Copyright © 2002 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
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 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.
This document is a Proposed Recommendations (PR) of the W3C. The IETF/W3C XML Signature Working Group (W3C Activity Statement) believes this specification incorporates the resolution of all last call issues and its specification is sufficient for the creation of independent interoperable implementations. The exit criteria of at least two interoperable implementations over every feature, one implementation of all features, reports of satisfactory performance and adequacy in an application context (e.g., SOAP, SAML, etc.) has been satisfied as documented in the Interoperability Report.
W3C Advisory Committee Members are invited to send formal review comments to the W3C Team until 21 June 2002 at [email protected]. The public is invited to send comments to the editors and the public mailing list [email protected] (archive ).After the review the Director will announce the document's disposition. This announcement should not be expected sooner than 14 days after the end of the review.
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.
Publication as a Proposed Recommendation does not imply endorsement by the W3C membership. This is still a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite a W3C Proposed Recommendations as other than a "work in progress." A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.
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:
An understanding of the Canonical XML Recommendation [XML-C14N] is required.
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 given 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).
Pictorial Representation:
Textual Representation:
A-+-b
`-c-+-d
`-E-+-f
`-G
The following characteristics apply:
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:
xml:lang and
xml:space are not imported into orphan nodes of the document
subset, and
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.
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.
Exclusive XML Canonicalization has the limitations of Canonical XML [XML-C14N] plus two additional limitations as follows:
xml:lang, xml:space, and
xml:base 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.
<number xsi:type="xsd:decimal">10.09</number>.
To avoid problems with such namespace declarations,
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.
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">
content
</n1:elem1>
this is then enveloped in another document:
<n0:pdu xmlns:n0="http://a.example">
<n1:elem1 xmlns:n1="http://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="http://a.example"
xmlns:n1="http://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.
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="http://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="http://example.com"
xmlns:n2="http://foo.example"
xml:lang="fr"
xml:space="retain">
<n1:elem2 xmlns:n1="http://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="http://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="http://example.net"
xmlns:n2="http://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="http://example.net"
xml:lang="en">
<n3:stuff xmlns:n3="ftp://example.org"></n3:stuff>
</n1:elem2>
in both cases.
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:
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.
One method (non-normative) for implementing the Exclusive XML Canonicalization method is as follows:
xml: namespace attributes into output apex element nodes is
not done.)
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.
ns_rendered of
namespace nodes rendered by output ancestors. Push ns_rendered on
state stack and recurse.
state 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.)
Exclusive Canonicalization may be used as a Transform or
CanonicalizationMethod algorithm in XML Digital Signature [XML-DSig] and XML Encryption [XML-Enc].
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="http://www.w3.org/2001/10/xml-exc-c14n#">
<ec:InclusiveNamespaces PrefixList="dsig soap #default"
xmlns:ec="http://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" "http://www.w3.org/2001/XMLSchema.dtd"
[
<!ATTLIST schema
xmlns:ec CDATA #FIXED 'http://www.w3.org/2001/10/xml-exc-c14n#'>
<!ENTITY ec 'http://www.w3.org/2001/10/xml-exc-c14n#'>
<!ENTITY % p ''>
<!ENTITY % s ''>
]>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:ec="http://www.w3.org/2001/10/xml-exc-c14n#"
targetNamespace="http://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>
</schema>
DTD:
<!ELEMENT InclusiveNamespaces EMPTY >
<!ATTLIST InclusiveNamespaces
PrefixList NMTOKENS #REQUIRED >
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 an XML declaration;
implementations of this specification only render attributes from the
"xml" namespace (e.g., xml:lang, xml:space, and
xml:base) when they are in in the subset being serialized; and
implementations of this specification do not consider the appearance of a
namespace prefix within an attribute value to be visibly utilized. 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 unambiguous in its application context. The Limitations of this specification, the Resolutions
of [XML-C14N], and the Security
Considerations of [XML-DSig] should be carefully
attended to.
Consider an application that might use this specification or [XML-C14N] to serialize a single attribute node. An implementation of either specification will not emit a namespace declaration for that single attribute node. Consequently, a "carefully constructed" transform should create a nodeset containing the attribute and the relevant namespace declaration for serialization.
We provide this example to caution that as one moves beyond well-formed [XML] and then well-balanced XML [XML-Fragment], it becomes increasingly difficult to create a result that "is meaningful and unambiguous in its application context."
The following people provided valuable feedback that improved the quality of this specification: