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</style><link href="http://www.w3.org/StyleSheets/TR/W3C-WD" rel="stylesheet" type="text/css" charset="utf-8"></head><body style="display: inherit; "><div class="head"><p><a href="http://www.w3.org/"><img width="72" height="48" src="http://www.w3.org/Icons/w3c_home" alt="W3C"></a></p><h1 class="title" id="title">XML Signature Streaming Profile of XPath 1.0</h1><h2 id="w3c-working-draft-21-april-2011">W3C Working Draft 21 April 2011</h2><dl><dt>This version:</dt><dd><a href="http://www.w3.org/TR/2011/WD-xmldsig-xpath-20110421/">http://www.w3.org/TR/2011/WD-xmldsig-xpath-20110421/</a></dd><dt>Latest published version:</dt><dd><a href="http://www.w3.org/TR/xmldsig-xpath/">http://www.w3.org/TR/xmldsig-xpath/</a></dd><dt>Latest editor's draft:</dt><dd><a href="http://www.w3.org/2008/xmlsec/Drafts/xmldsig-xpath/">http://www.w3.org/2008/xmlsec/Drafts/xmldsig-xpath/</a></dd><dt>Previous version:</dt><dd><a href="http://www.w3.org/TR/2010/WD-xmldsig-xpath-20100831/">http://www.w3.org/TR/2010/WD-xmldsig-xpath-20100831/</a></dd><dt>Editors:</dt><dd><span>Pratik Datta</span>, <span class="ed_mailto"><a href="mailto:pratik.datta@oracle.com">pratik.datta@oracle.com</a></span> </dd>
<dd><span>Frederick Hirsch</span>, <span class="ed_mailto"><a href="mailto:frederick.hirsch@nokia.com">frederick.hirsch@nokia.com</a></span> </dd>
<dd><span>Meiko Jensen</span>, <span class="ed_mailto"><a href="mailto:Meiko.Jensen@ruhr-uni-bochum.de">Meiko.Jensen@ruhr-uni-bochum.de</a></span> </dd>
</dl><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2011 <a href="http://www.w3.org/"><acronym title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.</p><hr></div>
    <div id="abstract" class="introductory section"><h2>Abstract</h2>
      <p>This document defines a streamable profile of XPath 1.0
      suitable for use 
      with XML Signature 2.0. 
      </p>
    </div><div id="sotd" class="introductory section"><h2>Status of This Document</h2><p><em>This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the <a href="http://www.w3.org/TR/">W3C technical reports index</a> at http://www.w3.org/TR/.</em></p>
               <p>
            This is a W3C Last Call Working Draft of "XML Signature Streaming
            Profile of XPath 1.0". 
               </p>
               <p>A <a href="Overview-pub-diff.html">diff-marked version</a> of this 
                 specification that highlights changes against  the <a href="http://www.w3.org/TR/2010/WD-xmldsig-xpath-20100831/">previous
                   version</a> is available. Major changes in this version include:</p>
               <ul>
                 <li>Update XPath profile based on review with W3C XSLT/XQuery working
                   groups, including various simplifications and corrections.</li>
                 <li>Modified grammar and split into sections:
                 top-level, LocationPaths and Predicates.</li>
                 <li>Add  clarifications related to  1-pass, top-level expressions and predicate.</li>
                 <li>Add security considerations.</li>
                 <li>Editorial formatting and cleanup. Updated editors list.</li>
               </ul>
               <p>
                 This document was originally derived from material in an earlier
                 publication of XML Signature 2.0,  
                 see <a href="http://www.w3.org/TR/2010/WD-xmldsig-core2-20100304/#sec-XPath-2.0">http://www.w3.org/TR/2010/WD-xmldsig-core2-20100304/#sec-XPath-2.0</a>.
               </p>
               
    <p>This document was published by the <a href="http://www.w3.org/2008/xmlsec/">XML Security Working Group</a> as a Last Call Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to <a href="mailto:public-xmlsec@w3.org">public-xmlsec@w3.org</a> (<a href="mailto:public-xmlsec-request@w3.org?subject=subscribe">subscribe</a>, <a href="http://lists.w3.org/Archives/Public/public-xmlsec/">archives</a>). The Last Call period ends 31 May 2011. All feedback is welcome.</p><p>Publication as a Working Draft 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 this document as other than work in progress.</p><p>This is a Last Call Working Draft and thus the Working Group has determined that this document has satisfied the relevant technical requirements and is sufficiently stable to advance through the Technical Recommendation process.</p><p>This document was produced by a group operating under the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent Policy</a>. W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/42458/status" rel="disclosure">public list of any patent disclosures</a> made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the W3C Patent Policy</a>.</p></div><div id="toc" class="section"><h2 class="introductory">Table of Contents</h2><ul class="toc"><li class="tocline"><a href="#sec-Introduction" class="tocxref"><span class="secno">1. </span>Introduction</a></li><li class="tocline"><a href="#sec-Streamable" class="tocxref"><span class="secno">2. </span>Streamable</a><ul class="toc"><li class="tocline"><a href="#sec-Streaming-Signatures" class="tocxref"><span class="secno">2.1 </span>Streaming for XML signatures</a></li><li class="tocline"><a href="#sec-One-Pass-Streaming" class="tocxref"><span class="secno">2.2 </span>Limitations with one-pass streaming</a></li></ul></li><li class="tocline"><a href="#sec-other-requirements" class="tocxref"><span class="secno">3. </span>Other requirements</a></li><li class="tocline"><a href="#sec-definition" class="tocxref"><span class="secno">4. </span>Definition of the Profile</a><ul class="toc"><li class="tocline"><a href="#sec-definition-toplevel" class="tocxref"><span class="secno">4.1 </span>Top level XPath expression</a></li><li class="tocline"><a href="#sec-definition-LocationPath" class="tocxref"><span class="secno">4.2 </span>LocationPaths</a></li><li class="tocline"><a href="#sec-definition-RestrictedPredicate" class="tocxref"><span class="secno">4.3 </span>Predicates</a></li></ul></li><li class="tocline"><a href="#sec-examples" class="tocxref"><span class="secno">5. </span>Examples of XPath expression that are part of this profile</a></li><li class="tocline"><a href="#sec-Algorithm" class="tocxref"><span class="secno">A. </span>Algorithm</a></li><li class="tocline"><a href="#references" class="tocxref"><span class="secno">B. </span>References</a><ul class="toc"><li class="tocline"><a href="#normative-references" class="tocxref"><span class="secno">B.1 </span>Normative references</a></li><li class="tocline"><a href="#informative-references" class="tocxref"><span class="secno">B.2 </span>Informative references</a></li></ul></li></ul></div>
    
    <div id="sec-Introduction" class="section">
      <!--OddPage--><h2><span class="secno">1. </span>Introduction</h2>
      <p>This document specifies a streamable profile of XPath 1.0
      [<cite><a class="bibref" rel="biblioentry" href="#bib-XPATH">XPATH</a></cite>] for use in XML 
      Signature 2.0 [<cite><a class="bibref" rel="biblioentry" href="#bib-XMLDSIG-CORE2">XMLDSIG-CORE2</a></cite>]. It is a proper subset of XPath 1.0, i.e. any XPath expression that is 
      part of this subset is also a valid XPath 1.0 expression, and when evaluated using the streaming algorithm
      mentioned here, produces exactly the same results as those produced by a regular DOM based XPath engine.
      Although this XPath subset has been designed in the context of XML Signature 2.0, it is 
      a general purpose subset and can be used in other contexts too.
      </p>
      <p>
      The motivation for introducing this 
      profile is outlined here. </p>
      <p>XML Signature lets one
      sign parts of the XML document. In 1.x version of XML Signature [<cite><a class="bibref" rel="biblioentry" href="#bib-XMLDSIG-CORE1">XMLDSIG-CORE1</a></cite>],
      the part to be signed can be identified  
      in one of the following ways: 
</p><ol>
<li><p><b>ID based references</b> This is the simplest and the most popular mechanism. An ID attribute is added 
to the element to be signed, and the signature refers to this element
    by this ID. However this approach has certain 
problems: 
</p><ol style="list-style-type:lower-alpha">
<li> the document needs to be changed to add the id, this may not be possible in all situations, 
especially if the schema does not allow an ID attribute for that element. </li>
<li> ID based references are subject
to wrapping attacks, these attacks can be avoided by using XPath expressions.</li>
<li> this mechanism can only sign complete subtrees, i.e. parts of a subtree cannot be excluded.</li>
</ol>
<p></p></li>
<li><p><b>XPath Filter Transform</b> This is the original XPath mechanism in XML Signature 1.0. It solves the three 
problems mentioned above, but it introduces new problems:
</p><ol style="list-style-type:lower-alpha"><li>it is extremely inefficient, because the XPath needs to be 
evaluated for every node of the document.</li> 
<li>the XPath is not a "normal" XPath, because it has to be of the
form of a boolean expression that is evaluated with
every node as the context node. For example the XPath <code>//chapter/</code> (i.e. all <code>chapter</code>)
descendants has to be expressed as <code>ancestor-or-self::chapter</code> (i.e. a boolean expression which evaluates 
to true for a node, if that node has an ancestor call <code>chapter</code>). Not only is this very hard to understand
, but also some XPaths cannot be expressed like this at all. For example it is not possible to express 
<code>/book/chapter[3]</code> in this model.</li>
</ol><p></p></li>
<li><p><b>XPath Filter 2 Transform</b> This was introduced to solve the problems in above problems [<cite><a class="bibref" rel="biblioentry" href="#bib-XMLDSIG-XPATH-FILTER2">XMLDSIG-XPATH-FILTER2</a></cite>]. However it has a few problems
of its own:
</p><ol style="list-style-type:lower-alpha"><li>although there are implementations of it, it was not popular because it is marked as optional to implement.</li>
<li>it is not streamable in general, although some limited set may be streamable. Note ID based references are streamable.</li>
</ol>
<p></p></li>
<li><p><b>XPointer</b> At the time the XML Signature 1.0 was written, XPointer supported a full XPath model,
but it was still under development. Later on it split up into multiple specs, the full XPath support remained as a Working draft [<cite><a class="bibref" rel="biblioentry" href="#bib-XPTR-XPOINTER">XPTR-XPOINTER</a></cite>],
and only the XPointer element scheme [<cite><a class="bibref" rel="biblioentry" href="#bib-XPTR-ELEMENT">XPTR-ELEMENT</a></cite>] became an
    official W3C Recommendation. The XPointer 
element scheme does not support generic XPaths, it only
allows basic addressing of XML elements e.g. <code>element(/1/2)</code> identifies the 2nd child of the root element.
</p><ol style="list-style-type:lower-alpha">
<li>there are almost no implementations of XPointer with XPath, in fact the XML Signature specification actively discourages it.</li>
<li>this mechanism can only sign complete subtrees, with no exclusions.</li>
</ol>
<p></p></li>
</ol>
      <p></p>
      
      <p>XML Signature 2.0, retains all 1.x mechanisms for backwards compatibility, but it introduces a new mechanism 
      <code>&lt;ds2:Selection&gt;</code> which can be viewed as a very simplified form of XPath Filter 2 Transform. It 
      consists of <code>URI</code> which can be used for ID based references and an <code>IncludedXPath</code> and <code>ExcludedXPath</code>
      for inclusions and exclusions. The result of the selection is subtrees identified by included XPath, minus the subtrees 
      identified by excluded XPath. These <code>IncludedXPath</code> and <code>ExcludedXPath</code> take a profile of XPath, and 
      and it is this profile that this document describes.
      </p>
      <p>This 2.0 selection mechanism, has all the advantages of XPath Filter 2 transform, and additionally it is designed to
      support streaming. However it does the restrict the kind 
      of selection - only subtrees with one round of subtree exclusion can be selected. This restriction is required for high performance.
      </p> 
    </div>

<div id="sec-Streamable" class="section">
  <!--OddPage--><h2><span class="secno">2. </span>Streamable</h2>
<p>The XPath profile defined in this document is streamable with single-pass pre-order XPath recognition.  This means 
</p><ul>
  <li><p>All the XPaths to be evaluated are known at the beginning of the one-pass.</p></li>
  <li><p>It should be possible to evaluate these XPaths on large XML documents without having having to load the entire document into memory. 
  The implementation should read the XML one chunk at a time, and do a single forward only pass over the document.</p></li>
  
  <li><p>More specifically, the XPath engine should work off a streaming XML parser. A streaming parser is a software module that reads the XML document, and 
  constructs a stream of XML events like "beginElement", "text", "endElement" etc. [<cite><a class="bibref" rel="biblioentry" href="#bib-XML-PARSER-STAX">XML-PARSER-STAX</a></cite>] is an example of a streaming parser. The
  <a href="#sec-Algorithm">Algorithm Section</a> describes one
  possible algorithm to evaluate this XPath subset with a streaming
  parser. In this algorithm 
 each of the XPaths is converted into a state machine
  during initialization then as 
 the XML input document is parsed with a streaming parser
  the resultant XML events are fed as inputs into these state
  machines. The state machines 
  determine whether the current XML event is accepted by the XPath or not. </p></li>
  
  <li><p>The XPath itself might select a large portion of the document, or even the whole document. The result of the XPath evaluation 
  should not be loaded all into memory if it is large. Instead it
  should be pipelined to the processing stage. </p></li> 
   
  <li><p>The XML document may have very large text nodes. The XPath
  engine should not be required to load such large nodes in their
  entirety. Instead they should be split up into multiple text nodes, and processed one by one. </p></li> 

</ul>
<p></p>


<div id="sec-Streaming-Signatures" class="section">
 <h3><span class="secno">2.1 </span>Streaming for XML signatures</h3>

<p>In the context of XML Signature, streaming is absolutely essential for XML gateways which need to perform XML Signature and Encryption 
operations for messages on the wire. It is also important for performance sensitive applications; as streaming improves performance by conserving memory,
which greatly reduces temporary memory allocation, resulting in far less memory garbage collection. 
</p> 
 
<p>For XML Signatures it is not only the XPath expressions that need
to be evaluated in streaming, but the rest of the signature processing
as well (e.g. canonicalization and digesting also need to be performed
in streaming mode). For example a streaming Signature processor could
compute 
Reference 
digests for 2.0 Signatures as follows:
 </p><ul> 
  <li><p>Initialize XPath engines for each of the <code>&lt;IncludedXPath&gt;</code> and <code>&lt;ExcludedXPath&gt;</code> for each <code>&lt;Reference&gt;</code>. </p> </li>
  <li><p>Initialize a "Selector", "Canonicalizer" and a "Digestor" for each Reference and put them into a pipeline.
    </p><ol>
      <li><p>The "Selector" takes as input XML events generated by the Parser. It checks if the XML event is either a 
      descendant of the subtree identified by the <code>&lt;Selection&gt;</code>'s <code>URI</code> attribute or accepted 
      by the IncludedXPath engine and not accepted by the ExcludedXPath engine. Exclusions always trump Inclusions, and Exclusions also apply to 
      ID references.
      If the check passes, it passes on the 
      XML event to the "Canonicalizer".  
      <br>
      Note a <code>&lt;Selection&gt;</code> can either contain the <code>URI</code> attribute or a <code>&lt;IncludedXPath&gt;</code> subelement, but not
      both. But in either case it can optionally contain the <code>&lt;ExcludedXPath&gt;</code> subelement.</p> </li>
      <li><p>The "Canonicalizer" inputs XML events passed on by the Selector, and emits byte arrays for each event, and sends them to the "Digestor".</p> </li>
      <li><p>The "Digestor" takes byte arrays and computes a running digest.</p> </li>
    </ol>
  <p></p> </li>   
  <li><p>Start parsing the current XML document using a streaming XML parser, and feed the XML events to each of the Reference processing pipelines.</p></li>
  <li><p>At the end of parsing the pipelines will contain the computed digest for each Reference. Note the same XML event 
  may be accepted by more than one Reference, and hence included is multiple digests.</p> </li>
 </ul>

<p>
For simplicity the above steps make a simplifying assumption that all the <code>&lt;Reference&gt;</code>s have a 
<code>&lt;Selection&gt;</code> of <code>Type="http://www.w3.org/2010/xmldsig2#xml"</code>, 
and each of the 
<code>&lt;Selection&gt;</code> <code>URI</code>s are same document references. If the <code>&lt;Selection&gt;</code> <code>URI</code>s 
refer to external resource, the URI should be dereferenced to fetch the external XML document,
and the XPaths be evaluated on this external document, instead of the current document.   
</p>
</div>

<div id="sec-One-Pass-Streaming" class="section">
 <h3><span class="secno">2.2 </span>Limitations with one-pass streaming</h3>

<p>
Note that it is not always possible to apply or verify XML Signatures in
a one-pass streaming fashion. For instance, the verification of an
enveloped signature requires an XPath for selection that matches an
element that is located prior to the XML Signature itself (in document
order). Hence, on signature verification, the selected elements are
processed by the streaming parser before the selection XPath itself gets
parsed. Example:
</p>

<pre class="example sh_xml sh_sourceCode"><span class="sh_keyword">&lt;Document&gt;</span>

  <span class="sh_keyword">&lt;DataBlock1</span> <span class="sh_keyword">/&gt;</span>

  <span class="sh_keyword">&lt;Signature</span> <span class="sh_type">xmlns</span><span class="sh_symbol">=</span><span class="sh_string">"http://www.w3.org/2000/09/xmldsig#"</span><span class="sh_keyword">&gt;</span>
    <span class="sh_keyword">&lt;SignedInfo&gt;</span> 
      [...]
      <span class="sh_keyword">&lt;Reference&gt;</span>
        <span class="sh_keyword">&lt;Transforms&gt;</span>
          <span class="sh_keyword">&lt;Transform</span> <span class="sh_type">Algorithm</span><span class="sh_symbol">=</span><span class="sh_string">"http://www.w3.org/2010/xmldsig2#transform"</span><span class="sh_keyword">&gt;</span>
            <span class="sh_keyword">&lt;dsig2:Selection</span> <span class="sh_type">xmlns:dsig2</span><span class="sh_symbol">=</span><span class="sh_string">"http://www.w3.org/2010/xmldsig2#"</span>
                 <span class="sh_type">type</span><span class="sh_symbol">=</span><span class="sh_string">"http://www.w3.org/2010/xmldsig2#xml"</span>  <span class="sh_type">URI</span><span class="sh_symbol">=</span><span class="sh_string">""</span> <span class="sh_keyword">/&gt;</span>
            [...]
          <span class="sh_keyword">&lt;/Transform&gt;</span>
        <span class="sh_keyword">&lt;/Transforms&gt;</span>
        [...]
      <span class="sh_keyword">&lt;/Reference&gt;</span>
    <span class="sh_keyword">&lt;/SignedInfo&gt;</span>
    [...]
  <span class="sh_keyword">&lt;/Signature&gt;</span>  

  <span class="sh_keyword">&lt;DataBlock2</span> <span class="sh_keyword">/&gt;</span>

<span class="sh_keyword">&lt;/Document&gt;</span></pre>

<p>
As can be seen, the XML Signature selects the whole document, hence all
XML elements therein must be processed on signature verification.
However, when parsing this document using a streaming approach, the
verifying application might not know in advance which parts of the
document are protected by the XML Signature. Hence, it will start
parsing the document to extract the XPath expressions used for
selection, but once it encounters that information, all the elements
processed before have already been processed and dismissed (such as the
&lt;Document&gt; and &lt;DataBlock1&gt; elements). Thus, these elements have not
been digested, and hence there is no way to verify such an XML Signature
in a one-pass streaming fashion.
</p>

<p>
Note that this impossibility of one-pass streaming is not only affecting
enveloping signatures. For instance, an XML Signature verification with
a selection of

</p><pre class="example sh_xml sh_sourceCode"><span class="sh_keyword">&lt;dsig2:Selection</span> <span class="sh_type">type</span><span class="sh_symbol">=</span><span class="sh_string">"http://www.w3.org/2010/xmldsig2#xml"</span>
   <span class="sh_type">xmlns:dsig2</span><span class="sh_symbol">=</span><span class="sh_string">"http://www.w3.org/2010/xmldsig2#"</span> <span class="sh_type">URI</span><span class="sh_symbol">=</span><span class="sh_string">""</span> <span class="sh_keyword">&gt;</span>
   <span class="sh_keyword">&lt;dsig2:IncludedXPath&gt;</span> //DataBlock1 <span class="sh_keyword">&lt;/dsig2:IncludedXPath&gt;</span>
<span class="sh_keyword">&lt;/dsig2:Selection&gt;</span></pre>

would have also failed due to the same issue, though not being an
enveloped signature. The same holds for ID-based selection if the
selected elements occur prior to the XML Signature in document order.
<p></p>

<p>
These problems can be alleviated by doing the verification in two passes, the
first pass merely scanning the document for the <code>&lt;Signature&gt;</code> and the second pass 
actually evaluating the XPath , canonicalizing and computing the digest.  

Applications that require pure one pass processing  <em class="rfc2119" title="should">should</em> avoid backward references of any kind.
</p>
</div>
</div>

<div id="sec-other-requirements" class="section">
 <!--OddPage--><h2><span class="secno">3. </span>Other requirements</h2>
<p>
Apart from streaming, the XPath profile also needs to satisfy the following requirements:
</p><ul>
<li>  <p>
The profile should produce results that are compatible with the C14N
2.0 data model, i.e. it should 
only result in element nodes or attribute nodes (but not xml: attribute and namespace
 attributes). </p></li>
<li><p>This XPath profile should include some of the known usages of XPath in XML Signatures.
</p><ol>
  <li>ebXML messages require a signature to exclude elements whose <code>@SOAP:actor</code> attribute matches
  a certain value. Refer section 4.1.3 of [<cite><a class="bibref" rel="biblioentry" href="#bib-EBXML-MSG">EBXML-MSG</a></cite>].</li>
  <li>UK government specification requires signature to include the <code>GovTalkMessage/Body</code>  subtree, but exclude the 
<code>GovTalkMessage/Body/IRevenvelope/IRHeader/IRmark</code>
subtree [<cite><a class="bibref" rel="biblioentry" href="#bib-HMRMC">HMRMC</a></cite>].</li> 
</ol>
<p></p></li>
</ul>
<p></p> 
</div>
   
<div id="sec-definition" class="section">
      <!--OddPage--><h2><span class="secno">4. </span>Definition of the Profile</h2>

<p>

The following tables define this XPath profile. It is a restricted version of grammar in [<cite><a class="bibref" rel="biblioentry" href="#bib-XPATH">XPATH</a></cite>]. Although this grammar 
appears to deviate from the [<cite><a class="bibref" rel="biblioentry" href="#bib-XPATH">XPATH</a></cite>] grammar, it is in fact a 
proper subset of  XPath 1.0, i.e. any XPath expression defined by this
grammar is also a valid XPath 1.0 expression and  
has exactly the same meaning as that defined by the XPath 1.0 specification.
</p>

<div id="sec-definition-toplevel" class="section">
<h3><span class="secno">4.1 </span>Top level XPath expression</h3>
  <table class="simple">
    <thead>
      <tr>
        <th valign="top">
          <p>Grammar</p>
        </th>

        <th valign="top">
          <p>Explanation</p>
        </th>
      </tr>
     </thead>
     <tbody>
      <tr>
        <td valign="top">
<pre><dfn id="dfn-xpath">XPath</dfn> ::=
      (<a title="AbsoluteLocationPath" href="#dfn-absolutelocationpath" class="internalDFN">AbsoluteLocationPath</a> '|' )* <a title="AbsoluteLocationPath" href="#dfn-absolutelocationpath" class="internalDFN">AbsoluteLocationPath</a>
</pre>
        </td>
        <td valign="top">
          At the top level it is union of absolute locationPaths. e.g. <code>/a/b | //a[@c]</code>. 
          <p>
          Note: [<cite><a class="bibref" rel="biblioentry" href="#bib-XPATH">XPATH</a></cite>] allows a generic
          Expr in top level, e.g  <code>count(/a/b) + string-length(/a)</code> is a perfectly valid XPath 1.0 expression, but these are not allowed 
          in this subset. That's because the whole goal of this subset is to select a set of subtrees, identified by subtree root elements. E.g. 
          the XPath <code>/a/b</code> returns all <code>b</code> child elements of the top level element <code>a</code>. But when this XPath
          expression is used in 
          an <code>IncludedXPath</code> in XML Signature 2.0, it means not only these <code>b</code> elements, but all <code>b</code>'s descendants 
          as well are to be selected for signing.
          </p><p>
          Relative Location Paths e.g. <code>../../a</code> are currently not allowed, This is because relative would be most useful 
          in Enveloped signatures in which the signature is contained inside the element to be signed. In such a situation, 
          the relative signature XPath would need to use the ancestor axis to reach the element to be signed, however the ancestor axis is not streamable. 
        </p></td>
      </tr>
    
  </tbody>
</table>
</div>

<div id="sec-definition-LocationPath" class="section">
<h3><span class="secno">4.2 </span>LocationPaths</h3>
      
  <table class="simple">
    <thead>
      <tr>
        <th valign="top">
          <p>Grammar</p>
        </th>

        <th valign="top">
          <p>Explanation</p>
        </th>
      </tr>
     </thead>
     <tbody>
    <tr>
     <td valign="top">
<pre>      
<dfn id="dfn-absolutelocationpath">AbsoluteLocationPath</dfn> ::=  
       '/' <a title="RelativeLocationPath" href="#dfn-relativelocationpath" class="internalDFN">RelativeLocationPath</a>? 
     | <a title="AbbreviatedAbsoluteLocationPath" href="#dfn-abbreviatedabsolutelocationpath" class="internalDFN">AbbreviatedAbsoluteLocationPath</a>

<dfn id="dfn-relativelocationpath">RelativeLocationPath</dfn> ::= 
       <a title="Step" href="#dfn-step" class="internalDFN">Step</a> 
     | <a title="RelativeLocationPath" href="#dfn-relativelocationpath" class="internalDFN">RelativeLocationPath</a> '/' <a title="Step" href="#dfn-step" class="internalDFN">Step</a>
     | <a title="AbbreviatedRelativeLocationPath" href="#dfn-abbreviatedrelativelocationpath" class="internalDFN">AbbreviatedRelativeLocationPath</a>

<dfn id="dfn-abbreviatedabsolutelocationpath">AbbreviatedAbsoluteLocationPath</dfn> ::=  
       '//' <a title="RelativeLocationPath" href="#dfn-relativelocationpath" class="internalDFN">RelativeLocationPath</a>

<dfn id="dfn-abbreviatedrelativelocationpath">AbbreviatedRelativeLocationPath</dfn> ::=  
       <a title="RelativeLocationPath" href="#dfn-relativelocationpath" class="internalDFN">RelativeLocationPath</a> '//' <a title="Step" href="#dfn-step" class="internalDFN">Step</a>
</pre>
    </td>
    <td valign="top">
    An absolute location path starts with a slash and has consists of one or many "steps" separated by slash. e.g. <code>/a/b</code>.
    Double slashes are also allowed, they are short for <code>/descendant-or-self::node()/</code>.
    </td>
   </tr>
   <tr>
     <td valign="top">
<pre>
<dfn id="dfn-step">Step</dfn> ::= 
      <a title="AxisSpecifier" href="#dfn-axisspecifier" class="internalDFN">AxisSpecifier</a> <a title="NameTest" href="#dfn-nametest" class="internalDFN">NameTest</a> <a title="RestrictedPredicate" href="#dfn-restrictedpredicate" class="internalDFN">RestrictedPredicate</a>*
    | <a title="AbbreviatedStep">AbbreviatedStep</a>
   

<dfn id="dfn-axisspecifier">AxisSpecifier</dfn> ::=  
      <a title="AxisName" href="#dfn-axisname" class="internalDFN">AxisName</a> '::' 
    | <a title="AbbreviatedAxisSpecifier" href="#dfn-abbreviatedaxisspecifier" class="internalDFN">AbbreviatedAxisSpecifier</a>

<dfn id="dfn-abbreviatedaxisspecifier">AbbreviatedAxisSpecifier</dfn> ::= 
       '@'?

<dfn id="dfn-axisname">AxisName</dfn> ::= 
      'attribute' 
    | 'child' 
    | 'descendant' 
    | 'descendant-or-self' 
    | 'following' 
    | 'following-sibling' 
    | 'self'
    
<dfn id="dfn-nametest">NameTest</dfn> ::= 
      '*' 
    | <a title="NCName">NCName</a> ':' '*' 
    | <a title="QName">QName</a>
   
</pre>
  </td>
  <td valign="top">
  A Step consists of an AxisSpecifier a NameTest, and zero or more predicates. The AxisSpecifier allows only forward axes; all the backward axes
   <code>ancestor</code>, <code>ancestor-or-self</code>,  
   <code>parent</code>, <code>preceding</code>, <code>preceding-sibling</code> are not streamable, and hence not allowed.
   The <code>namespace</code> is also not streamable, so it is disallowed too.
   <p>
   Examples of allowed Steps : <code>b[2]</code>, <code>*</code>, <code>following::chapter</code>.
   </p><p>
   This Step is a restricted form of the Step in XPath 1.0 in two aspects. First it only allows a restricted set of Predicate expressions
   that use attributes only, this restriction is described in the next section; and second it doesn't allow NodeTests. 
   e.g. <code>child::text()</code> is not an allowed Step. This is because some predicates involving text nodes are not streamable.
   </p><p>
    Consider
   this Step <code>child::text()[contains(.,"foo")]</code>, it selects all child text nodes that contain the string "foo". But as mentioned
   earlier, text nodes can be extremely long, so a streaming XPath processor will be processing text nodes in chunks. The "foo" substring may be 
   present at the very end of a large text node, so when the streaming XPath processor encounters the "foo", it would have already gotten past all 
   previous chunks of this text node, and with streaming there is no way to rewind back to the beginning of this text node.
   </p><p>
   Because of this possibility of large text nodes, this XPath subset eliminates all mechanisms of selecting text nodes, either directly 
   or through other mechanisms e.g. string-value.
   </p><p>
   Although the <code>node()</code> is not allowed, <code>//</code> which is an abbreviation for <code>/descendant-or-self::node()/</code>
   is still allowed, because <code>//</code> always need to be followed by Step, so it cannot be used to select text nodes.
  <code>
  </code></p></td>
</tr>
</tbody>
</table>
</div>

<div id="sec-definition-RestrictedPredicate" class="section">
<h3><span class="secno">4.3 </span>Predicates</h3>
Although this XPath subset allows predicates, with all kinds of arithmetic, relational and boolean operators, including functions and variables, 
these predicates are restricted to only referrring to the attributes of the current element.
So an XPath expression like <code>/a/b[c/d]</code> is not allowed. 
<p>
These kinds of XPath expressions cannot be streamed in general, e.g. in the
 <code>/a/b[c/d]</code> 'b' may have a lot of children, with 'c' being the last one. To determine if 'b' is included or not, the XPath processor
 needs to traverse through all the children of 'b', searching for the existence of 'c'. By the time it finds 'c', all the previous children of 'b' 
 have already been passed, and there is no way to rewind back to beginning of 'b'.
 
</p><table class="simple">
   <thead>
      <tr>
        <th valign="top">
          <p>Grammar</p>
        </th>

        <th valign="top">
          <p>Explanation</p>
        </th>
      </tr>
     </thead>
<tbody>
<tr>
  <td valign="top">

<pre><dfn id="dfn-restrictedpredicate">RestrictedPredicate</dfn> ::= 
     '[' <a title="AttributeExpr" href="#dfn-attributeexpr" class="internalDFN">AttributeExpr</a> ']'

<dfn id="dfn-attributeexpr">AttributeExpr</dfn> ::= 
      <a title="OrExpr" href="#dfn-orexpr" class="internalDFN">OrExpr</a>

<dfn id="dfn-orexpr">OrExpr</dfn> ::= 
      <a title="AndExpr" href="#dfn-andexpr" class="internalDFN">AndExpr</a> 
    | <a title="OrExpr" href="#dfn-orexpr" class="internalDFN">OrExpr</a> 'or' <a title="AndExpr" href="#dfn-andexpr" class="internalDFN">AndExpr</a>

<dfn id="dfn-andexpr">AndExpr</dfn> ::= 
      <a title="EqualityExpr" href="#dfn-equalityexpr" class="internalDFN">EqualityExpr</a> 
    | <a title="AndExpr" href="#dfn-andexpr" class="internalDFN">AndExpr</a> 'and' <a title="EqualityExpr" href="#dfn-equalityexpr" class="internalDFN">EqualityExpr</a>

<dfn id="dfn-equalityexpr">EqualityExpr</dfn> ::= 
      <a title="RelationalExpr" href="#dfn-relationalexpr" class="internalDFN">RelationalExpr</a>
    | <a title="EqualityExpr" href="#dfn-equalityexpr" class="internalDFN">EqualityExpr</a> '=' <a title="RelationalExpr" href="#dfn-relationalexpr" class="internalDFN">RelationalExpr</a>
    | <a title="EqualityExpr" href="#dfn-equalityexpr" class="internalDFN">EqualityExpr</a> '!=' <a title="RelationalExpr" href="#dfn-relationalexpr" class="internalDFN">RelationalExpr</a>
    
<dfn id="dfn-relationalexpr">RelationalExpr</dfn> ::= 
      <a title="AdditiveExpr" href="#dfn-additiveexpr" class="internalDFN">AdditiveExpr</a>
    | <a title="RelationalExpr" href="#dfn-relationalexpr" class="internalDFN">RelationalExpr</a> '&lt;' <a title="AdditiveExpr" href="#dfn-additiveexpr" class="internalDFN">AdditiveExpr</a>
    | <a title="RelationalExpr" href="#dfn-relationalexpr" class="internalDFN">RelationalExpr</a> '&gt;' <a title="AdditiveExpr" href="#dfn-additiveexpr" class="internalDFN">AdditiveExpr</a>
    | <a title="RelationalExpr" href="#dfn-relationalexpr" class="internalDFN">RelationalExpr</a> '&lt;=' <a title="AdditiveExpr" href="#dfn-additiveexpr" class="internalDFN">AdditiveExpr</a>
    | <a title="RelationalExpr" href="#dfn-relationalexpr" class="internalDFN">RelationalExpr</a> '&gt;=' <a title="AdditiveExpr" href="#dfn-additiveexpr" class="internalDFN">AdditiveExpr</a>

<dfn id="dfn-additiveexpr">AdditiveExpr</dfn> ::=
      <a title="MultiplicativeExpr" href="#dfn-multiplicativeexpr" class="internalDFN">MultiplicativeExpr</a>
    | <a title="AdditiveExpr" href="#dfn-additiveexpr" class="internalDFN">AdditiveExpr</a> '+' <a title="MultiplicativeExpr" href="#dfn-multiplicativeexpr" class="internalDFN">MultiplicativeExpr</a>
    | <a title="AdditiveExpr" href="#dfn-additiveexpr" class="internalDFN">AdditiveExpr</a> '-' <a title="MultiplicativeExpr" href="#dfn-multiplicativeexpr" class="internalDFN">MultiplicativeExpr</a>
    
<dfn id="dfn-multiplicativeexpr">MultiplicativeExpr</dfn> ::= 
      <a title="UnaryExpr" href="#dfn-unaryexpr" class="internalDFN">UnaryExpr</a>
    | <a title="MultiplicativeExpr" href="#dfn-multiplicativeexpr" class="internalDFN">MultiplicativeExpr</a> <a title="MultiplyOperator">MultiplyOperator</a> <a title="UnaryExpr" href="#dfn-unaryexpr" class="internalDFN">UnaryExpr</a>
    | <a title="MultiplicativeExpr" href="#dfn-multiplicativeexpr" class="internalDFN">MultiplicativeExpr</a> 'div' <a title="UnaryExpr" href="#dfn-unaryexpr" class="internalDFN">UnaryExpr</a>
    | <a title="MultiplicativeExpr" href="#dfn-multiplicativeexpr" class="internalDFN">MultiplicativeExpr</a> 'mod' <a title="UnaryExpr" href="#dfn-unaryexpr" class="internalDFN">UnaryExpr</a>

<dfn id="dfn-unaryexpr">UnaryExpr</dfn> ::= 
      <a title="PrimaryExpr" href="#dfn-primaryexpr" class="internalDFN">PrimaryExpr</a>
    | <a title="AttributeReference" href="#dfn-attributereference" class="internalDFN">AttributeReference</a>
    | '-' <a title="UnaryExpr" href="#dfn-unaryexpr" class="internalDFN">UnaryExpr</a>

<dfn id="dfn-attributereference">AttributeReference</dfn> ::=  
      'attribute' '::' <a title="NameTest" href="#dfn-nametest" class="internalDFN">NameTest</a>
    | '@' <a title="NameTest" href="#dfn-nametest" class="internalDFN">NameTest</a>

</pre>
 </td>
 <td valign="top">
 An AttributeExpression is an expression involving arithmentic, boolean and relational operators. It can only involve AttributeReferences or PrimaryExprs.
 <p>
 An AttributeReference is reference to an attribute of the current element. e.g. <code>@title</code>.
 </p></td>
 </tr>
 <tr>
 <td valign="top">
<pre> 
<dfn id="dfn-primaryexpr">PrimaryExpr</dfn> ::= 
      <a title="VariableReference" href="#dfn-variablereference" class="internalDFN">VariableReference</a>
    | '(' <a title="AttributeExpr" href="#dfn-attributeexpr" class="internalDFN">AttributeExpr</a> ')'
    | <a title="Literal" href="#dfn-literal" class="internalDFN">Literal</a>
    | <a title="Number" href="#dfn-number" class="internalDFN">Number</a>
    | <a title="FunctionCall" href="#dfn-functioncall" class="internalDFN">FunctionCall</a>


<dfn id="dfn-literal">Literal</dfn> ::= '"' [^"]* '"' | "'" [^']* "'"

<dfn id="dfn-number">Number</dfn> ::= <a title="Digits" href="#dfn-digits" class="internalDFN">Digits</a> ('.' <a title="Digits" href="#dfn-digits" class="internalDFN">Digits</a>?)?  | '.' <a title="Digits" href="#dfn-digits" class="internalDFN">Digits</a>

<dfn id="dfn-digits">Digits</dfn> ::= [0-9]+
 
<dfn id="dfn-functioncall">FunctionCall</dfn> ::= 
      <a title="FunctionName" href="#dfn-functionname" class="internalDFN">FunctionName</a> '(' ( <a title="Argument" href="#dfn-argument" class="internalDFN">Argument</a> ( ',' <a title="Argument" href="#dfn-argument" class="internalDFN">Argument</a> )* )? ')'

<dfn id="dfn-argument">Argument</dfn> ::= <a title="AttributeExpr" href="#dfn-attributeexpr" class="internalDFN">AttributeExpr</a>

<dfn id="dfn-functionname">FunctionName</dfn> ::= <a title="QName">QName</a> - <a title="NodeType">NodeType</a>

<dfn id="dfn-variablereference">VariableReference</dfn> ::= '$' <a title="QName">QName</a>


</pre>
  </td>
        <td valign="top">
A PrimaryExpr is either a literal e.g. <code>"foo"</code> or a number e.g. <code>23</code> or a variable reference e.g. <code>$var1</code> 
or a function call e.g. <code>sum(23, $price)</code>. It can also be a complete AttributeReference.
<p>
node(), comment(), text(), processing-instruction()  are reserved names, and cannot be used for naming functions.
        </p></td>
      </tr>

      <tr>
        <td valign="top">
          Node set functions

          <ul>
     
            <li>
              position()
            </li>

            <li>
              count(nodeset)
            </li>

            <li>
        local-name(nodeset)
            </li>

      <li>
        namespace-uri(nodeset)
            </li>

            <li>
        name(nodeset)
            </li>
          </ul>

          String functions

          <ul class="ul1">
            <li>
        string(object?)
            </li>

            <li>
        concat(string, string, string*)
            </li>

            <li>
        starts-with(string, string)
            </li>

            <li>
        contains(string, string)
            </li>

            <li>
        substring-before(string, string)
            </li>

            <li>
        substring-after(string, string)
            </li>

            <li>
        substring(string, number, number)
            </li>

            <li>
        string-length(string?)
            </li>

            <li>
        normalize-space(string?)
            </li>
          </ul>

          Boolean functions

          <ul class="ul1">
            <li>
              boolean(object)
            </li>

            <li>
              true()
            </li>

            <li>
              false()
            </li>

            <li>
              lang(string)
            </li>
          </ul>

          Number functions

          <ul class="ul1">
            <li>
              number(object?)
            </li>

            <li>
              sum(node-set)
            </li>

            <li>
              floor(number)
            </li>

            <li>
              ceiling(number)
            </li>

            <li>
              round(number)
            </li>
          </ul>
        </td>

        <td valign="top">
          <p>Note:</p>
          
          <p>All of these functions are only allowed inside a predicate. 
          </p>
          
          <p>A predicate's
          expression can only involve attribute nodes of the current element.  Functions can also be
          used inside this expression, but this function's arguments also have to be expressions involving 
          attribute nodes. There is no way to use elements, text nodes comments and processing instructions in predicate expressions.</p>

          <p>The "string-value" of an attribute node is the attributes value. This XPath subset is designed in such a way that "string-value" only
          need to be evaluted on attribute nodes, never on elements, text nodes or others. </p>

          <p>The <code>last()</code> function is not supported because that involves backtracking, streaming 
          parsers cannot do that.</p>

          <p>String, number and boolean functions are all supported. However the no argument forms of <code>string()</code>, 
            <code>string-length()</code> and <code>normalize-space()</code> are not supported, because they involve computing a string-value
            of the current node, and this is not streamable as it involves looking ahead to collecting all descendant text nodes. 
            Also as mentioned before some text nodes can be very large, and cannot be loaded all at once into memory. </p>
            
           <p>The no argument forms of <code>local-name()</code>, <code>namespace-uri()</code> and <code>name()</code> are 
           allowed. For example <code>/a/*[local-name()="foo"]</code> selects all "foo" children of "a" regardless of namespace. </p> 

        </td>
      </tr>
    </tbody>
  </table>

</div>

<p>
Note: The descendant and related axes can be exploited by a denial of service attacks. See section 
"XPath selection that causes denial of service in streaming mode" in [<cite><a class="bibref" rel="biblioentry" href="#bib-XMLDSIG-BESTPRACTICES">XMLDSIG-BESTPRACTICES</a></cite>]. 
</p>

</div>

<div id="sec-examples" class="section">
<!--OddPage--><h2><span class="secno">5. </span>Examples of XPath expression that are part of this profile</h2>
<p>This sections explains the profile with some XPath expressions that are part of this
profile and some that aren't.
</p>

<p> All the XPath examples below are based on the following XML document. </p>
<pre class="example sh_xml sh_sourceCode"><span class="sh_keyword">&lt;book&gt;</span>
  <span class="sh_keyword">&lt;foreword&gt;</span>
  <span class="sh_keyword">&lt;/foreword&gt;</span>
  
  <span class="sh_keyword">&lt;chapter</span> <span class="sh_type">type</span><span class="sh_symbol">=</span><span class="sh_string">"preface"</span><span class="sh_keyword">&gt;</span>
  <span class="sh_keyword">&lt;/chapter&gt;</span>

  <span class="sh_keyword">&lt;chapter&gt;</span>
    <span class="sh_keyword">&lt;title&gt;</span>Hybridism<span class="sh_keyword">&lt;/title&gt;</span>
  <span class="sh_keyword">&lt;/chapter&gt;</span>

  <span class="sh_keyword">&lt;chapter&gt;</span>
  <span class="sh_keyword">&lt;/chapter&gt;</span>

<span class="sh_keyword">&lt;/book&gt;</span></pre>

<p>Examples of XPath expression that are included in the profile.
</p>
	<table class="simple">
		<tbody><tr>
			<th style="text-align:center">#</th>
			<th style="text-align:center">Example</th>
			<th style="text-align:center">Description</th>
			<th style="text-align:center">XML Dsig 2.0</th>
		</tr>
		<tr>
			<td>1</td>
			<td><code>/book/chapter</code></td>
			<td>all <code>chapter</code> children of <code>book</code></td>
			<td>Y</td>
		</tr>
		<tr>
			<td>2</td>
			<td><code>/book/chapter[3]</code></td>
			<td>third <code>chapter</code> child of <code>book</code> </td>
			<td>Y</td>
		</tr>
		<tr>
			<td>3</td>
			<td><code>/book/chapter[@type="preface"]</code></td>
			<td>all <code>chapter</code> children of <code>book</code> that have a <code>type</code> attribute with value <code>preface</code></td>
			<td>Y</td>
		</tr>
		<tr>
			<td>4</td>
			<td><code>/book/chapter[@type="preface"][1]</code></td>
			<td>the first <code>chapter</code> child of <code>book</code> that has a <code>title</code> attribute with
			value preface.</td>
			<td>Y</td>
		</tr>
		<tr>
			<td>5</td>
			<td><code>/book/chapter[2]/title[1]</code></td>
			<td>the first <code>title</code> child of the second <code>chapter</code> child of <code>book</code>.</td>
			<td>Y</td>
		</tr>
		<tr>
			<td>6</td>
			<td><code>/book/chapter[contains(@type,"pre")]</code></td>
			<td>all <code>chapter</code> children of <code>book</code> that have an attribute
			<code>type</code> whose value contains the string "pre".</td>
			<td>Y</td>
		</tr>
		<tr>
			<td>7</td>
			<td><code>/child::book/child::chapter[contains(attribute::type,"pre")]</code></td>
			<td>non abbreviated form of the above</td>
			<td>Y</td>
		</tr>
		<tr>
			<td>8</td>
			<td><code>/book/chapter[position() mod 2 != 0]</code></td>
			<td><code>chapter</code> children of <code>book</code> whose position is an odd number, 
			i.e. the odd numbered chapters of the book.</td>
			<td>Y</td>
		</tr>
		<tr>
			<td>9</td>
			<td><code>/book/chapter[position() mod 2 != 0][@type="preface"]</code></td>
			<td>all the odd numbered chapters whose <code>type</code> is "preface".</td>			
			<td>Y</td>
		</tr>
		<tr>
			<td>10</td>
			<td><code>//chapter</code></td>
			<td>all <code>chapter</code> descendants</td>
			<td>Y</td>
		</tr>
		<tr>
			<td>11</td>
			<td><code>/book/chapter | /book/foreword </code></td>
			<td>all the <code>chapter</code> children of <code>book</code> and all the <code>foreword<code> children of book.</code></code></td>
			<td>Y</td>
		</tr>
		<tr>
			<td>12</td>
			<td><code>//* </code></td>
			<td>all the element nodes in the document.</td>
			<td>Y<br>Note: when this is used to identify a selection in XML Dsig 2.0, it is exactly
			equivalent to  "/*" which select only the document root element.</td>
		</tr>
</tbody></table>

<p>These are examples of XPath expressions that are NOT included in the profile.</p>
<table class="simple">
		<tbody><tr>
			<th style="text-align:center">#</th>
			<th style="text-align:center">Example</th>
			<th style="text-align:center">Description</th>
			<th style="text-align:center">XML Dsig 2.0</th>
		</tr>
		<tr>
			<td>1</td>
			<td><code>/book/chapter[title="Hybridism"]</code></td>
			<td>all <code>chapter</code> children of <code>book</code> that have a <code>title</code> sub element with value <code>Hybridism</code></td>
			<td>N<br>expressions can only involve attributes</td>
		</tr>
		<tr>
			<td>2</td>
			<td><code>(/book)/chapter</code></td>
			<td>Evaluate the (/book) expression and set that to the context node, and get the chapter child
			of that context node.</td>
			<td>N<br>the top level expression cannot have parenthesis or any other operators except the union operator "|"</td>
		</tr>
		<tr>
			<td>3</td>
			<td><code>count(/book/chapter)</code></td>
			<td>count the number of <code>chapter</code> children of <code>book</code> </td>
			<td>N<br></td>
		</tr>
		<tr>
			<td>4</td>
			<td><code>chapter</code></td>
			<td>all <code>chapter</code> element children of context node.</td>
			<td>N<br>relative location paths not allowed.</td>
		</tr>
		<tr>
			<td>5</td>
			<td><code>.</code></td>
			<td>The context node.</td>
			<td>N<br>relative location paths are not allowed.</td>
		</tr>
		<tr>
			<td>6</td>
			<td><code>/book/chapter/title/ancestor-or-self::chapter</code></td>
			<td>the <code>chapter</code> ancestor of <code>/book/chapter/title</code>.</td>
			<td>N<br>Only child, descendant and self axes are allowed.</td>
		</tr>
		<tr>
			<td>7</td>
			<td><code>/book/chapter/title/text()</code></td>
			<td>the text child of <code>/book/chapter/title</code>.</td>
			<td>N<br>text, comment and processing-instructions cannot be selected.</td>
		</tr>
		<tr>
			<td>8</td>
			<td><code>id("i1")</code></td>
			<td>elements that have ID, whose value is "i1".</td>
			<td>N<br>Functions are only allowed inside predicates, and also the <code>id()</code> function is not part of this subset.</td>
		</tr>
		<tr>
			<td>9</td>
			<td><code>/book[chapter/title]</code></td>
			<td>the <code>book</code> element if it has a <code>chapter/title</code> grandchild.</td>
			<td>N<br>only attributes are allowed in predicates.</td>
		</tr>
		<tr>
			<td>10</td>
			<td><code>/book/*[local-name(self::node()) = "chapter"]</code></td>
			<td>the children of <code>book</code> element whose local name is "chapter".</td>
			<td>N<br>Only attributes are allowed in predicates.</td>
		</tr>
		<tr>
			<td>11</td>
			<td><code>/book/chapter[2]/node()</code></td>
			<td>all the child elements of the second <code>chapter</code> of the <code>book</code>.</td>
			<td>N<br>node() test is not allowed, because it can select text nodes too.</td>
		</tr>
		<tr>
			<td>12</td>
			<td><code>/book/chapter or /book/foreword</code></td>
			<td>boolean result is true, i.e. either of the location paths evaluate to non empty.</td>
			<td>N<br>or operator is not allowed at top level. Top level expression can only be union 
			of location paths.</td>
		</tr>
</tbody></table>
</div>

<div id="sec-Algorithm" class="appendix section">
  <!--OddPage--><h2><span class="secno">A. </span>Algorithm</h2>
  
<p>This section outlines an algorithm for a Streaming XPath engine that can execute this XPath subset. It is 
NOT NORMATIVE.
</p>

<p> A streaming XML Parser e.g. [<cite><a class="bibref" rel="biblioentry" href="#bib-XML-PARSER-STAX">XML-PARSER-STAX</a></cite>] which will produce events like StartElement, EndElement, TextNode etc. This event stream will be 
the input for the XPath engine. The StartElement event includes all the attributes in that element. An streaming XML Parser may break up a large text node
into multiple TextNode events. </p>

<ol>
<li>Split up the union expression by <code>"|"</code>.   i.e.  
break up the  <code>locationPath | locationPath | .. </code>  into individual 
location paths.</li>

<li>For each location XPath create a state machine.</li>

<li>Drive the state machines using the events.</li>


</ol>

</div>





<div id="references" class="appendix section"><!--OddPage--><h2><span class="secno">B. </span>References</h2><p>Dated references below are to the latest known or appropriate edition of the referenced work.  The referenced works may be subject to revision, and conformant implementations may follow, and are encouraged to investigate the appropriateness of following, some or all more recent editions or replacements of the works cited. It is in each case implementation-defined which  editions are supported.</p><div id="normative-references" class="section"><h3><span class="secno">B.1 </span>Normative references</h3><dl class="bibliography"><dt id="bib-XPATH">[XPATH]</dt><dd>James Clark; Steven DeRose. <a href="http://www.w3.org/TR/1999/REC-xpath-19991116/"><cite>XML Path Language (XPath) Version 1.0.</cite></a> 16 November 1999. W3C Recommendation. URL: <a href="http://www.w3.org/TR/1999/REC-xpath-19991116/">http://www.w3.org/TR/1999/REC-xpath-19991116/</a> 
</dd></dl></div><div id="informative-references" class="section"><h3><span class="secno">B.2 </span>Informative references</h3><dl class="bibliography"><dt id="bib-EBXML-MSG">[EBXML-MSG]</dt><dd>Ian Jones; Brian Gibb; David Fischer. <a href="http://www.oasis-open.org/committees/download.php/272/ebMS_v2_0.pdf"><cite> OASIS ebXML Message Service Specification</cite></a> 1 April 2002. URL: <a href="http://www.oasis-open.org/committees/download.php/272/ebMS_v2_0.pdf">http://www.oasis-open.org/committees/download.php/272/ebMS_v2_0.pdf</a>
</dd><dt id="bib-HMRMC">[HMRMC]</dt><dd><a href="http://www.hmrc.gov.uk/softwaredevelopers/index.htm"><cite>HM Revenue and customs</cite></a> Her Majesty's Revenue and Customs. URL: <a href="http://www.hmrc.gov.uk/softwaredevelopers/index.htm">http://www.hmrc.gov.uk/softwaredevelopers/index.htm</a> <br> Sample response message with XML signature: <a href="http://www.hmrc.gov.uk/ebu/responsemessages.pdf">http://www.hmrc.gov.uk/ebu/responsemessages.pdf</a>
</dd><dt id="bib-XML-PARSER-STAX">[XML-PARSER-STAX]</dt><dd>Christopher Fry; <a href="http://jcp.org/en/jsr/detail?id=173"><cite>JSR 173: Streaming API for XML for Java Specification</cite></a> 8th October 2003. v1.0  URL: <a href="http://jcp.org/en/jsr/detail?id=173">http://jcp.org/en/jsr/detail?id=173</a>
</dd><dt id="bib-XMLDSIG-BESTPRACTICES">[XMLDSIG-BESTPRACTICES]</dt><dd>Pratik Datta; Frederick Hirsch. <a href="http://www.w3.org/TR/2010/WD-xmldsig-bestpractices-20100204/"><cite>XML Signature Best Practices.</cite></a> 4 February 2010. W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/2010/WD-xmldsig-bestpractices-20100204/">http://www.w3.org/TR/2010/WD-xmldsig-bestpractices-20100204/</a> 
</dd><dt id="bib-XMLDSIG-CORE1">[XMLDSIG-CORE1]</dt><dd>D. Eastlake, J. Reagle, D. Solo, F. Hirsch, T. Roessler, K. Yiu. <a href="http://www.w3.org/TR/2011/CR-xmldsig-core1-20110303/"><cite>XML Signature Syntax and Processing Version 1.1.</cite></a> 3 March 2011. W3C Candidate Recommendation. (Work in progress.) URL: <a href="http://www.w3.org/TR/2011/CR-xmldsig-core1-20110303/">http://www.w3.org/TR/2011/CR-xmldsig-core1-20110303/</a> 
</dd><dt id="bib-XMLDSIG-CORE2">[XMLDSIG-CORE2]</dt><dd>Mark Bartel; John Boyer; Barb Fox et al. <a href="http://www.w3.org/TR/2011/WD-xmldsig-core2-20110421/"><cite>XML Signature Syntax and Processing Version 2.0</cite></a>. 21 April 2011. W3C Last Call Working Draft. URL: <a href="http://www.w3.org/TR/2011/WD-xmldsig-core2-20110421/">http://www.w3.org/TR/2011/WD-xmldsig-core2-20110421/</a>
</dd><dt id="bib-XMLDSIG-XPATH-FILTER2">[XMLDSIG-XPATH-FILTER2]</dt><dd>Merlin Hughes; John Boyer; Joseph Reagle. <a href="http://www.w3.org/TR/2002/REC-xmldsig-filter2-20021108/"><cite>XML-Signature XPath Filter 2.0.</cite></a> 8 November 2002. W3C Recommendation. URL: <a href="http://www.w3.org/TR/2002/REC-xmldsig-filter2-20021108/">http://www.w3.org/TR/2002/REC-xmldsig-filter2-20021108/</a> 
</dd><dt id="bib-XPTR-ELEMENT">[XPTR-ELEMENT]</dt><dd>Norman Walsh; et al. <a href="http://www.w3.org/TR/2003/REC-xptr-element-20030325/"><cite>XPointer element() Scheme.</cite></a> 25 March 2003. W3C Recommendation. URL: <a href="http://www.w3.org/TR/2003/REC-xptr-element-20030325/">http://www.w3.org/TR/2003/REC-xptr-element-20030325/</a> 
</dd><dt id="bib-XPTR-XPOINTER">[XPTR-XPOINTER]</dt><dd>Ron Daniel Jr.; Eve Maler; Steven DeRose. <a href="http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219/"><cite>XPointer xpointer() Scheme.</cite></a> 19 December 2002. W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219/">http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219/</a> 
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