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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.
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The XML Recommendation [XML] specifies the syntax of a class of objects called XML documents. The Namespaces in XML Recommendation [Names] 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:
Complete familiarity with the Canonical XML Recommendation [XML-C14N] is assumed.
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:
xml:lang and
xml:space are not imported into orphan nodes of the document
subset, andThe 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 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.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 needs 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 two documents, the second of which is the first enveloped:
<n1:elem1 xmlns:n1="http://b.example">
content
</n1:elem1>
<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::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::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">
<ns3:stuff></ns3: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"/>
</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 for implementing the Exclusive XML Canonicalization method is as follows:
xml: namespace attributes into output apex
elements is NOT done.Exclusive Canonicalization may be used as a Transform or
CanonicalizationMethod algorithm in XML Digital Signature
[
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). An example is as follows:
<Transform
Algorithm="http://www.w3.org/TR/2001/xml-exc-c14n.html">
<InclusiveNamespacePrefix List="foo bar"/>
</Transform>
The schema for InclusiveNamespacePrefix is as follows:
Schema Definition:
<element name="InclusiveNamespacePrefix"
type="InclusiveNamespacePrefixType>
<comlexType name="InclusiveNamespacePrefix">
<attribute name="List" type="NMTOKENS">
</complexType>
DTD:
<!ELEMENT InclusiveNamespacePrefix EMPTY >
<!ATTLIST InclusiveNamespacePrefix
List NMTOKENS #REQUIRED >
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