Network Working Group J. Boyer
Request for Comments: 3653 PureEdge Solutions
Category: Informational M. Hughes
Betrusted, Inc.
J. Reagle
W3C
December 2003
XML-Signature XPath Filter 2.0
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
XML Signature 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Acknowledgements (Informative) . . . . . . . . . . . . 4
1.2. W3C Status . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology. . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Specification of Signature Filter Transform. . . . . . . . . 5
3.1. Algorithm Identifier . . . . . . . . . . . . . . . . . 5
3.2. Syntax of Signature Filter Transform . . . . . . . . . 5
3.3. Input and Evaluation Context of Signature Filter
Transform. . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Processing Model of Signature Filter Transform . . . . 7
4. Examples of Signature Filter Transform . . . . . . . . . . . 9
5. Normative References . . . . . . . . . . . . . . . . . . . . 13
6. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
7. Full Copyright Statement . . . . . . . . . . . . . . . . . . 15
1. Introduction
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.
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* 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.
NOTE: Since XPath Filter 2.0 depends on details of XPath, be sure to
take into account the XPath Errata at
.
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1.1. Acknowledgements (Informative)
The following people provided valuable feedback that improved the
quality of this specification:
* Christian Geuer-Pollmann, Universitat Siegen
* Donald Eastlake, 3rd, Motorola
* Gregor Karlinger, IAK TU Graz
* Aleksey Sanin
1.2. W3C Status
The World Wide Web Consortium Recommendation corresponding to this
RFC is at:
http://www.w3.org/TR/xmldsig-filter2/
2. 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 BCP 14, 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
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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.
3.1. Algorithm Identifier
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:
Algorithm Identifier
http://www.w3.org/2002/06/xmldsig-filter2
3.2. Syntax of Signature Filter Transform
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):
id("foo bar")
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Schema Definition:
]>
DTD:
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3.3. Input and Evaluation Context of Signature Filter Transform
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 function here() is defined as follows:
Function: node-set here()
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.
3.4. Processing Model of Signature Filter Transform
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:
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o Evaluate the XPath expression X. The result is a node-set
S.
o 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.
o 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'.
o 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'.
o 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'.
o 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:
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o 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.
o 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.
4. Examples of Signature Filter Transform
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.
here()/ancestor::dsig:Signature[1]
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.
id("PrimaryBorrowerSig")
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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:
...
//ToBeSigned
//NotToBeSigned
//ReallyToBeSigned
...
...
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:
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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.
(ancestor-or-self::ToBeSigned and
not (ancestor-or-self::NotToBeSigned))
or ancestor-or-self::ReallyToBeSigned
...
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
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.
5. Normative References
[Keywords] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[URI] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[XML] "Extensible Markup Language (XML) 1.0 (Second Edition)",
T. Bray, E. Maler, J. Paoli, and C. M. Sperberg-
McQueen. W3C Recommendation, October 2000. Available at
.
[XML-C14N] Boyer, J., "Canonical XML", RFC 3076, March 2001. Also a
W3C Recommendation available at
.
[XML-DSig] Eastlake, J., Reagle, J. and D. Solo, "XML-Signature
Syntax and Processing", RFC 3275, March 2002. Also a W3C
Recommendation available at
. (Note
also XPath Errata at .)
[XPointer] "XML Pointer Language (XPointer)", S. DeRose, R. Daniel,
and E. Maler. W3C Candidate Recommendation, January
2001. Available at
RFC, FYI, BCP