as default attributes and attributes of various types (such as ID and enumerated) without having to declare all attributes for all elements in the document. As well, one example contains an element that deliberately violates a validity constraint (because it is still well-formed).

3.1 PIs, Comments, and Outside of Document Element

Input Document <?xml version="1.0"?>

<?xml-stylesheet   href="doc.xsl"
   type="text/xsl"   ?>

<!DOCTYPE doc SYSTEM "doc.dtd">

<doc>Hello, world!<!-- Comment 1 --></doc>

<?pi-without-data     ?>

<!-- Comment 2 -->

<!-- Comment 3 -->
Canonical Form (uncommented) <?xml-stylesheet href="doc.xsl"
   type="text/xsl"   ?>
<doc>Hello, world!</doc>
Canonical Form (commented) <?xml-stylesheet href="doc.xsl"
   type="text/xsl"   ?>
<doc>Hello, world!<!-- Comment 1 --></doc>
<!-- Comment 2 -->
<!-- Comment 3 -->


3.2 Whitespace in Document Content

Input Document <doc>
   <clean>   </clean>
   <dirty>   A   B   </dirty>
      <clean>   </clean>
      <dirty>   A   B   </dirty>
Canonical Form <doc>
   <clean>   </clean>
   <dirty>   A   B   </dirty>
      <clean>   </clean>
      <dirty>   A   B   </dirty>


Note: In this example, the input document and canonical form are identical. Both end with '>' character.

3.3 Start and End Tags

Input Document <!DOCTYPE doc [<!ATTLIST e9 attr CDATA "default">]>
   <e1   />
   <e2   ></e2>
   <e3   name = "elem3"   id="elem3"   />
   <e4   name="elem4"   id="elem4"   ></e4>
   <e5 a:attr="out" b:attr="sorted" attr2="all" attr="I'm"
   <e6 xmlns="" xmlns:a="http://www.w3.org">
      <e7 xmlns="http://www.ietf.org">
         <e8 xmlns="" xmlns:a="http://www.w3.org">
            <e9 xmlns="" xmlns:a="http://www.ietf.org"/>
Canonical Form <doc>
   <e3 id="elem3" name="elem3"></e3>
   <e4 id="elem4" name="elem4"></e4>
   <e5 xmlns="http://www.uvic.ca" xmlns:a="http://www.w3.org" xmlns:b="http://www.ietf.org" attr="I'm" attr2="all" b:attr="sorted" a:attr="out"></e5>
   <e6 xmlns:a="http://www.w3.org">
      <e7 xmlns="http://www.ietf.org">
         <e8 xmlns="">
            <e9 xmlns:a="http://www.ietf.org" attr="default"></e9>


Note: Some start tags in the canonical form are very long, but each start tag in this example is entirely on a single line.

Note: In e5, b:attr precedes a:attr because the primary key is namespace URI not namespace prefix, and attr2 precedes b:attr because the default namespace is not applied to unqualified attributes (so the namespace URI for attr2 is empty).

3.4 Character Modifications and Character References

Input Document <!DOCTYPE doc [
   <text>First line&#x0d;&#10;Second line</text>
   <compute><![CDATA[value>"0" && value<"10" ?"valid":"error"]]></compute>
   <compute expr='value>"0" &amp;&amp; value&lt;"10" ?"valid":"error"'>valid</compute>
   <norm attr=' &apos;   &#x20;&#13;&#xa;&#9;   &apos; '/>
   <normNames attr='   A   &#x20;&#13;&#xa;&#9;   B   '/>
   <normId id=' &apos;   &#x20;&#13;&#xa;&#9;   &apos; '/>
Canonical Form <doc>
   <text>First line&#xD;
Second line</text>
   <compute>value&gt;"0" &amp;&amp; value&lt;"10" ?"valid":"error"</compute>
   <compute expr="value>&quot;0&quot; &amp;&amp; value&lt;&quot;10&quot; ?&quot;valid&quot;:&quot;error&quot;">valid</compute>
   <norm attr=" '    &#xD;&#xA;&#x9;   ' "></norm>
   <normNames attr="A &#xD;&#xA;&#x9; B"></normNames>
   <normId id="' &#xD;&#xA;&#x9; '"></normId>


Note: The last element, normId, is well-formed but violates a validity constraint for attributes of type ID. For testing canonical XML implementations based on validating processors, remove the line containing this element from the input and canonical form. In general, XML consumers should be discouraged from using this feature of XML.

Note: Whitespace character references other than &#x20; are not affected by attribute value normalization [XML].

Note: In the canonical form, the value of the attribute named attr in the element norm begins with a space, an apostrophe (single quote), then four spaces before the first character reference.

Note: The expr attribute of the second compute element contains no line breaks.

3.5 Entity References

Input Document <!DOCTYPE doc [
<!ENTITY ent1 "Hello">
<!ENTITY ent2 SYSTEM "world.txt">
<!ENTITY entExt SYSTEM "earth.gif" NDATA gif>
<!NOTATION gif SYSTEM "viewgif.exe">
<doc attrExtEnt="entExt">
   &ent1;, &ent2;!

<!-- Let world.txt contain "world" (excluding the quotes) -->
Canonical Form (uncommented) <doc attrExtEnt="entExt">
   Hello, world!


3.6 UTF-8 Encoding

Input Document <?xml version="1.0" encoding="ISO-8859-1"?>
Canonical Form <doc>#xC2#xA9</doc>


Note: The content of the doc element is NOT the string #xC2#xA9 but rather the two octets whose hexadecimal values are C2 and A9, which is the UTF-8 encoding of the UCS codepoint for the copyright sign (©).

3.7 Document Subsets

Input Document <!DOCTYPE doc [
<!ATTLIST e2 xml:space (default|preserve) 'preserve'>
<doc xmlns="http://www.ietf.org" xmlns:w3c="http://www.w3.org">
      <e2 xmlns="">
         <e3 id="E3"/>
Document Subset Expression <!-- Evaluate with declaration xmlns:ietf="http://www.ietf.org" -->

(//. | //@* | //namespace::*)
   self::ietf:e1 or (parent::ietf:e1 and not(self::text() or self::e2))
   count(id("E3")|ancestor-or-self::node()) = count(ancestor-or-self::node())
Canonical Form <e1 xmlns="http://www.ietf.org" xmlns:w3c="http://www.w3.org"><e3 xmlns="" id="E3" xml:space="preserve"></e3></e1>


Note: In the document subset expression, the subexpression (//. | //@* | //namespace::*) selects all nodes in the input document, subjecting each to the predicate expression in square brackets. The expression is true for e1 and its implicit namespace nodes, and it is true if the element identified by E3 is in the ancestor-or-self path of the context node (such that ancestor-or-self stays the same size under union with the element identified by E3).

Note: The canonical form contains no line delimiters.

4 Resolutions

This section discusses a number of key decision points as well as a rationale for each decision. Although this specification now defines XML canonicalization in terms of the XPath data model rather than XML Infoset, the canonical form described in this document is quite similar in most respects to the canonical form described in the January 2000 Canonical XML draft [C14N-20000119]. However, some differences exist, and a number of the subsections discuss the changes.

4.1 No XML Declaration

The XML declaration, including version number and character encoding is omitted from the canonical form. The encoding is not needed since the canonical form is encoded in UTF-8. The version is not needed since the absence of a version number unambiguously indicates XML 1.0.

Future versions of XML will be required to include an XML declaration to indicate the version number. However, canonicalization method described in this specification may not be applicable to future versions of XML without some modifications. When canonicalization of a new version of XML is required, this specification could be updated to include the XML declaration as presumably the absence of the XML declaration from the XPath data model can be remedied by that time (e.g. by reissuing a new XPath based on the Infoset data model).

4.2 No Character Model Normalization

The Unicode standard [Unicode] allows multiple different representations of certain "precomposed characters" (a simple example is "ç"). Thus two XML documents with content that is equivalent for the purposes of most applications may contain differing character sequences. The W3C is preparing a normalized representation [CharModel]. The C14N-20000119 Canonical XML draft used this normalized form. However, many XML 1.0 processors do not perform this normalization. Furthermore, applications that must solve this problem typically enforce character model normalization at all times starting when character content is created in order to avoid processing failures that could otherwise result (e.g. see example from Cowan). Therefore, character model normalization has been moved out of scope for XML canonicalization. However, the XML processor used to prepare the XPath data model input is required (by the Data Model) to use Normalization Form C [NFC] when converting an XML document to the UCS character domain from any encoding that is not UCS-based (currently, UCS-based encodings include UTF-8, UTF-16, UTF-16BE, and UTF-16LE, UCS-2, and UCS-4).

4.3 Handling of Whitespace Outside Document Element

The C14N-20000119 Canonical XML draft placed a #xA after each PI outside of the document element as well as a #xA after the end tag of the document element. The method in this specification performs the same function except for omitting the final #xA after the last PI (or comment or end tag of the document element). This technique ensures that PI (and comment) children of the root are separated from markup by a line feed even if root node or the document element are omitted from the output node-set.

4.4 No Namespace Prefix Rewriting

The C14N-20000119 Canonical XML draft described a method for rewriting namespace prefixes such that two documents having logically equivalent namespace declarations would also have identical namespace prefixes. The goal was to eliminate dependence on the particular namespace prefixes in a document when testing for logical equivalence. However, there now exist a number of contexts in which namespace prefixes can impart information value in an XML document. For example, an XPath expression in an attribute value or element content can reference a namespace prefix. Thus, rewriting the namespace prefixes would damage such a document by changing its meaning (and it cannot be logically equivalent if its meaning has changed).

More formally, let D1 be a document containing an XPath in an attribute value or element content that refers to namespace prefixes used in D1. Further assume that the namespace prefixes in D1 will all be rewritten by the canonicalization method. Let D2 = D1, then modify the namespace prefixes in D2 and modify the XPath expression's references to namespace prefixes such that D2 and D1 remain logically equivalent. Since namespace rewriting does not include occurrences of namespace references in attribute values and element content, the canonical form of D1 does not equal the canonical form of D2 because the XPath will be different. Thus, although namespace rewriting normalizes the namespace declarations, the goal eliminating dependence on the particular namespace prefixes in the document is not achieved.

Moreover, it is possible to prove that namespace rewriting is harmful, rather than simply ineffective. Let D1 be a document containing an XPath in an attribute value or element content that refers to namespace prefixes used in D1. Further assume that the namespace prefixes in D1 will all be rewritten by the canonicalization method. Now let D2 be the canonical form of D1. Clearly, the canonical forms of D1 and D2 are equivalent (since D2 is the canonical form of the canonical form of D1), yet D1 and D2 are not logically equivalent because the aforementioned XPath works in D1 and doesn't work in D2.

Note that an argument similar to this can be leveled against the XML canonicalization method based on any of the cases in the Limitations, the problems cannot easily be fixed in those cases, whereas here we have an opportunity to avoid purposefully introducing such a limitation.

Applications that must test for logical equivalence must perform more sophisticated tests than mere octet stream comparison. However, this is quite likely to be necessary in any case in order to test for logical equivalencies based on application rules as well as rules from other XML-related recommendations, working drafts, and future works.

4.5 Order of Namespace Declarations and Attributes

The C14N-20000119 Canonical XML draft alternated between namespace declarations and attribute declarations. This is part of the namespace prefix rewriting scheme, which this specification eliminates. This specification follows the XPath data model of putting all namespace nodes before all attribute nodes.

4.6 Superfluous Namespace Declarations

Unnecessary namespace declarations are not made in the canonical form. Whether for an empty default namespace, a non-empty default namespace, or a namespace prefix binding, the XML canonicalization method omits a declaration if it determines that the immediate parent element in the canonical form has an equivalent declaration in scope. The root document element is handled specially since it has no parent element. All namespace declarations in it are retained, except the declaration of an empty default namespace is automatically omitted.

Relative to the method of simply rendering the entire namespace context of each element, implementations are not hindered by more than a constant factor in processing time and memory use. The advantages include:

Note that in document subsets, an element with omissions from its ancestral element chain will be rendered to the canonical form with namespace declarations that may have been made in its omitted ancestors, thus preserving the meaning of the element.

4.7 Propagation of Default Namespace Declaration in Document Subsets

The XPath data model represents an empty default namespace with the absence of a node, not with the presence of a default namespace node having an empty value. Thus, with respect to the fact that element e3 in the following examples is not namespace qualified, we cannot tell the difference between <e1 xmlns="a:b"><e2 xmlns=""><e3/></e2></e1> versus <e1 xmlns="a:b"><e2><e3 xmlns=""/></e2></e1>. All we know is that e3 was not namespace qualified on input, so we preserve this information on output if e2 is omitted so that e3 does not take on the default namespace qualification of e1.

4.8 Sorting Attributes by Namespace URI

Given the requirement to preserve the namespace prefixes declared in a document, sorting attributes with the prefix, rather than the namespace URI, as the primary key is viable and easier to implement. However, the namespace URI was selected as the primary key because this is closer to the intent of the Namespaces in XML specification, which is to identify namespaces by URI and local name, not by a prefix and local name. The effect of the sort is to group together all attributes that are in the same namespace.

5 References

Canonical XML Version 1.0, W3C Working Draft. T. Bray, J. Clark, J. Tauber, and J. Cowan. January 19, 2000. http://www.w3.org/TR/2000/WD-xml-c14n-20000119.html.
Character Model for the World Wide Web, W3C Working Draft. eds. Martin J. Dürst, François Yergeau. http://www.w3.org/TR/charmod/.
Example of Harmful Effect of Character Model Normalization, Letter in XML Signature Working Group Mail Archive. John Cowan, July 7, 2000 http://lists.w3.org/Archives/Public/w3c-ietf-xmldsig/2000JulSep/0038.html.
XML Information Set, W3C Working Draft. ed. John Cowan. http://www.w3.org/TR/xml-infoset.
ISO-8859-1 Latin 1 Character Set. http://www.utoronto.ca/webdocs/HTMLdocs/NewHTML/iso_table.html or http://www.iso.ch/cate/cat.html.
Key words for use in RFCs to Indicate Requirement Levels, IETF RFC 2119. S. Bradner. March 1997. http://www.ietf.org/rfc/rfc2119.txt.
Namespaces in XML, W3C Recommendation. eds. Tim Bray, Dave Hollander, and Andrew Layman. http://www.w3.org/TR/REC-xml-names/.
TR15, Unicode Normalization Forms. M. Davis, M. Dürst. Revision 18: November 1999. http://www.unicode.org/unicode/reports/tr15/tr15-18.html.
The Unicode Standard, version 3.0. The Unicode Consortium. ISBN 0-201-61633-5. http://www.unicode.org/unicode/standard/versions/Unicode3.0.html.
UTF-16, an encoding of ISO 10646, IETF RFC 2781. P. Hoffman , F. Yergeau. February 2000. http://www.ietf.org/rfc/rfc2781.txt.
UTF-8, a transformation format of ISO 10646, IETF RFC 2279. F. Yergeau. January 1998. http://www.ietf.org/rfc/rfc2279.txt.
Uniform Resource Identifiers (URI): Generic Syntax, IETF RFC 2396. T. Berners-Lee, R. Fielding, L. Masinter. August 1998 http://www.ietf.org/rfc/rfc2396.txt.
XML Base ed. Jonathan Marsh. 07 June 2000. http://www.w3.org/TR/xmlbase/.
Extensible Markup Language (XML) 1.0 (Second Edition), W3C Recommendation. eds. Tim Bray, Jean Paoli, C. M. Sperberg-McQueen and Eve Maler. 6 October 2000. http://www.w3.org/TR/REC-xml.
XML-Signature Syntax and Processing, IETF Draft/W3C Candidate Recommendation. D. Eastlake, J. Reagle, D. Solo, M. Bartel, J. Boyer, B. Fox, and E. Simon. 31 October 2000. http://www.w3.org/TR/xmldsig-core/.
XML Plenary Decision
W3C XML Plenary Decision on relative URI References In namespace declarations, W3C Document. 11 September 2000. http://lists.w3.org/Archives/Public/xml-uri/2000Sep/0083.html.
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.

6 Acknowledgements (Informative)

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