xmlenc-decrypt 60.5 KB

Raw Blame History Permalink

<?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>Decryption Transform for XML Signature</title>
  <style type="text/css">
<!--
/*<![CDATA[*/
u,ins,.ins            { background: white; color: red;}
del,strike,.strike    { background: white; color: silver; text-decoration: line-through;}
code                  { font-weight: normal; }
.def                  { background: #FFFFFF; font-weight: bold}
.link-def             { background: #FFFFFF; color: teal;  font-style: italic;}
.link-sec             { 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; border: none;}
.xml-dtd              { background: #efeff8; color: black;}
/*]]>*/
-->


  </style>
  <link rel="stylesheet" type="text/css"
  href="http://www.w3.org/StyleSheets/TR/W3C-REC.css" />
</head>

<body xml:lang="en" lang="en">

<div class="head">
<p><a href="http://www.w3.org/"><img height="48" width="72" alt="W3C"
src="http://www.w3.org/Icons/w3c_home" /></a></p>

<h1 class="notoc">Decryption Transform for XML Signature</h1>

<h2 class="notoc">W3C Recommendation 10 December 2002</h2>
<dl>
  <dt>This version:</dt>
    <dd><a
      href="http://www.w3.org/TR/2002/REC-xmlenc-decrypt-20021210">http://www.w3.org/TR/2002/REC-xmlenc-decrypt-20021210</a></dd>
  <dt>Latest version:</dt>
    <dd><a
      href="http://www.w3.org/TR/xmlenc-decrypt">http://www.w3.org/TR/xmlenc-decrypt</a></dd>
  <dt>Previous version:</dt>
    <dd><a
      href="http://www.w3.org/TR/2002/PR-xmlenc-decrypt-20021003">http://www.w3.org/TR/2002/PR-xmlenc-decrypt-20021003</a></dd>
  <dt>Editors</dt>
    <dd>Merlin Hughes &lt;<a
      href="mailto:merlin@baltimore.ie">merlin@baltimore.ie</a>&gt;</dd>
    <dd>Takeshi Imamura &lt;<a
      href="mailto:imamu@jp.ibm.com">imamu@jp.ibm.com</a>&gt;</dd>
    <dd>Hiroshi Maruyama &lt;<a
      href="mailto:maruyama@jp.ibm.com">maruyama@jp.ibm.com</a>&gt;</dd>
  <dt>Contributors</dt>
    <dd>See <a href="#sec-acknlowledgements">Acknlowledgements</a></dd>
</dl>

<p>Please see the <a
href="http://www.w3.org/Encryption/2002/12-xmlenc-decrypt-errata"><strong>errata</strong></a>
for this document, which may include some normative corrections. See also <a
href="http://www.w3.org/Encryption/2002/12-xmlenc-translations"><strong>translations</strong></a>.</p>

<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>Abstract</h2>

<p>This document specifies an XML Signature "decryption transform" that
enables XML Signature applications to distinguish between those XML
Encryption structures that were encrypted before signing (and must not be
decrypted) and those that were encrypted after signing (and must be
decrypted) for the signature to validate.</p>

<h2 class="notoc"><a name="sec-Status" id="sec-Status">Status of this
document</a></h2>

<div class="">
<p>This document is the W3C Decryption Transform for XML Signature <a
href="http://www.w3.org/Consortium/Process-20010719/process.html#RecsW3C">Recommendation
(REC)</a>. 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.</p>

<p>This specification was produced by the W3C <a
href="http://www.w3.org/Encryption/2001/Overview.html">XML Encryption Working
Group</a> (<a
href="http://www.w3.org/Encryption/2001/Activity.html">Activity</a>) which
believes the specification is sufficient for the creation of independent
interoperable implementations as demonstrated in the <a
href="http://www.w3.org/Encryption/2002/02-xenc-interop.html#decryption-transform">Interoperablity
Report.</a></p>

<p>Patent disclosures relevant to this specification may be found on the
Working Group's <a
href="http://www.w3.org/Encryption/2001/Disclosures.html">patent disclosure
page</a> in conformance with W3C policy.</p>

<p>Pleae report errors in this document to <a
href="mailto:xml-encryption@w3.org">xml-encryption@w3.org</a> (<a
href="http://lists.w3.org/Archives/Public/xml-encryption/">public
archive</a>).</p>

<p>The list of known errors in this specification is available at <a
href="http://www.w3.org/Encryption/2002/12-xmlenc-decrypt-errata">http://www.w3.org/Encryption/2002/12-xmlenc-decrypt-errata</a>.</p>

<p>The English version of this specification is the only normative version.
Information about translations of this document (if any) is available <a
href="http://www.w3.org/Encryption/2002/12-xmlenc-translations">http://www.w3.org/Encryption/2002/12-xmlenc-translations</a>.</p>

<p>A list of current W3C Recommendations and other technical documents can be
found at <a href="http://www.w3.org/TR/">http://www.w3.org/TR/</a>.</p>
</div>

<h2><a name="sec-ToC" id="sec-ToC">Table of Contents</a></h2>
<ol>
  <li><a href="#sec-introduction">Introduction</a>
    <ol>
      <li><a href="#sec-purpose">Purpose</a></li>
      <li><a href="#sec-conventions">Editorial Conventions</a></li>
      <li><a href="#sec-acknlowledgements">Acknlowledgments</a></li>
    </ol>
  </li>
  <li><a href="#sec-syntax">Decryption Transform Syntax</a></li>
  <li><a href="#sec-xml-mode">XML-Mode Decryption Transform</a>
    <ol>
      <li><a href="#sec-xml-processing">Processing Rules</a></li>
      <li><a href="#sec-xml-creation">Transform Creation
      (Non-Normative)</a></li>
      <li><a href="#sec-xml-example">Example</a></li>
      <li><a href="#sec-xml-restrictions">Restrictions and Limitations</a>
        <ol>
          <li><a href="#sec-well-formed">The Constraint of Well-Formed
            Data</a></li>
          <li><a href="#sec-interiting-xml-attributes">Inheriting Attributes
            from the XML Namespace</a></li>
          <li><a href="#sec-references-structural-changes">References and
            Structural Changes</a></li>
          <li><a href="#sec-references-super-encryption">References and
            Super-Encryption</a></li>
          <li><a href="#sec-References-Non-Barename">References Using
            Non-barename XPointers</a></li>
          <li><a href="#sec-interactions">Interactions with Other
          Filters</a></li>
          <li><a href="#sec-encryptedkey">EncryptedKey is Out of
          Scope</a></li>
        </ol>
      </li>
    </ol>
  </li>
  <li><a href="#sec-binary-mode">Binary-Mode Decryption Transform</a>
    <ol>
      <li><a href="#sec-binary-processing">Processing Rules</a></li>
      <li><a href="#sec-binary-example">Example</a></li>
    </ol>
  </li>
  <li><a href="#sec-security">Security Considerations</a>
    <ol>
      <li><a href="#sec-security-reveal">Signatures Over Encrypted Data May
        Reveal Information</a></li>
      <li><a href="#sec-sign-what-you-see">"Sign What You See"</a></li>
    </ol>
  </li>
  <li><a href="#sec-references">References</a></li>
</ol>
<hr />

<h2><a id="sec-introduction" name="sec-introduction">1 Introduction</a></h2>

<h3><a id="sec-purpose" name="sec-purpose">1.1 Purpose</a></h3>

<p>It has been noted by David Solo in [<a href="#Solo">Solo</a>] that both
signature [<a href="#XML-Signature">XML-Signature</a>] and encryption [<a
href="#XML-Encryption">XML-Encryption</a>] operations may be performed on an
XML document at any time and in any order, especially in scenarios such as
workflow. For example, Alice wishes to order and pay for a book from Bob
using the mutually trusted payment system ZipPay. Bob creates an order form
including the book title, price and his account info. He wants to sign all of
this information, but will subsequently encrypt his account info for ZipPay
only. He sends this to Alice who affirms the book title and price, signs the
form and presents the twice-signed order with her own payment information to
ZipPay. To validate both signatures ZipPay will have to know that the cipher
data version of the encrypted information is necessary for validating Alice's
signature, but the plain data form is necessary for validating Bob's
signature. (See <a class="link-sec" href="#sec-sign-what-you-see">"Sign What
You See"</a> (section 5.2) for more on signing encrypted data.)</p>

<p>Since encryption operations applied to part of the signed content after a
signature operation cause a signature not to be verifiable, it is necessary
to decrypt the portions encrypted after signing before the signature is
verified. The "decryption transform" proposed in this document provides a
mechanism; decrypting only signed-then-encrypted portions (and ignoring
encrypted-then-signed ones). A signer can insert this transform in a
transform sequence (e.g., before Canonical XML [<a
href="#XML-C14N">XML-C14N</a>] or XPath [<a href="#XPath">XPath</a>]) if
there is a possibility that someone will encrypt portions of the
signature.</p>

<p>The transform defined in this document is intended to propose a resolution
to the decryption/verification ordering issue within signed resources. It is
out of scope of this document to deal with the cases where the ordering can
be derived from the context. For example, when a <code>ds:DigestValue</code>
element or a (part of) <code>ds:SignedInfo</code> element is encrypted, the
ordering is obvious (without decryption, signature verification is not
possible) and there is no need to introduce a new transform.</p>

<h3><a id="sec-conventions" name="sec-conventions">1.2 Editorial
Conventions</a></h3>

<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="#Keywords">Keywords</a>].</p>

<p>This document makes use of the [<a
href="#XML-Encryption">XML-Encryption</a>] and [<a
href="#XML-Signature">XML-Signature</a>] namespaces, and defines it own, with
the following prefixes:</p>
<pre>xmlns:xenc="http://www.w3.org/2001/04/xmlenc#"
xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
xmlns:dcrpt="http://www.w3.org/2002/07/decrypt#"</pre>

<p>While implementations MUST support XML and XML namespaces, the use of our
"<code>xenc</code>", "<code>ds</code>", and "<code>dcrpt</code>" XML
namespace prefixes is OPTIONAL; we use this facility to provide compact and
readable exposition. Additionally, the entity <code>&amp;xenc;</code> is
borrowed from [<a href="#XML-Encryption">XML-Encryption</a>] to provide
short-hand identifiers for URIs defined in that specification. For example
<code>"&amp;xenc;Element"</code> corresponds to
"http://www.w3.org/2001/04/xmlenc#Element".</p>

<h3><a id="sec-acknlowledgements" name="sec-acknlowledgements">1.3
Acknlowledgments</a></h3>

<p>The contributions of the following Working Group participants to this
specification are gratefully acknowledged:</p>
<ul>
  <li>Hal Finney, PGP Security</li>
  <li>Amir Herzberg, NewGenPay</li>
  <li>Takeshi Imamura, IBM</li>
  <li>Ari Kermaier, Phaos</li>
  <li>Hiroshi Maruyama, IBM</li>
  <li>Joseph Reagle, W3C</li>
  <li>David Solo, Citigroup</li>
</ul>

<h2><a id="sec-syntax" name="sec-syntax">2 Decryption Transform
Syntax</a></h2>

<p class="">This transform supports two modes of operation. In XML mode the
data is encrypted XML and the result of decryption is a node set. In binary
mode the data is an encrypted octet sequence and the result of decryption is
an octet sequence. In both modes, <code>xenc:EncryptedData</code> elements in
the input node-set can be excluded from processing using
<code>dcrpt:Except</code> elements. <code>dcrpt:Except</code> is defined
below via XML Schema [<a href="#XML-Schema">XML-Schema</a>] and appears as
direct child elements of the <code>ds:Transform</code> element.</p>

<p>The REQUIRED <code>URI</code> attribute value of the
<code>dcrpt:Except</code> element identifies <code>xenc:EncryptedData</code>
elements within the input to the transform. The value MUST be a non-empty
same-document [<a href="#URI">URI</a>] reference (i.e., a number sign '#'
character) followed by an XPointer expression [<a
href="#XPointer">XPointer</a>] as profiled by <a class="link-sec"
href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-ReferenceProcessingModel">The
Reference Processing Model</a> [<a href="#XML-Signature">XML-Signature</a>,
section 4.3.3.2].</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;!DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
    "http://www.w3.org/2001/XMLSchema.dtd" [
    &lt;!ATTLIST schema
      xmlns:dt CDATA #FIXED "http://www.w3.org/2002/07/decrypt#"&gt;
    &lt;!ENTITY % p ''&gt;
    &lt;!ENTITY % s ''&gt;
  ]&gt;

  &lt;schema xmlns="http://www.w3.org/2001/XMLSchema" version="0.1"
          xmlns:dt="http://www.w3.org/2002/07/decrypt#"
          targetNamespace="http://www.w3.org/2002/07/decrypt#"
          elementFormDefault="qualified"&gt;

    &lt;element name="Except" type="dt:ExceptType"/&gt;
    &lt;complexType name="ExceptType"&gt;
      &lt;attribute name="Id" type="ID" use="optional"/&gt;
      &lt;attribute name="URI" type="anyURI" use="required"/&gt;
    &lt;/complexType&gt;
  &lt;/schema&gt;</pre>

<h2><a id="sec-xml-mode" name="sec-xml-mode">3 XML Mode Decryption
Transform</a></h2>
<dl>
  <dt>Identifier:</dt>
    <dd><a name="XML" id="XML"
      href="http://www.w3.org/2002/07/decrypt#XML">http://www.w3.org/2002/07/decrypt#XML</a></dd>
</dl>

<p>This mode of the transform requires an XPath node-set [<a
href="#XPath">XPath</a>] for input. If an octet stream is given as input, it
MUST be converted to a node-set as described in <a class="link-sec"
href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-ReferenceProcessingModel">The
Reference Processing Model</a> (section 4.3.3.2) of the XML Signature
specification [<a href="#XML-Signature">XML-Signature</a>]. The transform
decrypts all the <code>xenc:EncryptedData</code> elements except for those
specified by <code>dcrpt:Except</code> elements. The output of the transform
is a node-set.</p>

<h3><a id="sec-xml-processing" name="sec-xml-processing">3.1 Processing
Rules</a></h3>

<p>This section describes the processing rules of the transform. The rules
are written as two functions; the inputs and outputs of the transform are
those of the <a href="#func-decryptXML" class="link-def">decryptXML()</a>
function, which itself calls the <a href="#func-decryptNodeSet"
class="link-def">decryptNodeSet()</a> function.</p>
<dl>
  <dt><a name="func-decryptXML" id="func-decryptXML" class="link-def">O =
  decryptXML(N, E)</a></dt>
    <dd><p>where <em>N</em> is a node-set and <em>E</em> is a set of
      exception URIs held by <code>URI</code> attributes of
      <code>dcrpt:Except</code> elements. <em>O</em> is a node-set, computed
      as follows:</p>
      <ol>
        <li>Let <em><strong>Y</strong></em>, a set of replacement node-sets,
          be <a href="#func-decryptNodeSet"
          class="link-def">decryptNodeSet(N, <em>E</em>)</a>.</li>
        <li><a class="def" name="def-Canonicalize-with-Replacement"
          id="def-Canonicalize-with-Replacement">Canonicalize-with-Replacement</a>:
          convert <em>N</em> to an octet stream <em>B</em>, (which MUST be <a
          href="#sec-well-formed">well-formed</a> (see section 3.4.1)), using
          [<a
          href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#ref-XML-C14N">XML-C14N</a>]
          as described in <a class="link-sec"
          href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-ReferenceProcessingModel">The
          Reference Processing Model</a> [<a
          href="#XML-Signature">XML-Signature</a>, section 4.3.3.2]; but, in
          place of any decrypted <code>xenc:EncryptedData</code> element
          <em>d</em> and its descendants, process the replacement node-set
          <em>O<sub>d</sub></em>, from <em><strong>Y</strong></em>. During
          this step, canonicalization of the replacement node-set MUST be <a
          href="#sec-interiting-xml-attributes">augmented</a> (see section
          3.4.2) as follows:
          <ul>
            <li>A namespace declaration <code>xmlns=""</code> MUST be emitted
              with every <a class="link-def"
              href="http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/#def-apex-node">apex</a>
              element that has no namespace node declaring a value for the
              default namespace as described in <a class="link-sec"
              href="http://www.w3.org/Encryption/2001/Drafts/xmlenc-core/#sec-Serializing-XML">Serializing
              XML</a> [<a href="#XML-Encryption">XML-Encryption</a>, section
              4.3.3].</li>
            <li>If a node-set is replacing an element from <em>N</em> whose
              parent element is not in <em>N</em>, then its apex elements
              MUST inherit attributes associated with the <a class=""
              href="http://www.w3.org/XML/1998/namespace">XML namespace</a>
              from the parent element., such as <code>xml:base</code>,
              <code>xml:lang</code> and <code>xml:space</code>.</li>
          </ul>
          <p><em>B</em> may not be in canonical form.</p>
        </li>
        <li>Let <em>O</em>, the output of this function, be a node-set
          converted from <em>B</em> as described in <a class="link-sec"
          href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-ReferenceProcessingModel">The
          Reference Processing Model</a> [<a
          href="#XML-Signature">XML-Signature</a>, section 4.3.3.2].
          <ul>
            <li>If parsing of <em>B</em> fails, then the implementation MUST
              signal a failure of the transform.</li>
            <li>Note that even if there are no decrypted
              <code>xenc:EncryptedData</code> elements, then <em>N</em> is
              still canonicalized and parsed.</li>
          </ul>
        </li>
      </ol>
    </dd>
</dl>
<dl>
  <dt><a name="func-decryptNodeSet" id="func-decryptNodeSet"
  class="link-def"><strong>Y</strong> = decryptNodeSet(N, E)</a></dt>
    <dd><p>where <em>N</em> is a node-set and <em>E</em> is a set of
      exception URIs held by <code>URI</code> attributes of
      <code>dcrpt:Except</code> elements. <em><strong>Y</strong></em> is a
      set of node-sets, computed as follows:</p>
      <ol>
        <li>Let <em>D</em> be a node-set containing all element nodes in
          <em>N</em> with the type <code>xenc:EncryptedData</code> that are
          not identified by any exception URI in <em>E</em>.
          <ul>
            <li class="">When dereferencing an exception URI in the context
              of the original input node set, the implementation MUST behave
              as if the document node of the input node-set is used to
              initialize the XPointer evaluation context [<a
              href="#XPointer">XPointer</a>], even if the node is not part of
              the node-set. Unlike XML Signature [<a
              href="#XML-Signature">XML-Signature</a>], the exception URI may
              be evaluated against a different document than the "root node
              of the XML document containing the URI attribute." If the input
              is a different document then, as per XPointer [<a
              href="#XPointer">XPointer</a>], use of the <a
              href="http://www.w3.org/TR/2001/CR-xptr-20010911/#N3568"
              class="link-def">here()</a> function is an error.</li>
            <li class="">When dereferencing an exception URI in the context
              of a replacement node-set, bare name [<a
              href="#XPointer">XPointer</a>] exception URIs are used to
              locate <code>xenc:EncryptedData</code> elements with matching
              <code>Id</code> attributes. Implementors MAY attempt to resolve
              full XPointers into replacement node-sets using appropriate
              techniques to take into account the location of the replacement
              node-set in the input document, see <a class="link-sec"
              href="#sec-References-Non-Barename">References Using
              Non-barename XPointers</a> (section 3.4.5).</li>
            <li class="">If an exception URI fails to dereference any nodes,
              then the resulting error MUST be ignored; it may be the result
              of part of the input document being encrypted.</li>
          </ul>
        </li>
        <li>Let <em><strong>Y</strong></em> be {}, an empty set.</li>
        <li>For each <code>xenc:EncryptedData</code> element <em>d</em> from
          <em>D</em>:
          <ol>
            <li>Decrypt <em>d</em>, without regard for which, if any, of its
              descendants are in <em>N</em>, and process it in accordance
              with the value of its <code>Type</code> attribute, resulting in
              a node-set <em>O<sub>d</sub></em>.
              <ul>
                <li>For example, processing of an
                  <code>xenc:EncryptedData</code> element with the
                  <code>Type</code> attribute whose value is <a
                  href="http://www.w3.org/2001/04/xmlenc#Element">&amp;xenc;Element</a>
                  or <a
                  href="http://www.w3.org/2001/04/xmlenc#Content">&amp;xenc;Content</a>
                  is specified in <a class="link-sec"
                  href="http://www.w3.org/Encryption/2001/Drafts/xmlenc-core/#sec-Decrypt-Imp">A
                  Decrypt Implementation</a> (section 4.3.1) of [<a
                  href="#XML-Encryption">XML-Encryption</a>], and the result
                  is a node-set.</li>
                <li>If the <code>Type</code> attribute is absent, is not
                  known to the decryptor, or the result of its processing is
                  not a node-set, then the implementation MUST signal a
                  failure of the transform.</li>
                <li>If decryption of any <code>xenc:EncryptedData</code>
                  element fails, then the implementation MUST signal a
                  failure of the transform.</li>
              </ul>
            </li>
            <li class="">Replace <strong><em>Y</em></strong> with
              <strong>Y</strong> &#x222a; {<em>O<sub>d</sub>}</em>.</li>
            <li class="">If <em>O</em><sub><em>d</em></sub> contains
              <code>xenc:EncryptedData</code> that are not in <em>E,</em>
              replace <em><strong>Y</strong></em> with
              <em><strong>Y</strong></em> &#x222a; <a
              href="#func-decryptNodeSet"
              class="link-def">decryptNodeSet(O<sub>d</sub>, E)</a>. This
              recursively decrypts <a class="link-def"
              href="http://www.w3.org/TR/xmlenc-core/#sec-eg-Super-Encryption">super-encrypted</a>
              data within the replacement node-set.</li>
          </ol>
        </li>
      </ol>
    </dd>
</dl>

<h3><a id="sec-xml-creation" name="sec-xml-creation">3.2 Transform Creation
(Non-Normative)</a></h3>

<p>This specification does not mandate a mechanism for creating a
<code>ds:Transform</code> element in a [<a
href="#XML-Signature">XML-Signature</a>] transform sequence. However, the
following is one (non-normative) approach:</p>
<ol>
  <li>Apply all the transforms being placed before this transform to a data
    object being signed.</li>
  <li>If the transform just before this transform outputs an octet stream,
    convert it to a node-set as described in <a class="link-sec"
    href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-ReferenceProcessingModel">The
    Reference Processing Model</a> [<a
    href="#XML-Signature">XML-Signature</a>, section 4.3.3.2].</li>
  <li>For each node in the node-set, if the node is an element node with the
    type <code>xenc:EncryptedData</code>, create an <code>dcrpt:Except</code>
    element referencing the node.</li>
  <li>Create a <code>ds:Transform</code> element, including the algorithm
    identifier of this transform and all the <code>dcrpt:Except</code>
    elements created in Step 3.</li>
</ol>

<h3><a id="sec-xml-example" name="sec-xml-example">3.3 Example</a></h3>

<p>Suppose that a part of the following XML document (<code>[02-14]</code>)
is to be signed. Note that a few parts of the document
(<code>[05,11,12]</code>) are already encrypted prior to signature. Also
suppose that the signer anticipates that additional parts of the document
will be encrypted after signing.</p>
<pre class="xml-example">  [01] &lt;Document&gt;
  [02]   &lt;ToBeSigned Id="tbs"&gt;
  [03]     &lt;Part number="1"&gt;
  [04]       &lt;Data&gt;...&lt;/Data&gt;
  [05]       &lt;xenc:EncryptedData Id="#secret-1" .../&gt;
  [06]     &lt;/Part&gt;
  [07]     &lt;Part number="2"&gt;
  [08]       &lt;Data&gt;...&lt;/Data&gt;
  [09]     &lt;/Part&gt;
  [10]     &lt;Secrets&gt;
  [11]       &lt;xenc:EncryptedData .../&gt;
  [12]       &lt;xenc:EncryptedData .../&gt;
  [13]     &lt;/Secrets&gt;
  [14]   &lt;/ToBeSigned&gt;
  [15] &lt;/Document&gt;</pre>

<p>In order to let the recipient know the proper order of decryption and
signature verification, the signer includes the decryption transform
(<code>[a19-a22]</code>) in the signature. The <code>dcrpt:Except</code>
elements (<code>[a20,a21]</code>) identify parts of the document that are
already encrypted.</p>
<pre class="xml-example">  [a01] &lt;Document&gt;
  [a02]   &lt;ToBeSigned Id="tbs"&gt;
  [a03]     &lt;Part number="1"&gt;
  [a04]       &lt;Data&gt;...&lt;/Data&gt;
  [a05]       &lt;xenc:EncryptedData Id="#secret-1" .../&gt;
  [a06]     &lt;/Part&gt;
  [a07]     &lt;Part number="2"&gt;
  [a08]       &lt;Data&gt;...&lt;/Data&gt;
  [a09]     &lt;/Part&gt;
  [a10]     &lt;Secrets&gt;
  [a11]       &lt;xenc:EncryptedData .../&gt;
  [a12]       &lt;xenc:EncryptedData .../&gt;
  [a13]     &lt;/Secrets&gt;
  [a14]   &lt;/ToBeSigned&gt;
  [a15]   &lt;dsig:Signature ...&gt;
  [a16]     ...
  [a17]     &lt;dsig:Reference URI="#tbs"&gt;
  [a18]       &lt;dsig:Transforms&gt;
  [a19]         &lt;dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#XML"&gt;
  [a20]           &lt;dcrpt:Except URI="#secret-1"/&gt;
  [a21]           &lt;dcrpt:Except URI="#xpointer(id('tbs')/Secrets/*)"/&gt;
  [a22]         &lt;/dsig:Transform&gt;
  [a23]       &lt;/dsig:Transforms&gt;
  [a24]       ...
  [a25]     &lt;/dsig:Reference&gt;
  [a26]     ...
  [a27]   &lt;/dsig:Signature&gt;
  [a28] &lt;/Document&gt;</pre>

<p>Consider that this document is subsequently encrypted by various
processes, resulting in the following:</p>
<pre class="xml-example">  [b01] &lt;Document&gt;
  [b02]   &lt;ToBeSigned Id="tbs"&gt;
  [b03]     &lt;xenc:EncryptedData Id="part-1" Type="&amp;enc;Element" .../&gt;
  [b04]     &lt;xenc:EncryptedData Id="part-2" Type="&amp;enc;Element" .../&gt;
  [b05]     &lt;Secrets&gt;
  [b06]       &lt;xenc:EncryptedData .../&gt;
  [b07]       &lt;xenc:EncryptedData .../&gt;
  [b08]     &lt;/Secrets&gt;
  [b09]   &lt;/ToBeSigned&gt;
  [b10]   &lt;dsig:Signature ...&gt;
  [b11]     ...
  [b12]     &lt;dsig:Reference URI="#tbs"&gt;
  [b13]       &lt;dsig:Transforms&gt;
  [b14]         &lt;dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#XML"&gt;
  [b15]           &lt;dcrpt:Except URI="#secret-1"/&gt;
  [b16]           &lt;dcrpt:Except URI="#xpointer(id('tbs')/Secrets/*)"/&gt;
  [b17]         &lt;/dsig:Transform&gt;
  [b18]       &lt;/dsig:Transforms&gt;
  [b19]       ...
  [b20]     &lt;/dsig:Reference&gt;
  [b21]     ...
  [b22]   &lt;/dsig:Signature&gt;
  [b23] &lt;/Document&gt;</pre>

<p>Execution of the decryption transform will proceed as follows:</p>
<ol>
  <li>The input to the transform, <em>N</em>, is a node-set containing the
    <code>ToBeSigned</code> element and its children, less comments
    (<code>[b02-b09]</code>). The parameter to the transform, <em>E</em>, is
    a set containing the two exception URIs (<code>[b15,b16]</code>).</li>
  <li>The first exception URI does not resolve in this document; the second
    resolves to the two children of the <code>Secrets</code> element
    (<code>[b06,b07]</code>).</li>
  <li>As a result, <em>D</em> for <em>N</em> is a node-set consisting of the
    two <code>xenc:EncryptedData</code> elements, <em>d<sub>part-1</sub></em>
    (<code>[d03]</code>) and <em>d<sub>part-2</sub></em>
    (<code>[d04]</code>). Each of these is decrypted, resulting in the
    following node-sets for <em>O<sub>part-1</sub></em> and
    <em>O<sub>part-2</sub></em>:
    <pre class="xml-example">  [c01] &lt;Part number="1"&gt;
  [c02]   &lt;Data&gt;...&lt;/Data&gt;
  [c03]   &lt;xenc:EncryptedData Id="#secret-1" .../&gt;
  [c04] &lt;/Part&gt;</pre>
    <pre class="xml-example">  [d01] &lt;Part number="2"&gt;
  [d02]   &lt;xenc:EncryptedData Id="#data-2" Type="&amp;enc;Element" .../&gt;
  [d03] &lt;/Part&gt;</pre>
  </li>
  <li>After this decryption stage, two new <code>xenc:EncryptedData</code>
    elements (<code>[c03]</code> and <code>[d02]</code>) have been revealed.
    However, the first matches an exception URI with a bare name and so is
    not considered further; hence, <em>D</em> for <em>O<sub>part-1</sub></em>
    is empty while <em>D</em> for <em>O<sub>part-2</sub></em> contains just
    the <code>xenc:EncryptedData</code> element <em>d<sub>data-2</sub></em>
    (<code>[d02]</code>). This is decrypted again, resulting in the following
    node-set <em>O<sub>data-2</sub></em>:
    <pre class="xml-example">  [e01] &lt;Data&gt;...&lt;/Data&gt;</pre>
  </li>
  <li>No new <code>xenc:EncryptedData</code> element are revealed, so
    <em>D</em> for <em>O<sub>data-2</sub></em> is empty and processing falls
    through to canonicalization.</li>
  <li>The canonicalization-with-replace operation canonicalizes the node-set
    <em>N</em>; but, in place of any <code>xenc:EncryptedData</code> elements
    that were decrypted, it canonicalizes the replacement node-sets.
    Similarly, it also replaces any decrypted <code>xenc:EncryptedData</code>
    elements in the replacement node-sets. Further, canonicalization of any
    replacement node-sets is augmented such that <code>xmlns=""</code> is
    emitted on any apex elements that have no namespace node declaring a
    value for the default namespace. The resulting canonicalized data are the
    following:
    <pre class="xml-example">  [f01] &lt;Document&gt;
  [f02]   &lt;ToBeSigned Id="tbs"&gt;
  [f03]     &lt;Part xmlns="" number="1"&gt;
  [f04]       &lt;Data&gt;...&lt;/Data&gt;
  [f05]       &lt;xenc:EncryptedData Id="#secret-1" .../&gt;
  [f06]     &lt;/Part&gt;
  [f07]     &lt;Part xmlns="" number="2"&gt;
  [f08]       &lt;Data xmlns=""&gt;...&lt;/Data&gt;
  [f09]     &lt;/Part&gt;
  [f10]     &lt;Secrets&gt;
  [f11]       &lt;xenc:EncryptedData .../&gt;
  [f12]       &lt;xenc:EncryptedData .../&gt;
  [f13]     &lt;/Secrets&gt;
  [f14]   &lt;/ToBeSigned&gt;
  [f15] &lt;/Document&gt;</pre>
  </li>
  <li>This octet stream is then parsed and returned as the output of the
    transform.</li>
</ol>

<h3><a id="sec-xml-restrictions" name="sec-xml-restrictions">3.4 Restrictions
and Limitations</a></h3>

<h4>3.4.1 <a name="sec-well-formed" id="sec-well-formed">The Constraint of
Well-Formed Data</a></h4>

<p>As specified in step 2 of the <a href="#func-decryptXML"
class="link-def">decryptXML()</a> function, the octet stream resulting from
canonicalization-with-replacement MUST be well-formed. Typically this will be
characterized by a <a href="#def-single-rooted"
class="link-def">single-rooted</a> input node-set, where a node-set is said
to be <a name="def-single-rooted" id="def-single-rooted"
class="link-def">single-rooted</a> if and only if all of its member nodes are
either (1) the first node in the node-set in the document order, (2) a
descendant node of the first node, or (3) an attribute node or a namespace
node of another node in the node-set. Additionally, if the input node-set
has, at its top level, an <code>xenc:EncryptedData</code> element being
decrypted, then this SHOULD correspond to an encrypted <a
href="#def-single-rooted" class="link-def">single-rooted</a> node-set.
However, this need not be the case: after decryption, multiple top-level
nodes may be well-formed if they consist of white space, comments, processing
instructions and a single element. No special processing is required to test
for this condition because ill-formed data will result in a parsing error.</p>

<h4>3.4.2 <a name="sec-interiting-xml-attributes"
id="sec-interiting-xml-attributes">Inheriting Attributes from the XML
Namespace</a></h4>

<p>As specified in step 2 of the <a href="#func-decryptXML"
class="link-def">decryptXML()</a> function, the canonicalization with
replacement step requires XML namespace attribute inheritance. One of the
features of the Canonical XML [<a href="#XML-C14N">XML-C14N</a>] algorithm,
which is automatically applied by the decryption transform, is that all
ancestral attributes from the XML namespace (e.g., <code>xml:lang</code>) are
inherited by any element whose parent node is not in the canonicalized
node-set. The inheritance in step 2 ensures these attributes are preserved
during decryption and replacement. For example, consider the following signed
document:</p>
<pre class="xml-example">  [01] &lt;Document xml:lang="ga"&gt;
  [02]   &lt;ToBeSigned Id="tbs"&gt;
  [03]     ...
  [04]   &lt;/ToBeSigned&gt;
  [05]   &lt;dsig:Signature ...&gt;
  [06]     ...
  [07]     &lt;dsig:Reference URI="#tbs"&gt;
  [08]       &lt;dsig:Transforms&gt;
  [09]         &lt;dsig:Transform Algorithm="http://www.w3.org/2001/04/decrypt#XML" /&gt;
  [10]       &lt;/dsig:Transforms&gt;
  [11]       ...
  [12]     &lt;/dsig:Reference&gt;
  [13]     ...
  [14]   &lt;/dsig:Signature&gt;
  [15] &lt;/Document&gt;</pre>

<p>The canonical form of the <code>ToBeSigned</code> element (the target of
the <code>#tbs</code> reference, <code>[02-04]</code>) is the following
(<code>[a01-a03]</code>):</p>
<pre class="xml-example">  [a01] &lt;ToBeSigned Id="tbs" xml:lang="ga"&gt;
  [a02]     ...
  [a03]   &lt;/ToBeSigned&gt;</pre>

<p>Consider, however, if this top-level signed element is subsequently
encrypted using an XML serialization algorithm that does not include
inherited attributes from the XML namespace (<code>[b02-b04]</code>):</p>
<pre class="xml-example">  [b01] &lt;Document xml:lang="ga"&gt;
  [b02]   &lt;xenc:EncryptedData Id="tbs" ...&gt;
  [b03]     ...
  [b04]   &lt;/xenc:EncryptedData&gt;
  [b05]   &lt;dsig:Signature ...&gt;
  [b06]     ...</pre>

<p>Upon decryption, the document that would be parsed to form the replacement
node set would be:</p>
<pre class="xml-example">  [c01] &lt;dummy&gt;&lt;ToBeSigned xmlns="" Id="tbs"&gt;
  [c02]     ...
  [c03]   &lt;/ToBeSigned&gt;&lt;/dummy&gt;</pre>

<p>Because this fragment is no longer in its original ancestral context, the
canonical form of the resulting <code>ToBeSigned</code> element
(<code>[d01-d03]</code>) would not match that which was originally signed and
the signature verification operation would fail.</p>
<pre class="xml-example">  [d01] &lt;ToBeSigned Id="tbs"&gt;
  [d02]     ...
  [d03]   &lt;/ToBeSigned&gt;</pre>

<p class="">As shown, this failure often occurs when a directly-signed
element was encrypted. The remedy is to augment the internal canonicalization
of the canonicalization-with-replacement step of <a href="#func-decryptXML"
class="link-def">decryptXML()</a>: node-sets that are replacing elements
whose parent node is not part of the original signed node-set are
canonicalized with attributes from the <a
href="http://www.w3.org/XML/1998/namespace">XML namespace</a> that would have
been inherited by the unencrypted element in its original document.</p>

<p>While this change is made to maintain the validity of signatures using [<a
href="#XML-C14N">XML-C14N</a>], it does not interfere with the validity of
signatures using [<a href="#ref-XML-exc-C14N">XML-exc-C14N</a>]. This
transform, and the inclusion of attributes from the <a
href="http://www.w3.org/XML/1998/namespace">XML namespace</a> (i.e., 'xml:'),
is performed during signature validation <em>and</em> generation.
Consequently, the exclusively canonicalized form of the element will maintain
these 'xml:' attributes &mdash; even if the exclusively canonicalized form of
the element would not have had them without this transform.</p>

<h4>3.4.3 <a name="sec-references-structural-changes"
id="sec-references-structural-changes">References and Structural
Changes</a></h4>

<p>URIs with a full XPointer or child sequence (whether in exceptions,
encrypted data or elsewhere) may fail to resolve if encryption results in a
structural change to part of the document relied upon by the reference. For
example, the URI <code>#xpointer(/ToBeSigned/*[3])</code> will no longer
function if the first two children of the <code>ToBeSigned</code> element are
encrypted together. Care SHOULD be taken when employing such references in
association with the decryption transform.</p>

<h4>3.4.4 <a name="sec-references-super-encryption"
id="sec-references-super-encryption">References and Super Encryption</a></h4>

<p>Super-encryption may cause problems if a super-encrypted
<code>xenc:EncryptedData</code> element uses same-document references, or if
an exceptional super-encrypted <code>xenc:EncryptedData</code> element is
referenced by a non-bare name XPointer URI. Super-encryption of signed data
under these conditions is NOT RECOMMENDED: super-encryption is precluded, or
such references should not be used.</p>

<p>As an example of where super-encryption over same-document references may
cause problems, consider the following signed document
(<code>[02-05]</code>):</p>
<pre class="xml-example">  [01] &lt;Document&gt;
  [02]   &lt;ToBeSigned Id="tbs"&gt;
  [03]     &lt;Data&gt;...&lt;/Data&gt;
  [04]     ...
  [05]   &lt;/ToBeSigned&gt;
  [06]   &lt;dsig:Signature ...&gt;
  [07]     ...
  [08]     &lt;dsig:Reference URI="#tbs"&gt;
  [09]       &lt;dsig:Transforms&gt;
  [10]         &lt;dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#XML" /&gt;
  [11]       &lt;/dsig:Transforms&gt;
  [12]       ...
  [13]     &lt;/dsig:Reference&gt;
  [14]     ...
  [15]   &lt;/dsig:Signature&gt;
  [16]   ...
  [17] &lt;/Document&gt;</pre>

<p>If the <code>Data</code> element (<code>[03]</code>) is subsequently
encrypted, along with other data elsewhere in the document, the new
<code>xenc:EncryptedData</code> element could use a same-document retrieval
method to identify shared keying information (<code>[a06]</code>):</p>
<pre class="xml-example">  [a01] &lt;Document&gt;
  [a02]   &lt;ToBeSigned Id="tbs"&gt;
  [a03]     &lt;xenc:EncryptedData ...&gt;
  [a04]       ...
  [a05]       &lt;dsig:KeyInfo ...&gt;
  [a06]         &lt;dsig:RetrievalMethod URI="#key-info"
                 Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey" .../&gt;
  [a07]       &lt;/dsig:KeyInfo&gt;
  [a08]       ...
  [a09]     &lt;/xenc:EncryptedData&gt;
  [a10]     ...
  [a11]   &lt;/ToBeSigned&gt;
  [a12]   &lt;xenc:EncryptedKey Id="key-info" .../&gt;
  [a13]   &lt;dsig:Signature ...&gt;
  [a14]     ...
  [a15]     &lt;dsig:Reference URI="#tbs"&gt;
  [a16]       &lt;dsig:Transforms&gt;
  [a17]         &lt;dsig:Transform
                 Algorithm="http://www.w3.org/2002/07/decrypt#XML"/&gt;
  [a18]       &lt;/dsig:Transforms&gt;
  [a19]       ...
  [a20]     &lt;/dsig:Reference&gt;
  [a21]     ...
  [a22]   &lt;/dsig:Signature&gt;
  [a23] &lt;/Document&gt;</pre>

<p>If this new <code>xenc:EncryptedData</code> is subsequently
super-encrypted (<code>[b02]</code>), the decryption transform will fail:
When the inner retrieval method is processed, it will be dereferenced within
the context of a new decrypted document that does not contain the referenced
keying information.</p>
<pre class="xml-example">  [b01] &lt;Document&gt;
  [b02]   &lt;xenc:EncryptedData Id="tbs" .../&gt;
  [b03]   &lt;xenc:EncryptedKey Id="key-info" .../&gt;
  [b04]   &lt;dsig:Signature ...&gt;
  [b05]     ...
  [b06]     &lt;dsig:Reference URI="#tbs"&gt;
  [b07]       &lt;dsig:Transforms&gt;
  [b08]         &lt;dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#XML" /&gt;
  [b09]       &lt;/dsig:Transforms&gt;
  [b10]       ...
  [b11]     &lt;/dsig:Reference&gt;
  [b12]     ...
  [b13]   &lt;/dsig:Signature&gt;
  [b14] &lt;/Document&gt;</pre>

<p class="">Upon decryption, the document that would be parsed to form the
replacement node set would be:</p>
<pre class="xml-example">  [d01] &lt;dummy&gt;&lt;ToBeSigned Id="tbs"&gt;
  [d02]   &lt;xenc:EncryptedData ...&gt;
  [d03]     ...
  [d04]     &lt;dsig:KeyInfo ...&gt;
  [d05]       &lt;dsig:RetrievalMethod URI="#key-info"
               Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey"
               .../&gt;
  [d06]     &lt;/dsig:KeyInfo&gt;
  [d07]     ...
  [d08]   &lt;/xenc:EncryptedData&gt;
  [d09]   ...
  [d10] &lt;/ToBeSigned&gt;&lt;/dummy&gt;</pre>

<h4>3.4.5 <a name="sec-References-Non-Barename"
id="sec-References-Non-Barename">References Using Non-barename
XPointers</a></h4>

<p>As an example of where non-barename XPointers may fail, consider the
following signed document (<code>[02-07]</code>) which uses a full XPointer
(<code>[13]</code>) to identify data that was already encrypted when the
signature was generated (<code>[05]</code>), and so should not be processed
by the decryption transform:</p>
<pre class="xml-example">  [01] &lt;Document&gt;
  [02]   &lt;ToBeSigned Id="tbs"&gt;
  [03]     &lt;Data&gt;...&lt;/Data&gt;
  [04]     &lt;Secrets&gt;
  [05]       &lt;xenc:EncryptedData Id="#secret-1" .../&gt;
  [06]     &lt;/Secrets&gt;
  [07]   &lt;/ToBeSigned&gt;
  [08]   &lt;dsig:Signature ...&gt;
  [09]     ...
  [10]     &lt;dsig:Reference URI="#tbs"&gt;
  [11]       &lt;dsig:Transforms&gt;
  [12]         &lt;dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#XML"&gt;
  [13]           &lt;dcrpt:Except URI="#xpointer(/Document/ToBeSigned/Secrets/*)"/&gt;
  [14]         &lt;/dsig:Transform&gt;
  [15]       &lt;/dsig:Transforms&gt;
  [16]       ...
  [17]     &lt;/dsig:Reference&gt;
  [18]     ...
  [19]   &lt;/dsig:Signature&gt;
  [20] &lt;/Document&gt;</pre>

<p>If the <code>Secrets</code> element is subsequently encrypted, as shown in
the following example (<code>[a04]</code>); i.e., the existing
<code>xenc:EncryptedData</code> is super-encrypted, then the XPointer
exception URI will no longer resolve. As a result, the decryption transform
will attempt to perform super-decryption of the inner
<code>xenc:EncryptedData</code> element and processing will fail.</p>
<pre class="xml-example">  [a01] &lt;Document&gt;
  [a02]   &lt;ToBeSigned Id="tbs"&gt;
  [a03]     &lt;Data&gt;...&lt;/Data&gt;
  [a04]     &lt;xenc:EncryptedData Id="#secrets" .../&gt;
  [a05]   &lt;/ToBeSigned&gt;
  [a06]   &lt;dsig:Signature ...&gt;
  [a07]     ...
  [a08]     &lt;dsig:Reference URI="#tbs"&gt;
  [a09]       &lt;dsig:Transforms&gt;
  [a10]         &lt;dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#XML"&gt;
  [a11]           &lt;dcrpt:Except URI="#xpointer(/Document/ToBeSigned/Secrets/*)"/&gt;
  [a12]         &lt;/dsig:Transform&gt;
  [a13]       &lt;/dsig:Transforms&gt;
  [a14]       ...
  [a15]     &lt;/dsig:Reference&gt;
  [a16]     ...
  [a17]   &lt;/dsig:Signature&gt;
  [a18] &lt;/Document&gt;</pre>

<p>The reason that the full XPointer cannot be processed is that the document
which results from decrypting the outer <code>xenc:EncryptedData</code>
element will have the following form (<code>[b01-b05]</code>):</p>
<pre class="xml-example">  [b01] &lt;dummy&gt;
  [b02]   &lt;Secrets&gt;
  [b03]     &lt;xenc:EncryptedData Id="#secret-1" .../&gt;
  [b04]   &lt;/Secrets&gt;
  [b05] &lt;/dummy&gt;</pre>

<h4>3.4.6 <a name="sec-interactions" id="sec-interactions">Interactions with
Other Filters</a></h4>

<p>The XML signature reference processing model allows transforms to remove
parts of a node-set undergoing transformation. It is RECOMMENDED that any
such transforms appear <em>before</em> the decryption transform. Otherwise,
encrypted data that appear in parts of the document that are to be removed
and cannot be processed by the recipient will cause the decryption transform
to fail.</p>

<p>For example, consider the following document which contains parts that are
to be signed for receipt by different individuals. The XPath transform
<code>ancestor-or-self::*[@For="job"]</code> selects only the subset
<code>[02-04]</code>. If this XPath transform occurs after the decryption
transform, and another part of the document contains encrypted data (e.g.,
<code>[07]</code>), whether created before or after the signature, then the
decryption transform may fail to decrypt them and processing will fail. If
the XPath transform occurs first, then the encrypted data will not be
considered by the decryption transform.</p>
<pre class="xml-example">  [01] &lt;Document&gt;
  [02]   &lt;ToBeSigned For="job"&gt;
  [03]     ...
  [04]   &lt;/ToBeSigned&gt;
  [05]   &lt;ToBeSigned For="bob"&gt;
  [06]     ...
  [07]     &lt;xenc:EncryptedData .../&gt;
  [08]   &lt;/ToBeSigned&gt;
  [09] &lt;/Document&gt;</pre>

<h4>3.4.7 <a name="sec-encryptedkey" id="sec-encryptedkey">EncryptedKey is
Out of Scope</a></h4>

<p>This transform does not include any <code>xenc:EncryptedKey</code>
elements within its scope of specifically indicating elements, and their
exceptions, that should be decrypted. An <code>xenc:EncryptedKey</code>
element that exists as a descendent of <code>xenc:EncryptedData</code>
element might be decrypted and will be removed from the original document as
part of processing its ancestor <code>xenc:EncryptedData</code> element with
the transform. However, a lone <code>xenc:EncryptedKey</code> element will be
processed like any other data: a signature is presumed to be over that actual
element and not its decrypted form. Consequently, we RECOMMEND that
<code>xenc:EncryptedKey</code> elements always be children of an
<code>xenc:EncryptedData</code> element's <code>ds:KeyInfo</code> element
when they fall within the scope of a signature.</p>

<h2><a id="sec-binary-mode" name="sec-binary-mode"></a>4 Binary Mode
Decryption Transform</h2>
<dl>
  <dt>Identifier:</dt>
    <dd><a name="Binary" id="Binary"
      href="http://www.w3.org/2002/07/decrypt#Binary">http://www.w3.org/2002/07/decrypt#Binary</a></dd>
</dl>

<p>This mode of the transform requires an XPath node-set [<a
href="#XPath">XPath</a>] for input. If an octet stream is given as input, it
MUST be converted to a node-set as described in <a class="link-sec"
href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-ReferenceProcessingModel">The
Reference Processing Model</a> (section 4.3.3.2) of the XML Signature
specification [<a href="#XML-Signature">XML-Signature</a>]. The transform
decrypts all the <code>xenc:EncryptedData</code> elements except for those
specified by <code>dcrpt:Except</code> elements. The output of the transform
is an octet-stream.</p>

<h3><a id="sec-binary-processing" name="sec-binary-processing">4.1 Processing
Rules</a></h3>

<p>The binary mode of operation is intended for use when generating a
signature over binary data that are to be encrypted for transmission to the
recipient. Use of this mode of the transform allows a signature to be
computed over the plaintext form of the data, rather than the opaque
ciphertext. This further allows the ciphertext to be stored elsewhere,
identified by a cipher reference, without the need for the signature to take
this into account.</p>

<p>This section describes the processing rules of the binary mode of this
transform. The inputs and outputs of the transform are those of the <a
href="#func-decryptBinary" class="link-def">decryptBinary()</a> function.</p>
<dl>
  <dt><a name="func-decryptBinary" id="func-decryptBinary" class="link-def">O
  = decryptBinary(N, E)</a></dt>
    <dd><p>where <em>N</em> is a node-set and <em>E</em> is a set of
      exception URIs held by <code>URI</code> attributes of
      <code>dcrpt:Except</code> elements. <em>O</em> is an octet stream,
      computed as follows:</p>
      <ol>
        <li>Let <em>D</em> be a node-set containing all element nodes in
          <em>N</em> with the type <code>xenc:EncryptedData</code> that are
          not identified by any exception URI in <em>E</em>.
          <ul>
            <li>When dereferencing an exception URI, the implementation MUST
              behave as if the document node of the input node-set is used to
              initialize the XPointer evaluation context [<a
              href="#XPointer">XPointer</a>], even if the node is not part of
              the node-set. Unlike XML Signature [<a
              href="#XML-Signature">XML-Signature</a>], the exception URI may
              be evaluated against a different document from the signature
              document. If the input is a different document then, as per
              XPointer [<a href="#XPointer">XPointer</a>], use of the <a
              href="http://www.w3.org/TR/2001/CR-xptr-20010911/#N3568"
              class="link-def">here()</a> function is an error.</li>
            <li>If an exception URI fails to dereference any nodes, then the
              resulting error MUST be ignored; it may be the result of part
              of the input document being encrypted.</li>
          </ul>
        </li>
        <li>For each <code>xenc:EncryptedData</code> element <em>d</em> from
          <em>D</em>, decrypt <em>d</em>, without regard for which, if any,
          of its descendants are in <em>N</em>, and without consideration of
          its <code>Type</code> attribute, resulting in an octet-stream
          <em>O<sub>d</sub></em>.</li>
        <li>Let <em>O</em>, the output of this transform, be the
          concatenation of the octet streams <em>O<sub>d</sub></em>, ordered
          in the document order of <em>d</em>.</li>
        <li>If there are no <code>EncryptedData</code> elements in
          <em>D</em>, then the result is a zero-length octet stream.</li>
      </ol>
    </dd>
</dl>

<h3><a id="sec-binary-example" name="sec-binary-example">4.2 Example</a></h3>

<p>Consider the following example signed document:</p>
<pre class="xml-example">  &lt;Document&gt;
    &lt;xenc:EncryptedData Id="image" MimeType="image/png" ...&gt;
      ...
      &lt;!-- image data --&gt;
      ...
    &lt;/xenc:EncryptedData&gt;
    &lt;dsig:Signature ...&gt;
      ...
      &lt;dsig:Reference URI="#image"&gt;
        &lt;dsig:Transforms&gt;
          &lt;dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#Binary" /&gt;
        &lt;/dsig:Transforms&gt;
        ...
      &lt;/dsig:Reference&gt;
      ...
    &lt;/dsig:Signature&gt;
  &lt;/Document&gt;</pre>

<p>Much of the encrypted data and signature are elided; the implication of
the comment in the encrypted data is that the encrypted content is a binary
image.</p>

<p>Execution of the decryption transform will proceed as follows:</p>
<ul>
  <li>The input to the transform, <em>N</em>, is a node set containing the
    <code>EncryptedData</code> element and its children, less comments. The
    parameter to the transform, <em>E</em>, is empty.</li>
  <li>As a result, <em>D</em> is a node set consisting of the one
    <code>EncryptedData</code> element, <em>d<sub>image</sub></em>. This is
    decrypted, resulting in an octet string <em>O<sub>image</sub></em>
    containing the plaintext of the binary image.</li>
  <li>There are no other data to decrypt, so the result of this transform is
    the plaintext obtained in the previous step. This will be used directly
    as input to the digest algorithm.</li>
</ul>

<h2><a id="sec-security" name="sec-security">5 Security
Considerations</a></h2>

<h3><a id="sec-security-reveal" name="sec-security-reveal">5.1 Signatures
Over Encrypted Data May Reveal Information</a></h3>

<p>When this algorithm is used to facilitate subsequent encryption of data
already signed, the digest value of the signed resource still appears in
clear text in a <code>ds:Reference</code> element. As noted by Hal Finney in
[<a href="#Finney">Finney</a>], such a signature may reveal information (via
the digest value) over encrypted data that increases the encryption's
vulnerability to plain-text-guessing attacks. This consideration is out of
scope of this document and (if relevant) should be addressed by applications.
For example, as proposed by Amir Herzberg in [<a
href="#Herzberg">Herzberg</a>], one may include a random 'salt' in a resource
being signed to increase its entropy.</p>

<p>Another approach is that when a signature referent is encrypted, one may
also encrypt the signature (or at least the <code>ds:DigestValue</code>
elements). As noted by Joseph Reagle in [<a href="#Reagle">Reagle</a>], this
latter solution works only if signature and encryption are known to each
other. For example, the signature may not be known of because it is detached.
Or, it may be already encrypted! Consider, Alice encrypts element A and the
signature over the parent of A. Bob encrypts element B (sibling of A) but not
the signature since he doesn't know about it. Alice then decrypts A and its
signature, which may provide information to a subsequent plain text attack on
the encrypted B.</p>

<h3><a id="sec-sign-what-you-see" name="sec-sign-what-you-see">5.2 "Sign What
You See"</a></h3>

<p>This specification serves scenarios in which a person might sign encrypted
data. Because XML Signature [<a href="#XML-Signature">XML-Signature</a>] has
only a simple semantic whereby a key is associated with some data &mdash; and
nothing more &mdash; the signing of encrypted data is a legitimate process.
For example, someone might run a content-neutral time stamp service that will
sign any data sent to it with its time-stamping key under the semantic, "I
received this on $date $time." However, applications often explicitly or
implicitly associate more substantive semantics (e.g., authorizes, agrees,
authors) with a signature. No one should be asked to apply a signature and
its semantic to data he or she did not see. Just as the principles of <a
class="link-sec"
href="http://www.w3.org/TR/2001/PR-xmldsig-core-20010820/#sec-Seen">Only What
is 'Seen' Should be Signed</a> and <a class="link-sec"
href="http://www.w3.org/TR/2001/PR-xmldsig-core-20010820/#sec-See">'See' What
is Signed</a> are important for understanding the import of an XML Signature,
they are doubly important when semantics are associated with that signature:
one MUST NOT infer that a signature over encrypted data is also a signature
over its plain text form, nor that the meaning of that signature over the
encrypted data also applies to the plain text. If one wishes to sign the
plain text form of data which is later encrypted, use the transform specified
in this document!</p>

<h2><a id="sec-references" name="sec-references">6 References</a></h2>
<dl>
  <dt><a id="Finney" name="Finney">Finney</a></dt>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Nov/0064">Re:
      Combining signing and encrypting</a>. H. Finney. XML Encryption mailing
      list, 2000.</dd>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Nov/0064">http://lists.w3.org/Archives/Public/xml-encryption/2000Nov/0064</a></dd>
  <dt><a id="Herzberg" name="Herzberg">Herzberg</a></dt>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2001Mar/0025">Signing
      encrypted data</a>. A. Herzberg. XML Encryption mailing list, 2001.</dd>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2001Mar/0025">http://lists.w3.org/Archives/Public/xml-encryption/2001Mar/0025</a></dd>
  <dt><a id="Keywords" name="Keywords">Keywords</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2119.txt">RFC 2119: Key words for
      use in RFCs to Indicate Requirement Levels</a>. S. Bradner. Best
      Current Practices, 1997.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a></dd>
  <dt><a id="Reagle" name="Reagle">Reagle</a></dt>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2001Jan/0100">Re:
      Signing and Encryption</a>. J. Reagle. XML Encryption mailing list,
      2001.</dd>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2001Jan/0100">http://lists.w3.org/Archives/Public/xml-encryption/2001Jan/0100</a></dd>
  <dt><a id="Solo" name="Solo">Solo</a></dt>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Nov/0058">Combining
      signing and encrypting</a>. D. Solo. XML Encryption mailing list,
    2000.</dd>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Nov/0058">http://lists.w3.org/Archives/Public/xml-encryption/2000Nov/0058</a></dd>
  <dt><a id="URI" name="URI">URI</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2396.txt">RFC 2396: Uniform
      Resource Identifiers (URI): Generic Syntax</a>. T. Berners-Lee, R.
      Fielding, and L. Masinter. Standards Track, 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>. T. Bray, J. Paoli, C.
      M. Sperberg-McQueen, and E. Maler. W3C Recommendation, 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="XML-C14N" name="XML-C14N">XML-C14N</a></dt>
    <dd><a href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315">Canonical
      XML Version 1.0</a>. J. Boyer. W3C Recommendation, 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>
  <dt><a name="ref-XML-exc-C14N" id="ref-XML-exc-C14N">XML-exc-C14N</a></dt>
    <dd><a
      href="http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/">Exclusive
      XML Canonicalization</a>. J. Boyer, D. Eastlake, and J. Reagle. W3C
      Recommendation, 2002.</dd>
    <dd><a
      href="http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/">http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/</a></dd>
  <dt><a id="XML-Encryption" name="XML-Encryption">XML-Encryption</a></dt>
    <dd><a href="http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/">XML
      Encryption Syntax and Processing</a>. D. Eastlake and J. Reagle. W3C
      Recommendation, 2002.</dd>
    <dd><a
      href="http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/">http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/</a></dd>
  <dt><a id="XML-Schema" name="XML-Schema">XML-Schema</a></dt>
    <dd><a href="http://www.w3.org/TR/2001/REC-xmlschema-1-20010502/">XML
      Schema Part 1: Structures</a>. H. Thompson, D. Beech, M. Maloney, and
      N. Mendelsohn. W3C Recommendation, 2001.</dd>
    <dd><a
      href="http://www.w3.org/TR/2001/REC-xmlschema-1-20010502/">http://www.w3.org/TR/2001/REC-xmlschema-1-20010502/</a></dd>
    <dd><a href="http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/">XML
      Schema Part 2: Datatypes</a>. P. Biron and A. Malhotra. W3C
      Recommendation, 2001.</dd>
    <dd><a
      href="http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/">http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/</a></dd>
  <dt><a id="XML-Signature" name="XML-Signature">XML-Signature</a></dt>
    <dd><a
      href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/">XML-Signature
      Syntax and Processing</a>. D. Eastlake, J. Reagle, and D. Solo. W3C
      Recommendation, 2002.</dd>
    <dd><a
      href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/">http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/</a></dd>
  <dt><a id="XPath" name="XPath">XPath</a></dt>
    <dd><a href="http://www.w3.org/TR/1999/REC-xpath-19991116">XML Path
      Language (XPath) Version 1.0</a>. J. Clark and S. DeRose. W3C
      Recommendation, 1999.</dd>
    <dd><a
      href="http://www.w3.org/TR/1999/REC-xpath-19991116">http://www.w3.org/TR/1999/REC-xpath-19991116</a></dd>
  <dt><a id="XPointer" name="XPointer">XPointer</a></dt>
    <dd><a href="http://www.w3.org/TR/2001/CR-xptr-20010911/">XML Pointer
      Language (XPointer)</a>. S. DeRose, R. Daniel, and E. Maler. W3C
      Candidate Recommendation, 2001.</dd>
    <dd><a
      href="http://www.w3.org/TR/2001/CR-xptr-20010911/">http://www.w3.org/TR/2001/CR-xptr-20010911/</a></dd>
</dl>
</body>
</html>