Re: POLL: Fwd: Re: XPath filter 2.0
merlin <[email protected]> Tue, 11 Jun 2002 04:33:33 +0100
| Newsgroups | gmane.ietf.xmldsig |
|---|---|
| Message-ID | <[email protected]> |
r/[email protected]/2002.06.10/17:16:48 >So to conclude the thread I'd like to know what people prefer: >1. Move forward with what we have presently [3]. >2. Specify Merlin's proposal (The timing of this will be >dependent on properly representing the proposal (e.g., Merlin's time ><smile/>) and then reviewing and iterating on it a few times to make sure >we have it clear.) I believe that I've unilaterally eliminated anybody's choice in this matter, for which I apologize. Attached is, probably not as clear as it could be, my alternative proposal in more explicit language, along with interop samples. Note that intersect/subtract are unchanged from before; there is just the benefit that multiple ops can be done in a single transform. However; union is different: It is effectively applied against just the input node set. You'll see the language in the spec, along with verbiage on optimizing N XPath operations into a single sweep through the document. This also solves the bizarre null UNION foo from before. It's late, so I've checked neither the spec nor the examples for sanity. Merlin
WD-xmldsig-filter2-20020610.html
(text/html, 34.9 KB)
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>XML-Signature XPath Filter 2.0</title>
<style type="text/css">
<!--
/*<![CDATA[*/
em {font-weight: bold; font-style: normal;}
u,ins,.ins { background: white; color: red;}
del,strike,.strike { background: white; color: silver; text-decoration: line-through;}
code {font-weight: normal; }
.link-def { background: #FFFFFF; color: teal; font-style: italic;}
.comment { background: #FFFFF5; color: black; padding: .7em; border:
navy thin solid;}
.discuss { color: blue; background: yellow; }
.xml-example,.xml-dtd { margin-left: -1em; padding: .5em; white-space:
pre; border: none;}
.xml-dtd { background: #efeff8; color: black;}
/*]]>*/
-->
</style>
<link href="http://www.w3.org/StyleSheets/TR/W3C-WD" type="text/css"
rel="stylesheet" />
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
</head>
<body xml:lang="en" lang="en">
<p><a href="http://www.w3.org/"><img src="http://www.w3.org/Icons/w3c_home"
alt="W3C" border="0" height="48" width="72" /></a></p>
<div class="head">
<h1 class="notoc">XML-Signature XPath Filter 2.0</h1>
<h2 class="notoc">W3C Working Draft 10 June 2002</h2>
<dl>
<dt>This version:</dt>
<dd><a
href="http://www.w3.org/TR/2002/WD-xmldsig-filter2-20020610/">http://www.w3.org/TR/2002/WD-xmldsig-filter2-20020610/</a></dd>
<dt>Latest version:</dt>
<dd><a
href="http://www.w3.org/TR/xmldsig-filter2/">http://www.w3.org/TR/xmldsig-filter2/</a></dd>
<dt>Previous version:</dt>
<dd><a
href="http://www.w3.org/TR/2002/WD-xmldsig-filter2-20020425/">http://www.w3.org/TR/2002/WD-xmldsig-filter2-20020425/</a></dd>
<dt>Authors/Editors:</dt>
<dd>John Boyer, PureEdge Solutions Inc., <<a
href="mailto:[email protected]">[email protected]</a>></dd>
<dd>Merlin Hughes, Baltimore Technologies Ltd., <<a
href="mailto:[email protected]">[email protected]</a>></dd>
<dd>Joseph Reagle, W3C, <<a
href="mailto:[email protected]">[email protected]</a>></dd>
</dl>
<p class="copyright"><a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Copyright">Copyright</a>
© 2002 <a href="http://www.w3.org/"><abbr
title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
href="http://www.lcs.mit.edu/"><abbr
title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
href="http://www.inria.fr/"><abbr xml:lang="fr" lang="fr"
title="Institut National de Recherche en Informatique et Automatique">INRIA</abbr></a>,
<a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#W3C_Trademarks">trademark</a>,
<a
href="http://www.w3.org/Consortium/Legal/copyright-documents-19990405">document
use</a> and <a
href="http://www.w3.org/Consortium/Legal/copyright-software-19980720">software
licensing</a> rules apply.</p>
<hr title="Separator from Header" />
</div>
<h2 class="notoc">Abstract</h2>
<p>XML Signature [<a href="#ref-XML-DSig">XML-DSig</a>] 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 technologies that interoperate
under similar performance profiles.</p>
<h2><a id="status" name="status">Status of this document</a></h2>
<p>This document has no status; it is an unadopted proposed revision.</p>
<div class="strike">
<p>This is the first publication for the "Signature Filter Transform" Working
Draft from the <a href="http://www.w3.org/Signature/Overview.html">IETF/W3C
XML Signature Working Group</a> (<a
href="http://www.w3.org/Signature/Activity.html">Activity Statement</a>).
This specification defines an alternative to behaviour of the XPath transform
of the XML Signature Recommendation [<a href="#ref-XML-DSig">XML-DSig</a>].
The goal is to (1) more easily specify XPath transforms and (2) more
efficiently process those transforms. This specification has already received
a large amount of discussion and implementation within the Working Group. We
hope to move to and through Last Call and then Candidate Recommendation very
quickly.</p>
<p>Publication of this document does not imply endorsement by the W3C
membership. This is a draft document and may be updated, replaced, or
obsoleted by other documents at any time. It is inappropriate to cite a W3C
Working Draft as anything other than a "work in progress." Please send
comments to the editors and cc: the list <a
href="mailto:[email protected]">[email protected]</a> (<a
href="http://lists.w3.org/Archives/Public/w3c-ietf-xmldsig/">archives</a>).</p>
<p>Patent disclosures relevant to this specification may be found on the
Working Group's patent disclosure page in conformance with W3C.</p>
<p>A list of current W3C working drafts can be found at <a
href="http://www.w3.org/TR/">http://www.w3.org/TR/</a>.</p>
</div>
<h2><a id="contents" name="contents">Table of Contents</a></h2>
<ol>
<li><a href="#sec-Intro">Introduction</a></li>
<li><a href="#sec-Terminology">Terminology</a></li>
<li><a href="#sec-Specification">Specification of Signature Filter
Transform</a></li>
<li style="list-style: none"><ol>
<li><a href="#sec-Algorithm-Identifier">Algorithm Identifier</a></li>
<li><a href="#sec-Syntax">Syntax of Signature Filter Transform</a></li>
<li><a href="#sec-EvalContext">Input and Evaluation Context of
Signature Filter Transform</a></li>
<li><a href="#sec-ProcModel">Processing Model of Signature Filter
Transform</a></li>
</ol>
</li>
<li><a href="#sec-Examples">Examples of Signature Filter Transform</a></li>
<li><a href="#sec-References">References</a></li>
<li><a href="#sec-Acknowledgements">Acknowledgements</a></li>
</ol>
<hr />
<!-- =============================================================================== -->
<h2><a id="sec-Intro" name="sec-Intro"></a>1. Introduction</h2>
<p>The XML Recommendation [<a href="#ref-XML">XML</a>] specifies the syntax
of a class of objects called XML documents. The Namespaces in XML
Recommendation [<a href="#ref-XML-NS">XML-NS</a>] specifies additional syntax
and semantics for XML documents. The XML Signature Recommendation [<a
href="#ref-XML-DSig">XML-DSig</a>] 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.</p>
<p>This specification describes a new signature filter transform that, like
the <a href="http://www.w3.org/TR/xmldsig-core/#sec-XPath">XPath
transform</a> [<a href="#ref-XML-DSig">XML-DSig</a>, 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 <a
href="#ref-XPath">[XPath]</a>). Instead, a sequence of XPath expressions are
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:</p>
<ul>
<li>A sequence of XPath operations can be executed in a single transform,
allowing complex filters to be more-easily expressed and optimized.</li>
<li>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.</li>
<li>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.</li>
<li>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.</li>
</ul>
<p>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.</p>
<p>Consider the motivating scenario where an application wishes to affix two
<a class="link-def"
href="http://www.w3.org/TR/xmldsig-core/#def-SignatureEnveloped">enveloped
signatures</a> 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 <em>own</em>
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.</p>
<p>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.</p>
<p>Complete familiarity with the first XML Signature <a
href="http://www.w3.org/TR/xmldsig-core/#sec-XPath">XPath Transform</a> [<a
href="#ref-XML-DSig">XML-DSig</a>, section 6.6.3] is required.</p>
<!-- =============================================================================== -->
<h2><a id="sec-Terminology" name="sec-Terminology">2. Terminology</a></h2>
<p>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 <a
href="#ref-Keywords">[Keywords]</a>.</p>
<p>The XPath 1.0 Recommendation [<a href="#ref-XPath">XPath</a>] defines the
term <a class="link-def"
href="http://www.w3.org/TR/1999/REC-xpath-19991116#node-sets">node-set</a> 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).</p>
<p>An <a class="link-def" name="def-input-document"
id="def-input-document">input document</a> is the document that contains all
the nodes available to processing by this transform. A <a class="link-def"
name="def-document-subset" id="def-document-subset">document subset</a> 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 <a
class="link-def" name="def-input-node-set" id="def-input-node-set">input node
set</a> is a collection of XPath nodes from the <a class="link-def"
href="#def-input-document">input document</a> that is passed as a parameter
to this transform. A <a class="link-def" name="def-subtree" id="def-subtree">subtree</a>
rooted by a given node is a document subset containing the given node and
every node having the given node as an ancestor.
<a class="link-def" name="def-subtree-expansion">Subtree expansion</a> 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 the attribute nodes and namespaces nodes of any descendant elements.</p>
<p>The XML Signature Recommendation <a href="#ref-XML-DSig">[XML-DSig]</a>
defines a <a class="link-def" name="def-reference"
id="def-reference">reference</a> as a sequence of steps performed to obtain
an octet stream to be digitally signed. A <a class="link-def"
name="def-transform" id="def-transform">transform</a> 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).</p>
<!-- =============================================================================== -->
<h2><a id="sec-Specification" name="sec-Specification"></a>3. Specification
of Signature Filter Transform</h2>
<p>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.<p>
<p>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, <a href="#def-subtree-expansion">subtree-expanded</a>,
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.</p>
<h3><a id="sec-Algorithm-Identifier" name="sec-Algorithm-Identifier">3.1
Algorithm Identifier</a></h3>
<p>The XML Signature Recommendation <a href="#ref-XML-DSig">[XML-DSig]</a>
uses a URI <a href="#ref-URI">[URI]</a> to identify each algorithm to be
performed when creating or validating a signature. The signature filter
transform is identified as follows:</p>
<dl>
<dt>Algorithm Identifier</dt>
<dd><a
href="http://www.w3.org/2002/06/xmldsig-filter2">http://www.w3.org/2002/06/xmldsig-filter2</a></dd>
</dl>
<h3><a id="sec-Syntax" name="sec-Syntax">3.2 Syntax of Signature Filter
Transform</a></h3>
<p>The signature filter transform shall be represented by a sequence of
one or more elements named <code>XPath</code>. The content of
<code>XPath</code> is character data containing an XPath expression.
The <code>XPath</code> has an attribute named <code>Filter</code> whose
possible values are <code>intersect</code>, <code>subtract</code>, and
<code>union</code>. The <code>Filter</code> 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
<em>foo</em> and <em>bar</em> (and all nodes with one of those elements as an
ancestor):</p>
<pre class="xml-example"> <XPath Filter="subtract"
xmlns="http://www.w3.org/2002/06/xmldsig-filter2">
id("foo bar")
</XPath>
</pre>
<pre class="xml-dtd"> <a href="xmldsig-filter2.xsd">Schema Definition</a>:
<?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:xx CDATA #FIXED 'http://www.w3.org/2002/06/xmldsig-filter2'>
<!ENTITY xx 'http://www.w3.org/2002/06/xmldsig-filter2'>
<!ENTITY % p ''>
<!ENTITY % s ''>
]>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:xx="http://www.w3.org/2002/06/xmldsig-filter2"
targetNamespace="http://www.w3.org/2002/06/xmldsig-filter2"
version="0.1" elementFormDefault="qualified">
<element name="XPath"
type="xx:XPathType"/>
<complexType name="XPathType" content="textOnly">
<attribute name="Filter">
<simpleType>
<restriction base="NOTATION">
<enumeration value="intersect"/>
<enumeration value="subtract"/>
<enumeration value="union"/>
</restriction>
</simpleType>
</attribute>
</complexType>
</schema>
</pre>
<pre class="xml-dtd"> DTD:
<!ELEMENT XPath (#PCDATA) >
<!ATTLIST XPath
Filter (intersect|subtract|union) #REQUIRED >
</pre>
<h3><a id="sec-EvalContext" name="sec-EvalContext">3.3 Input and Evaluation
Context of Signature Filter Transform</a></h3>
<p>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:</p>
<ul>
<li>A <strong>context node</strong> 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.</li>
<li>A <strong>context position</strong>, initialized to 1.</li>
<li>A <strong>context size</strong>, initialized to 1.</li>
<li>A <strong>library of functions</strong> equal to the function set
defined in <a href="#ref-XPath">XPath</a> plus a function named
<strong><a href="#function-here">here</a>()</strong>.</li>
<li>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.</li>
<li>The set of namespace declarations in scope for the <code>XPath</code>
element.</li>
</ul>
<p><strong>The function <code>here()</code> is defined as
follows:</strong></p>
<p><a name="function-here" id="function-here"><strong>Function:</strong>
<em>node-set</em> <strong>here</strong>()</a></p>
<p>The <strong><a href="#function-here">here</a>()</strong> 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 <code>XPath</code> 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.</p>
<h3><a id="sec-ProcModel" name="sec-ProcModel">3.4 Processing Model of
Signature Filter Transform</a></h3>
<p>Using the aforementioned <a href="#sec-EvalContext">evaluation
context</a>, the signature filter transform evaluates the XPath expressions
appearing in the character content of the <code>XPath</code> elements and
uses these to compute a filter node set <em>F</em>, which is then used to
filter the input node set <em>I</em> resulting in an output node set
<em>O</em>:</p>
<div class="">
<ul>
<li>Initialize the filter node set <em>F</em> to consist of all nodes in
the input document.</li>
<li>Iterate through each XPath expression, <em>X</em>, in
sequence, and update the filter node set <em>F</em> as follows:</li>
<ul>
<li>Evaluate the XPath expression <em>X</em>. The result is a node set
<em>S</em>.
<li>Compute the set <em>S'</em> consisting of all nodes in the input
document that are either present in <em>S</em> or that have an ancestor
in <em>S</em>. This is equal to the union of all the document subtrees
rooted by a node in <em>S</em>.</li>
<li>If the <code>Filter</code> attribute value is <em>intersect</em>, then
compute the intersection of the selected subtrees, <em>S'</em>, with the
filter node set <em>F</em>. The result will include only those nodes that
are in both the filter node set and the selected subtrees:
<em>F' = F INTERSECT S'</em></li>
<li>If the <code>Filter</code> attribute value is <em>subtract</em>, then
compute the subtraction of the selected subtrees, <em>S'</em>, from the
filter node set <em>F</em>. The result will include only those nodes that
are in the filter node set, but not the selected subtrees:
<em>F' = F - S'</em></li>
<li>Otherwise, if the <code>Filter</code> attribute value is <em>union</em>,
then compute the union the selected subtrees, <em>S'</em>, with the filter
node set <em>F</em>. The result will include all those nodes that are in
either the filter node set, the selected subtrees, or both:
<em>F' = F UNION S'</em></li>
<li>Update the filter node set <em>F</em> to be the new node set <em>F'</em>.
</ul>
<li>Finally, after applying all the XPath expressions, compute the output
node set <em>O</em> to be the intersection of the computed filter node set,
<em>F</em>, with the input node set, <em>I</em>. The result will include all
nodes from the input node set that are also in the filter node set:
<em>O = I INTERSECT F</em></li>
<li>An empty input node set will always result in an empty output node set.</li>
</ul>
<p>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:</p>
<ul>
<li>For each XPath expression <em>X</em>, in sequence, evaluate the
expression and store the resulting node set, <em>S</em>, along with
the associated set operation.</li>
<li>Prepend a node set consisting of just the document node, along with
the operation <em>union</em>.
<li>Create a new, empty filter node set.
<li>Iterate through the input document in document order, adding each node
that is encountered to the filter node set <em>F</em> if a flag <em>Z</em>
is true. This flag is computed as follows:</li>
<ul>
<li>Any time a node is encountered that is in any evaluated node set <em>S</em>,
update <em>Z</em> to be true if and only if the node is present in any
subtree-expanded <em>union</em> node set and all subsequent subtree-expanded
<em>intersect</em> node sets but no subsequent subtree-expanded
<em>subtract</em> node sets, or false otherwise. If there are no
subsequent <em>intersect</em> or <em>subtract</em> node sets, then that
part of the test is automatically passed.</li>
<li>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.</li>
<li>The initial value of <em>Z</em> is irrelevant because it will be
automatically computed when the first document node is processed.</li>
</ul>
</ul>
<p>Implementers MAY further observe that, if this transform is followed by
a canonicalization operation, the described filter computation can be efficiently
commingled with the document-order canonicalization processing.</p>
</div>
<h3><a id="sec-Examples" name="sec-Examples">4. Examples of Signature Filter
Transform</a></h3>
<p>The example below illustrates one way to create an enveloped signature
with the signature filter transform. The function <em><strong>here</strong>()</em>
identifies the <code>XPath</code> element, and the subsequent location path obtains the
nearest ancestor <code>Signature</code> element. Due to the <em>subtract</em>
value of the <code>Filter</code> 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 <code>Signature</code> element
containing the example signature filter transform below.</p>
<pre class="xml-example"> <XPath Filter="subtract"
xmlns="http://www.w3.org/2002/06/xmldsig-filter2"
xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">
here()/ancestor::dsig:Signature[1]
</XPath>
</pre>
<div class="">
<p>A suitable signature reference URI to use with this subtract filter would
be <code>URI=""</code> (the entire signature document, without comments),
<code>URI="#xpointer(/)"</code> (the entire signature document, with
comments) or any same-document reference that includes the signature
itself.</p>
<p>An example of an intersect filter is a signature that co-signs another
signature. In this example, a <code>Signature</code> element identified by
<em>PrimaryBorrowSig</em> 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.</p>
<pre class="xml-example"> <XPath Filter="intersect"
xmlns="http://www.w3.org/2002/06/xmldsig-filter2">
id("PrimaryBorrowerSig")
</XPath>
</pre>
<p>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
<code>URI="document.xml"</code>, then this document will be automatically
parsed and just the identified element and its descendants will be signed.</p>
<p>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.</p>
<p>Consider the following document which contains a same-document
enveloped signature reference with an XPath filter containing three
XPath operations:</p>
<pre class="xml-example"> <Document>
<ToBeSigned>
<!-- comment -->
<Data />
<NotToBeSigned>
<ReallyToBeSigned>
<!-- comment -->
<Data />
</ReallyToBeSigned>
</NotToBeSigned>
</ToBeSigned>
<ToBeSigned>
<Data />
<NotToBeSigned>
<Data />
</NotToBeSigned>
</ToBeSigned>
<dsig:Signature
xmlns:dsig="http://www.w3.org/2000/09/xmldsig#"
xmlns:dsig-xpath="http://www.w3.org/2002/06/xmldsig-filter2">
<dsig:SignedInfo>
...
<dsig:Reference URI="">
<dsig:Transforms>
<dsig:Transform
Algorithm="http://www.w3.org/2002/06/xmldsig-filter2">
<dsig-xpath:XPath Filter="intersect"> //ToBeSigned </dsig-xpath:XPath>
<dsig-xpath:XPath Filter="subtract"> //NotToBeSigned </dsig-xpath:XPath>
<dsig-xpath:XPath Filter="union"> //ReallyToBeSigned </dsig-xpath:XPath>
</dsig:Transform>
</dsig:Transforms>
...
</dsig:Reference>
</dsig:SignedInfo>
...
</dsig:Signature>
</Document>
</pre>
<p>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 <code>ToBeSigned</code> elements; these are expanded to include
all their descendents and intersected with the filter node set, resulting in
the following:</p>
<pre class="xml-example"> <ToBeSigned>
<!-- comment -->
<Data />
<NotToBeSigned>
<ReallyToBeSigned>
<!-- comment -->
<Data />
</ReallyToBeSigned>
</NotToBeSigned>
</ToBeSigned><ToBeSigned>
<Data />
<NotToBeSigned>
<Data />
</NotToBeSigned>
</ToBeSigned>
</pre>
<p>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 <code>NotToBeSigned</code> elements; these are expanded
to include all their descendents and subtracted from the filter node set:</p>
<pre class="xml-example"> <ToBeSigned>
<!-- comment -->
<Data />
</ToBeSigned><ToBeSigned>
<Data />
</ToBeSigned>
</pre>
<p>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 <code>ReallyToBeSigned</code> element; this is expanded to
include all its descendents and added to the filter node set:</p>
<pre class="xml-example"> <ToBeSigned>
<!-- comment -->
<Data />
<ReallyToBeSigned>
<!-- comment -->
<Data />
</ReallyToBeSigned>
</ToBeSigned><ToBeSigned>
<Data />
</ToBeSigned>
</pre>
<p>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:</p>
<pre class="xml-example"> <ToBeSigned>
<Data />
<ReallyToBeSigned>
<Data />
</ReallyToBeSigned>
</ToBeSigned><ToBeSigned>
<Data />
</ToBeSigned>
</pre>
<p>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.</p>
<p>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.</p>
<pre class="xml-example"> <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>
</pre>
<p>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, whereas the former 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.</p>
</div>
<h2>5. <a id="sec-References" name="sec-References">References</a></h2>
<dl>
<dt><a id="ref-Keywords" name="ref-Keywords">Keywords</a></dt>
<dd><a href="http://www.ietf.org/rfc/rfc2119.txt">RFC 2119.</a> <em>Key
words for use in RFCs to Indicate Requirement Levels</em>. Best Current
Practice. S. Bradner. March 1997.S. Bradner. March 1997.</dd>
<dd>http://www.ietf.org/rfc/rfc2119.txt</dd>
<dt><a id="ref-URI" name="ref-URI">URI</a></dt>
<dd><a href="http://www.ietf.org/rfc/rfc2396.txt">RFC 2396</a> .
<em>Uniform Resource Identifiers (URI): Generic Syntax.</em> T.
Berners-Lee, R. Fielding, L. Masinter. August 1998.</dd>
<dd><a
href="http://www.ietf.org/rfc/rfc2396.txt">http://www.ietf.org/rfc/rfc2396.txt</a></dd>
<dt><a id="ref-XML" name="ref-XML">XML</a></dt>
<dd><a href="http://www.w3.org/TR/2000/REC-xml-20001006">Extensible
Markup Language (XML) 1.0 (Second Edition).</a> W3C Recommendation. T.
Bray, E. Maler, J. Paoli, C. M. Sperberg-McQueen. October 2000.</dd>
<dd><a
href="http://www.w3.org/TR/2000/REC-xml-20001006">http://www.w3.org/TR/2000/REC-xml-20001006</a>
.</dd>
<dt><a id="ref-XML-C14N" name="ref-XML-C14N">XML-C14N</a></dt>
<dd><a href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315">Canonical
XML.</a> W3C Recommendation. J. Boyer. March 2001.</dd>
<dd><a
href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315">http://www.w3.org/TR/2001/REC-xml-c14n-20010315</a></dd>
<dd><a
href="http://www.ietf.org/rfc/rfc3076.txt">http://www.ietf.org/rfc/rfc3076.txt</a></dd>
<dt><a id="ref-XML-DSig" name="ref-XML-DSig">XML DSig</a></dt>
<dd><a
href="http://www.w3.org/TR/2001/PR-xmldsig-core-20010820/">XML-Signature
Syntax and Processing</a>. IETF Draft/W3C Proposed Recommendation. D.
Eastlake, J. Reagle, and D. Solo. 31 August 2001.</dd>
<dd><a
href="http://www.w3.org/TR/2001/PR-xmldsig-core-20010820/">http://www.w3.org/TR/2001/PR-xmldsig-core-20010820/</a></dd>
<dt><a id="ref-XML-NS" name="ref-XML-NS">XML-NS</a></dt>
<dd><a
href="http://www.w3.org/TR/1999/REC-xml-names-19990114/">Namespaces in
XML</a>. Recommendation. T. Bray, D. Hollander, and A. Layman. Janaury
1999.</dd>
<dd><a
href="http://www.w3.org/TR/1999/REC-xml-names-19990114/">http://www.w3.org/TR/1999/REC-xml-names-19990114/</a></dd>
<dt><a id="ref-XPath" name="ref-XPath">XPath</a></dt>
<dd><a href="http://www.w3.org/TR/1999/REC-xpath-19991116">XML Path
Language (XPath) Version 1.0</a> , W3C Recommendation. eds. James Clark
and Steven DeRose. 16 November 1999. <a
href="http://www.w3.org/TR/1999/REC-xpath-19991116">http://www.w3.org/TR/1999/REC-xpath-19991116</a>.</dd>
</dl>
<!-- =============================================================================== -->
<h2><a id="sec-Acknowledgements" name="sec-Acknowledgements"></a>6.
Acknowledgements (Informative)</h2>
<p>The following people provided valuable feedback that improved the quality
of this specification:</p>
<ul>
<li>Donald Eastlake, Motorola</li>
<li>Christian Geuer-Pollmann, Universität Siegen</li>
</ul>
</body>
</html>
merlin-xpath-filter2-three.tar.gz
(application/x-gzip, 23.6 KB) - not displayed