XML Signature [XML-DSig] recommends a standard
means for specifying information content to be digitally signed and for
representing the resulting digital signatures in XML. Some applications
require the ability to specify a subset of a given XML document as the
information content to be signed. The XML Signature specification meets this
requirement with the XPath transform. However, this transform can be
difficult to implement efficiently with existing technologies. This
specification defines a new XML Signature transform to facilitate the
development of efficient document subsetting implementations that
interoperate under similar performance profiles.
This document is the W3C XML Signature XPath-Filter 2.0 Recommendation.
This document has been reviewed by W3C Members and other interested parties
and has been endorsed by the Director as a W3C Recommendation. It is a stable
document and may be used as reference material or cited as a normative
reference from another document. W3C's role in making the Recommendation is
to draw attention to the specification and to promote its widespread
deployment. This enhances the functionality and interoperability of the
Web.
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. At the time of publication,
there are no declarations specific to this document.
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. The XML Signature Recommendation [XML-DSig] defines standard means for specifying
information content to be digitally signed, including the ability to select a
portion of an XML document to be signed using an XPath transform.
This specification describes a new signature filter transform that, like
the XPath
transform [XML-DSig, section 6.6.3], provides
a method for computing a portion of a document to be signed. In the interest
of simplifying the creation of efficient implementations, the architecture of
this transform is not based on evaluating an [XPath]
expression for every node of the XML parse tree (as defined by the [XPath] data model). Instead, a sequence of XPath
expressions is used to select the roots of document subtrees — location
sets, in the language of [XPointer] — which
are combined using set intersection, subtraction and union, and then used to
filter the input node-set. The principal differences from the XPath transform
are:
A sequence of XPath operations can be executed in a single transform,
allowing complex filters to be more easily expressed and optimized.
The XPath expressions are evaluated against the input document
resulting in a set of nodes, instead of being used as a boolean test
against each node of the input node-set.
To increase efficiency, the expansion of a given node to include all
nodes having the given node as an ancestor is now implicit so it can be
performed by faster means than the evaluation of an XPath expression for
each document node.
The resulting node-sets can be combined using the three fundamental set
operations (intersection, subtraction, and union), and then applied as a
filter against the input node-set, allowing operations such as signing an
entire document except for a specified subset, to be expressed more
clearly and efficiently.
As with the original XPath transform, the primary purpose of this
transform is to ensure that only specifically defined changes to the input
XML document are permitted after the signature is affixed. This can be done
by excluding precisely those nodes that are allowed to change once the
signature is affixed, and including all other input nodes in the output. It
is the responsibility of the signature filter transform author to ensure that
nodes are not excluded which could affect the interpretation of the transform
output in the application context.
Consider the motivating scenario where an application wishes to affix two
enveloped
signatures to the document; any other change to the document must cause
the signatures to be invalid. When the application creates the first
signature that signature is automatically omitted from its own
digest calculations. However, it will also be necessary to exclude the
subsequent (second) signature element from the digest calculations of the
first signature. This specification can be used to efficiently satisfy this
requirement using the set subtraction operation.
This transform also supports the ability to specify a set of nodes that
will be included in a signature, with all non-specified nodes being excluded.
This formulation is useful for isolating a portion of a document, such as a
chapter of a document, or a payload in a protocol message, and can be
expressed using the set intersection operation.
Complete familiarity with the first XML Signature XPath Transform [XML-DSig, section 6.6.3] is required.
1.1.
Acknowledgements (Informative)
The following people provided valuable feedback that improved the quality
of this specification:
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 as
"(an unordered collection of nodes without duplicates)" 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).
An input document is the document that contains all
the nodes available to processing by this transform. A document subset is a
portion of an XML document indicated by an XPath node-set, which may not
include all of the nodes in the document. For example, the input
node-set is a collection of XPath nodes from the input document that is passed as a parameter
to this transform. A subtree rooted by a given node is a document subset
containing the given node and every node having the given node as an
ancestor. Subtree expansion is the process of
expanding a node-set to include all subtrees rooted at any node in the
node-set. For example, the subtree expansion of a node-set consisting of just
a single element node would be a node-set containing that element, its
attribute nodes, namespace nodes, and all its descendants including their
attribute nodes and namespaces nodes.
The XML Signature Recommendation [XML-DSig]
defines a reference as a sequence of steps performed to obtain
an octet stream to be digitally signed. A transform is an identified
algorithm to be used as a step in the reference processing model. A transform
takes an octet stream or XPath node-set as input, and it produces an octet
stream or XPath node-set as output (the reference processing model
automatically converts the final output to an octet stream if it is an XPath
node-set).
3. Specification
of Signature Filter Transform
The transform operates by computing a node-set that is used to filter the
input node-set: The output node-set consists of only those nodes in both the
input node-set and the filter node-set. In other words, the output node-set
is the intersection of the input node-set and the computed filter
node-set.
The filter node-set is computed by evaluating a sequence of XPath
expressions and combining their results. A node-set is initially computed
containing the entire input document. In sequence, each XPath expression is
then evaluated, subtree-expanded, and
then used to transform the filter node-set according to a specified set
operation; intersection, subtraction, or union. After all XPaths have been
applied, the resulting node-set is used as the filter node-set.
The XML Signature Recommendation [XML-DSig]
uses a [URI] to identify each algorithm to be
performed when creating or validating a signature. The signature filter
transform is identified as follows:
The signature filter transform shall be represented by a sequence of one
or more elements named XPath. The content of XPath
is character data containing an XPath expression. The XPath has
an attribute named Filter whose possible values are
intersect, subtract, and union. The
Filter attribute indicates the set operation that is performed
with the resulting node-set when computing the filter node-set. The following
is an example of markup for a signature filter that signs the entire input
node-set except for elements with identifier foo and bar
(and all nodes with one of those elements as an ancestor):
The input required by this transform is an XPath node-set over the input
document. If the input document is an octet stream, then the application MUST
convert the octet stream to an XPath node-set that contains all of the
document nodes (including comment nodes). The evaluation context for the
XPath expressions in the filter transform will be:
A context node equal to the root node of the document
whose node-set was provided as input to this transform. The root node is
the parent of the document element and any comment and processing
instruction nodes outside of the document element.
A context position, initialized to 1.
A context size, initialized to 1.
A library of functions equal to the function set
defined in [XPath] plus a function named
here().
A set of variable bindings. No means for initializing these is defined.
Thus, the set of variable bindings used when evaluating the XPath
expression is empty, and use of a variable reference in the XPath
expression results in an error.
The set of namespace declarations in scope for the XPath
element.
The here() function returns
a node-set containing the attribute or processing instruction node or the
parent element of the text node that directly bears the XPath expression. In
this transform, this will be the XPath element. This expression
results in an error if the containing XPath expression does not appear in the
same XML document against which the XPath expression is being evaluated.
Using the aforementioned evaluation
context, the signature filter transform evaluates the XPath expressions
appearing in the character content of the XPath elements and
uses these to compute a filter node-set F, which is then used to
filter the input node-set I resulting in an output node-set
O:
Initialize the filter node-set F to consist of all nodes in
the input document.
Iterate through each XPath expression, X, in sequence, and
update the filter node-set F as follows:
Evaluate the XPath expression X. The result is a node-set
S.
Compute the set S' consisting of all nodes in the input
document that are either present in S or that have an
ancestor in S. This is equal to the union of all the
document subtrees rooted by a node in S.
If the Filter attribute value is intersect,
then compute the intersection of the selected subtrees, S',
with the filter node-set F. The result will include only
those nodes that are in both the filter node-set and the selected
subtrees: F' = F INTERSECT S'.
If the Filter attribute value is subtract,
then compute the subtraction of the selected subtrees, S',
from the filter node-set F. The result will include only
those nodes that are in the filter node-set, but not the selected
subtrees: F' = F - S'.
Otherwise, if the Filter attribute value is
union, then compute the union the selected subtrees,
S', with the filter node-set F. The result will
include all those nodes that are in either the filter node-set, the
selected subtrees, or both: F' = F UNION S'.
Update the filter node-set F to be the new node-set
F'.
Finally, after applying all the XPath expressions, compute the output
node-set O to be the intersection of the computed filter
node-set, F, with the input node-set, I. The result
will include all nodes from the input node-set that are also in the
filter node-set: O = I INTERSECT F.
An empty input node-set will always result in an empty output
node-set.
In this processing model, the conversion from a subtree interpretation of
the XPath expressions to a node-set containing all nodes that must be used
during the set operation, along with actual performance of the set operation,
is described explicitly. Implementors SHOULD observe that it is possible to
compute the effective result of this operation in a single pass through the
input document without performing subtree expansion or any set operations:
For each XPath expression X, in sequence, evaluate the
expression and store the resulting node-set, S, along with the
associated set operation.
Prepend a node-set consisting of just the document node, along with the
operation union.
Create a new, empty filter node-set.
Process each node in the input node-set document, adding each
node to the output node-set F if a flag
Z is true. The flag is computed as follows:
Z is true if and only if the node is present in
any subtree-expanded union node-set and all
subsequent subtree-expanded intersect node-sets but
no subsequent subtree-expanded subtract node-sets,
or false otherwise. If there are no subsequent
intersect or subtract node-sets,
then that part of the test is automatically passed.
Presence in a subtree-expanded node-set can be efficiently
determined without actually expanding the node-set, by simply
maintaining a stack or count that identifies whether any nodes from
that node-set are an ancestor of the node being processed.
Implementers MAY further observe that, if this transform is followed by a
canonicalization operation (e.g., [XML-C14N]),
the described filter computation can be efficiently commingled with the
document-order canonicalization processing.
The example below illustrates one way to create an enveloped signature
with the signature filter transform. The function
here() identifies the XPath element,
and the subsequent location path obtains the nearest ancestor
Signature element. Due to the subtract value of the
Filter attribute, the output of the signature filter transform
is a node-set containing every node from the input node-set except the nodes
in the subtree rooted by the Signature element containing the
example signature filter transform below.
A suitable signature reference URI to use with this subtract filter would
be URI="" (the entire signature document, without comments),
URI="#xpointer(/)" (the entire signature document, with
comments) or any same-document reference that includes the signature
itself.
An example of an intersect filter is a signature that co-signs another
signature. In this example, a Signature element identified by
PrimaryBorrowSig must be signed. The XPath expression obtains the
element node, and the transform expands the output node-set to contain all
nodes from the input node-set that are also in the subtree rooted by the
element node.
This type of intersect filter is useful for efficiently signing subsets of
a document, whether this is the same document as the signature or an external
document. For example, if the signature reference URI is
URI="document.xml", then this document will be automatically
parsed and just the identified element and its descendants will be signed.
Union filters, by themselves are of no particular use: The initial filter
node-set consists of the entire input document; any union with this will have
no effect, so the output of the transform will be identical to the input. The
union operation is intended to follow a subtract operation, to allow a
subtree to be removed, with the exception of a lower subtree which is still
included in the output.
Consider the following document which contains a same-document enveloped
signature reference with an XPath filter containing three XPath
operations:
The intersect operation computes the intersection of the XPath-selected
subtrees with the filter node-set. In this case, the filter node-set
initially contains the entire input document, and the XPath expression
evaluates to the two ToBeSigned elements; these are expanded to
include all their descendents and intersected with the filter node-set,
resulting in the following:
The subtract filter computes the subtraction of the XPath-selected
subtrees from the filter node-set. In this case, the XPath expression
evaluates to the two NotToBeSigned elements; these are expanded
to include all their descendents and subtracted from the filter node-set:
Next, the union filter computes the union of the XPath-selected subtrees
with the filter node-set. In this case, the XPath expression evaluates to the
ReallyToBeSigned element; this is expanded to include all its
descendents and added to the filter node-set:
Finally, this resulting filter node-set is used to transform the input
node-set. In this example, the input node-set is the entire document, with
comments removed. The transformed node-set will thus be all those nodes from
the input document, less comments, that are also in the filter node-set:
Note that the result contains no nodes that were not in the input
node-set. Although the filter node-set included comments, these were not
present in the input node-set so they are not present in the output
node-set.
This signature filter does not provide any increased capability over the
original XPath transform. For example, this reference could be replicated
using the XPath transform as follows.
<dsig:Reference URI="">
<dsig:Transforms>
<dsig:Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
<dsig:XPath>
(ancestor-or-self::ToBeSigned and
not (ancestor-or-self::NotToBeSigned))
or ancestor-or-self::ReallyToBeSigned
</dsig:XPath>
</dsig:Transform>
</dsig:Transforms>
...
</dsig:Reference>
The advantage of the signature filter transform over the XPath transform
is that the latter requires evaluation of a potentially-complex expression
against every node in the input set, which has has proved costly in practice
for many useful operations. This specification's filter requires evaluation
of simple XPath expressions and then the execution of some basic set
operations or their equivalent, which can be implemented significantly more
efficiently.