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<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">XML Encryption Syntax and Processing</h1>

<h2 class="notoc">W3C Candidate Recommendation 04 March 2002</h2>
<dl>
  <dt>This version:</dt>
    <dd><a
      href="http://www.w3.org/TR/2002/CR-xmlenc-core-20020304/">http://www.w3.org/TR/2002/CR-xmlenc-core-20020304/</a></dd>
  <dt>Latest version:</dt>
    <dd><a
      href="http://www.w3.org/TR/xmlenc-core/">http://www.w3.org/TR/xmlenc-core/</a></dd>
  <dt>Previous version:</dt>
    <dd><a
      href="http://www.w3.org/TR/2001/WD-xmlenc-core-20011018/">http://www.w3.org/TR/2001/WD-xmlenc-core-20011018/</a></dd>
  <dt><a
  href="http://www.w3.org/Encryption/2001/Contributor.html#Editor">Editors</a></dt>
    <dd>Donald Eastlake &lt;dee3@torque.pothole.com&gt;</dd>
    <dd>Joseph Reagle &lt;reagle@w3.org&gt;</dd>
  <dt><a
  href="http://www.w3.org/Encryption/2001/Contributor.html#Author">Authors</a></dt>
    <dd>Takeshi Imamura &lt;IMAMU@jp.ibm.com&gt;</dd>
    <dd>Blair Dillaway &lt;blaird@microsoft.com&gt;</dd>
    <dd>Ed Simon &lt;edsimon@xmlsec.com&gt;</dd>
  <dt><a
  href="http://www.w3.org/Encryption/2001/Contributor.html#Contributor">Contributors</a></dt>
    <dd>See <a
      href="http://www.w3.org/Encryption/2001/Participants.html">participants</a>.</dd>
</dl>

<p class="copyright"><a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Copyright">Copyright</a>
© 2002 <a href="http://www.w3.org/"><abbr
title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
href="http://www.lcs.mit.edu/"><abbr
title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
href="http://www.inria.fr/"><abbr xml:lang="fr" lang="fr"
title="Institut National de Recherche en Informatique et Automatique">INRIA</abbr></a>,
<a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#W3C_Trademarks">trademark</a>,
<a
href="http://www.w3.org/Consortium/Legal/copyright-documents-19990405">document
use</a> and <a
href="http://www.w3.org/Consortium/Legal/copyright-software-19980720">software
licensing</a> rules apply.</p>
<hr title="Separator from Header" />
</div>

<h2 class="notoc">Abstract</h2>

<p class="notoc">This document specifies a process for encrypting data and
representing the result in XML. The data may be arbitrary data (including an
XML document), an XML element, or XML element content. The result of
encrypting data is an XML Encryption element which contains or references the
cipher data.</p>

<h2 class="notoc">Status of this document</h2>

<div class="">
<p>This specification from the <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>) is a <a
class="loc"
href="http://www.w3.org/Consortium/Process/Process-19991111/process.html#RecsCR">Candidate
Recommendation</a> of the W3C. The Working Group believes this specification
incorporates the resolution of all <a
href="http://www.w3.org/Encryption/2001/11/last-call-issues.html">last call
issues</a>; furthermore it considers the specification to be stable and
invites implementation feedback during this period.</p>

<p>The exit criteria for this phase is at least two interoperable
implementations over every feature, one implementation of all features, and
one report of satisfaction in an application context (e.g., SOAP, SAML, etc.)
Note, this specification already has significant implementation experience
which will soon be reflected in its <a
href="http://www.w3.org/Encryption/2002/02-xenc-interop.html">Interoperability
Report</a>. We expect to meet all requirements of that report within the two
month Candidate Recommendation period (closing April 25). Specific areas
where we would appreciate further experience are:</p>
<ul>
  <li>Do implementations achieve satisfactory performance?</li>
  <li>Does the specification satisfy application scenario requirements for
    encrypting portions of XML, particularly as they relate to <a
    href="http://lists.w3.org/Archives/Public/xml-encryption/2002Feb/0033.html">document
    validity</a>?</li>
</ul>

<p>Publication of this document does not imply endorsement by the W3C
membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite a W3C
Working Draft as anything other than a "work in progress." A list of current
W3C working drafts can be found at <a
href="http://www.w3.org/TR/">http://www.w3.org/TR/</a>.</p>

<p>Please send comments to the editors (&lt;<a
href="mailto:reagle@w3.org">reagle@w3.org</a>&gt;, &lt;<a
href="mailto:dee3@torque.pothole.com">dee3@torque.pothole.com</a>&gt;) and
cc: the list <a href="mailto:xml-encryption@w3.org">xml-encryption@w3.org</a>
(publicly <a
href="http://lists.w3.org/Archives/Public/xml-encryption/">archived</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>
</div>

<h2>Table of Contents</h2>
<ol>
  <li><a href="#sec-Introduction">Introduction</a>
    <ol>
      <li><a href="#sec-Editorial">Editorial and Conformance
      Conventions</a></li>
      <li><a href="#sec-Design">Design Philosophy</a></li>
      <li><a href="#sec-Versions">Versions, Namespaces URIs, and
        Identifiers</a></li>
      <li><a href="#sec-Acknowledgements">Acknowledgements</a></li>
    </ol>
  </li>
  <li><a href="#sec-Overview">Encryption Overview and Examples</a>
    <ol>
      <li><a href="#sec-eg-Granularity">Encryption Granularity</a>
        <ol>
          <li><a href="#sec-eg-Element">Encrypting an XML Element</a></li>
          <li><a href="#sec-eg-Element-Content">Encrypting XML Element
            Content (Elements)</a></li>
          <li><a href="#sec-eg-Element-Content-Character">Encrypting XML
            Element Content (Character Data)</a></li>
          <li><a href="#sec-eg-Arbitrary-Data">Encrypting Arbitrary Data and
            XML Documents</a></li>
          <li><a href="#sec-eg-Super-Encryption">Super-Encryption: Encrypting
            <code>EncryptedData</code></a></li>
        </ol>
      </li>
      <li><a href="#sec-Usage"><code>EncryptedData</code> and
        <code>EncryptedKey</code> Usage</a>
        <ol>
          <li><a href="#sec-eg-Symmetric-Key"><code>EncryptedData</code> with
            Symmetric Key  (<code>KeyName</code>)</a></li>
          <li><a href="#sec-eg-EncryptedKey"><code>EncryptedKey</code>
            (<code>ReferenceList</code>,
            <code>ds:RetrievalMethod</code>,<code>CarriedKeyName</code>)</a></li>
        </ol>
      </li>
    </ol>
  </li>
  <li><a href="#sec-Encryption-Syntax">Encryption Syntax</a>
    <ol>
      <li><a href="#sec-EncryptedType">The <code>EncryptedType</code></a></li>
      <li><a href="#sec-EncryptionMethod">The <code>EncryptionMethod</code>
        Element</a></li>
      <li><a href="#sec-CipherData">The <code>CipherData</code> Element</a>
        <ol>
          <li><a href="#sec-CipherReference">The <code>CipherReference</code>
            Element</a></li>
        </ol>
      </li>
      <li><a href="#sec-EncryptedData">The <code>EncryptedData</code>
        Element</a></li>
      <li><a href="#sec-Extensions-to-KeyInfo">Extensions to
        <code>ds:KeyInfo</code> Element</a>
        <ol>
          <li><a href="#sec-EncryptedKey">The <code>EncryptedKey</code>
            Element</a></li>
          <li><a href="#sec-ds:RetrievalMethod">The
            <code>ds:RetrievalMethod</code> Element</a></li>
        </ol>
      </li>
      <li><a href="#sec-ReferenceList">The <code>ReferenceList</code>
        Element</a></li>
      <li><a href="#sec-EncryptionProperties">The
        <code>EncryptionProperties</code> Element</a></li>
    </ol>
  </li>
  <li><a href="#sec-Processing">Processing Rules</a>
    <ol>
      <li><a href="#sec-Processing-Encryption">Encryption</a></li>
      <li><a href="#sec-Processing-Decryption">Decryption</a></li>
      <li><a href="#sec-Processing-XML">Encrypting XML</a></li>
    </ol>
  </li>
  <li><a href="#sec-Algorithms">Algorithms</a>
    <ol>
      <li><a href="#sec-AlgID">Algorithm Identifiers and Implementation
        Requirements</a></li>
      <li><a href="#sec-Alg-Block">Block Encryption Algorithms</a></li>
      <li><a href="#sec-Alg-Stream">Stream Encryption Algorithms</a></li>
      <li><a href="#sec-Alg-KeyTransport">Key Transport</a></li>
      <li><a href="#sec-Alg-KeyAgreement">Key Agreement</a></li>
      <li><a href="#sec-Alg-SymmetricKeyWrap">Symmetric Key Wrap</a></li>
      <li><a href="#sec-Alg-MessageDigest">Message Digest</a></li>
      <li><a href="#sec-Alg-MessageAuthentication">Message
      Authentication</a></li>
      <li><a href="#sec-Alg-Canonicalition">Canonicalization</a></li>
    </ol>
  </li>
  <li><a href="#sec-Security">Security Considerations</a></li>
  <li><a href="#sec-Schema">Schema and Valid Examples</a></li>
  <li><a href="#sec-References">References</a></li>
</ol>
<hr />

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

<p>This document specifies a process for encrypting data and representing the
result in XML. The data may be arbitrary data (including an XML document), an
XML element, or XML element content. The result of encrypting data is an XML
Encryption <code>EncryptedData</code> element which contains (via one of its
children's content) or identifies (via a URI reference) the cipher data.</p>

<p>When encrypting an XML element or element content the
<code>EncryptedData</code> element replaces the element or content
(respectively) in the encrypted version of the XML document.</p>

<p>When encrypting arbitrary data (including entire XML documents), the
<code>EncryptedData</code> element may become the root of a new XML document
or become a child element in an application-chosen XML document.</p>

<h3>1.1 <a id="sec-Editorial" name="sec-Editorial">Editorial</a> and
Conformance Conventions</h3>

<p>This specification uses XML schemas [<a
href="#ref-XML-schema">XML-schema</a>] to describe the content model.</p>

<p>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
specification are to be interpreted as described in <a
href="http://www.ietf.org/rfc/rfc2119.txt">RFC2119</a> [<a
href="#ref-KEYWORDS">KEYWORDS</a>]:</p>

<blockquote>
  <p>"they MUST only be used where it is actually required for interoperation
  or to limit behavior which has potential for causing harm (e.g., limiting
  retransmissions)"</p>
</blockquote>

<p>Consequently, we use these capitalized keywords to unambiguously specify
requirements over protocol and application features and behavior that affect
the interoperability and security of implementations. These key words are not
used (capitalized) to describe XML grammar; schema definitions unambiguously
describe such requirements and we wish to reserve the prominence of these
terms for the natural language descriptions of protocols and features. For
instance, an XML attribute might be described as being "optional." Compliance
with the XML-namespace specification [<a href="#ref-XML-NS">XML-NS</a>] is
described as "REQUIRED."</p>

<h3>1.2 <a id="sec-Design" name="sec-Design">Design</a> Philosophy</h3>

<p>The design philosophy and requirements of this specification are addressed
in the <a href="http://www.w3.org/TR/xml-encryption-req">XML Encryption
Requirements document</a> [<a href="#ref-EncReq">EncReq</a>].</p>

<h3>1.3 <a id="sec-Versions" name="sec-Versions">Versions</a>, Namespaces,
URIs, and Identifiers</h3>

<p>No provision is made for an explicit version number in this syntax. If a
future version is needed, it will use a different namespace. The experimental
XML namespace [<a href="#ref-XML-NS">XML-NS</a>] URI that MUST be used by
implementations of this (dated) specification is:</p>
<pre class="xml-example">   xmlns:xenc='http://www.w3.org/2001/04/xmlenc#'
</pre>

<p>This namespace is also used as the prefix for algorithm identifiers used
by this specification. While applications MUST support XML and XML
namespaces, the use of <a
href="http://www.w3.org/TR/REC-xml#sec-internal-ent">internal entities</a>
[<a href="#ref-XML">XML</a>, section 4.2.1] or our <code>&amp;xenc;</code>
entity or "<code>xenc</code>" XML <a
href="http://www.w3.org/TR/1999/REC-xml-names-19990114/#dt-prefix">namespace
prefix</a> [<a href="#ref-XML-NS">XML-NS</a>, section 2] and
defaulting/scoping conventions are OPTIONAL; we use these facilities to
provide compact and readable examples.</p>

<p>This specification makes use of the XML Signature [<a
href="#ref-XML-DSIG">XML-DSIG</a>] namespace and schema definitions</p>
<pre class="xml-example">   xmlns:ds='http://www.w3.org/2000/09/xmldsig#'
</pre>

<p>URIs [<a href="#ref-URI">URI</a>] MUST abide by the [<a
href="#ref-XML-schema">XML-Schema</a>] <code>anyURI</code> type definition
and the [<a href="#ref-XML-DSIG">XML-DSIG</a>, <a
href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-URI">4.3.3.1
The URI Attribute</a>] specification (i.e., permitted characters, character
escaping, scheme support, etc.).</p>

<h3>1.4  <a id="sec-Acknowledgements"
name="sec-Acknowledgements">Acknowledgements</a></h3>

<p>The contributions of the following Working Group members to this
specification are gratefully acknowledged in accordance with the <a
href="http://www.w3.org/Encryption/2001/Contributor.html">contributor
policies</a> and the active <a
href="http://www.w3.org/Encryption/2001/Participants.html">WG roster</a>.</p>
<ul>
  <li>Joseph Ashwood</li>
  <li>Katherine Betz, IBM</li>
  <li>Simon Blake-Wilson, Certicom</li>
  <li>Frank D. Cavallito, BEA System</li>
  <li>Eric Cohen, PricewaterhouseCoopers</li>
  <li>Blair Dillaway, Microsoft (Author)</li>
  <li>Blake Dournaee, RSA Security</li>
  <li>Donald Eastlake, Motorola (Editor)</li>
  <li>Barb Fox, Microsoft</li>
  <li>Christian Geuer-Pollmann, University of Siegen</li>
  <li>Jiandong Guo, Phaos</li>
  <li>Phillip Hallam-Baker, Verisign</li>
  <li>Amir Herzberg, NewGenPay</li>
  <li>Merlin Hughes, Baltimore</li>
  <li>Frederick Hirsch, Zolera</li>
  <li>Maryann Hondo, IBM</li>
  <li>Takeshi Imamura, IBM (Author)</li>
  <li>Mike Just, Entrust, Inc.</li>
  <li>Brian LaMacchia, Microsoft</li>
  <li>Hiroshi Maruyama, IBM</li>
  <li>John Messing, Law-on-Line</li>
  <li>Shivaram Mysore, Sun Microsystems</li>
  <li>Thane Plambeck, Verisign</li>
  <li>Joseph Reagle, W3C (Chair, Editor)</li>
  <li>Jim Schaad, Soaring Hawk Consulting</li>
  <li>Ed Simon, XMLsec (Author)</li>
  <li>Daniel Toth, Ford</li>
  <li>Yongge Wang, Certicom</li>
  <li>Steve Wiley, myProof</li>
</ul>

<p>Additionally, we thank the following for their comments during and
subsequent to Last Call:</p>
<ul>
  <li>Martin Duerst, W3C</li>
  <li>Dan Lanz, Zolera</li>
  <li>Susan Lesch, W3C</li>
  <li>David Orchard, BEA Systems</li>
  <li>Ronald Rivest, MIT</li>
</ul>

<h2>2 <a id="sec-Overview" name="sec-Overview">Encryption Overview</a> and
Examples (Non-normative)</h2>

<p>This section provides an overview and examples of XML Encryption syntax.
The formal syntax is found in <a href="#sec-Encryption-Syntax">Encryption
Syntax</a> (section 3); the specific processing is given in <a
href="http://www.w3.org/TR/2000/WD-xmldsig-core-20000104/#sec-Processing">Processing
Rules</a> (section 4).</p>

<p>Expressed in shorthand form, the <code><a
href="#sec-EncryptedData">EncryptedData</a></code> element has the following
structure (where "?" denotes zero or one occurrence; "+" denotes one or more
occurrences; "*" denotes zero or more occurrences; and the empty element tag
means the element must be empty ):</p>
<pre class="xml-example">  &lt;EncryptedData Id? Type?&gt;
    &lt;EncryptionMethod/&gt;?
    &lt;ds:KeyInfo&gt;
      &lt;EncryptedKey&gt;?
      &lt;AgreementMethod&gt;?
      &lt;ds:KeyName&gt;?
      &lt;ds:RetrievalMethod&gt;?
      &lt;ds:*&gt;?
    &lt;/ds:KeyInfo&gt;?
    &lt;CipherData&gt;
      &lt;CipherValue&gt;?
      &lt;CipherReference URI?&gt;?
    &lt;/CipherData&gt;
    &lt;EncryptionProperties&gt;?
  &lt;/EncryptedData&gt;
</pre>

<p>The <code>CipherData</code> element envelopes or references the raw
encrypted data. If enveloping, the raw encrypted data is the
<code>CipherValue</code> element's content; if referencing, the
<code>CipherReference</code> element's <code>URI</code> attribute points to
the location of the raw encrypted data</p>

<h3>2.1 Encryption <a name="sec-eg-Granularity"
id="sec-eg-Granularity">Granularity</a></h3>

<p>Consider the following fictitious payment information, which includes
identification information and information appropriate to a payment method
(e.g., credit card, money transfer, or electronic check):</p>
<pre class="xml-example">  &lt;?xml version='1.0'?&gt;
  &lt;PaymentInfo xmlns='http://example.org/paymentv2'&gt;
    &lt;Name&gt;John Smith&lt;/Name&gt;
    &lt;CreditCard Limit='5,000' Currency='USD'&gt;
      &lt;Number&gt;4019 2445 0277 5567&lt;/Number&gt;
      &lt;Issuer&gt;Example Bank&lt;/Issuer&gt;
      &lt;Expiration&gt;04/02&lt;/Expiration&gt;
    &lt;/<code>CreditCar</code>d&gt;
  &lt;/<code>PaymentInf</code>o&gt;
</pre>

<p>This markup represents that John Smith is using his credit card with a
limit of $5,000USD.</p>

<h4>2.1.1 Encrypting an XML <a name="sec-eg-Element"
id="sec-eg-Element">Element</a></h4>

<p>Smith's credit card number is sensitive information! If the application
wishes to keep that information confidential, it can encrypt the
<code>CreditCard</code> element:</p>
<pre class="xml-example">  &lt;?xml version='1.0'?&gt;
  &lt;PaymentInfo xmlns='http://example.org/paymentv2'&gt;
    &lt;Name&gt;John Smith&lt;/Name&gt;
    &lt;EncryptedData Type='http://www.w3.org/2001/04/xmlenc#Element'
     xmlns='http://www.w3.org/2001/04/xmlenc#'&gt;
      &lt;CipherData&gt;
        &lt;CipherValue&gt;A23B45C56&lt;/CipherValue&gt;
      &lt;/<code>CipherDat</code>a&gt;
    &lt;/<code>EncryptedDat</code>a&gt;
  &lt;/<code>PaymentInf</code>o&gt;
</pre>

<p>By encrypting the entire <code>CreditCard</code> element from its start to
end tags, the identity of the element itself is hidden. (An eavesdropper
doesn't know whether he used a credit card or money transfer.) The
<code>CipherData</code> element contains the encrypted serialization of the
<code>CreditCard</code> element.</p>

<h4>2.1.2 Encrypting XML <a name="sec-eg-Element-Content"
id="sec-eg-Element-Content">Element Content</a> (Elements)</h4>

<p>As an alternative scenario, it may be useful for intermediate agents to
know that John used a credit card with a particular limit, but not the card's
number, issuer, and expiration date. In this case, the content (character
data or children elements) of the <code>CreditCard</code> element is
encrypted:</p>
<pre class="xml-example">  &lt;?xml version='1.0'?&gt;
  &lt;PaymentInfo xmlns='http://example.org/paymentv2'&gt;
    &lt;Name&gt;John Smith&lt;/Name&gt;
    &lt;CreditCard Limit='5,000' Currency='USD'&gt;
      &lt;EncryptedData xmlns='http://www.w3.org/2001/04/xmlenc#'
       Type='http://www.w3.org/2001/04/xmlenc#Content'&gt;
        &lt;CipherData&gt;
          &lt;CipherValue&gt;A23B45C56&lt;/CipherValue&gt;
        &lt;/<code>CipherDat</code>a&gt;
      &lt;/<code>EncryptedDat</code>a&gt;
    &lt;/<code>CreditCar</code>d&gt;
  &lt;/<code>PaymentInf</code>o&gt;
</pre>

<h4>2.1.3 Encrypting XML <a name="sec-eg-Element-Content-Character"
id="sec-eg-Element-Content-Character">Element Content</a> (Character
Data)</h4>

<p>Or, consider the scenario in which all the information <em>except</em> the
actual credit card number can be in the clear, including the fact that the
Number element exists:</p>
<pre class="xml-example">  &lt;?xml version='1.0'?&gt;
  &lt;PaymentInfo xmlns='http://example.org/paymentv2'&gt;
    &lt;Name&gt;John Smith&lt;/Name&gt;
    &lt;CreditCard Limit='5,000' Currency='USD'&gt;
      &lt;Number&gt;
        &lt;EncryptedData xmlns='http://www.w3.org/2001/04/xmlenc#'
         Type='http://www.w3.org/2001/04/xmlenc#Content'&gt;
          &lt;CipherData&gt;
            &lt;CipherValue&gt;A23B45C56&lt;/CipherValue&gt;
          &lt;/<code>CipherDat</code>a&gt;
        &lt;/<code>EncryptedDat</code>a&gt;
      &lt;/Number&gt;
      &lt;Issuer&gt;Example Bank&lt;/Issuer&gt;
      &lt;Expiration&gt;04/02&lt;/Expiration&gt;
    &lt;/<code>CreditCar</code>d&gt;
  &lt;/<code>PaymentInf</code>o&gt;
</pre>

<p>Both <code>CreditCard</code> and <code>Number</code> are in the clear, but
the character data content of <code>Number</code> is encrypted.</p>

<h4>2.1.4 Encrypting <a name="sec-eg-Arbitrary-Data"
id="sec-eg-Arbitrary-Data">Arbitrary Data</a> and XML Documents</h4>

<p>If the application scenario requires all of the information to be
encrypted, the whole document is encrypted as an octet sequence. This applies
to arbitrary data including XML documents.</p>
<pre class="xml-example">  &lt;?xml version='1.0'?&gt;
  &lt;EncryptedData xmlns='http://www.w3.org/2001/04/xmlenc#'
   MimeType='text/xml'&gt;
    &lt;CipherData&gt;
      &lt;CipherValue&gt;A23B45C56&lt;/CipherValue&gt;
    &lt;/<code>CipherDat</code>a&gt;
  &lt;/<code>EncryptedDat</code>a&gt;
</pre>

<h4>2.1.5 <a name="sec-eg-Super-Encryption"
id="sec-eg-Super-Encryption">Super-Encryption</a>: Encrypting
EncryptedData</h4>

<p>An XML document may contain zero or more <code>EncryptedData</code>
elements. <code>EncryptedData</code> cannot be the parent or child of another
<code>EncryptedData</code> element. However, the actual data encrypted can be
anything, including <code>EncryptedData</code> and <code>EncryptedKey</code>
elements (i.e., super-encryption). During super-encryption of an
<code>EncryptedData</code> or <code>EncryptedKey</code> element, one must
encrypt the entire element. Encrypting only the content of these elements, or
encrypting selected child elements is an invalid instance under the provided
schema.<br />
For example, consider the following:</p>
<pre class="xml-example">  &lt;p<code>ay:PaymentInfo</code> xmlns:pay='http://example.org/paymentv2'&gt;
    &lt;EncryptedData Id='ED1' xmlns='http://www.w3.org/2001/04/xmlenc#'
     Type='<a href="http://www.w3.org/2001/04/xmlenc#Element">http://www.w3.org/2001/04/xmlenc#Element</a>'&gt;
      &lt;CipherData&gt;
        &lt;CipherValue&gt;<code>original</code>EncryptedData&lt;/CipherValue&gt;
      &lt;/<code>CipherDat</code>a&gt;
    &lt;/<code>EncryptedDat</code>a&gt;
  &lt;/<code>pay:PaymentInf</code>o&gt;
</pre>

<p>A valid super-encryption of "<code>//xenc:EncryptedData[@Id='ED1']</code>"
would be:</p>
<pre class="xml-example">  &lt;p<code>ay:PaymentInfo</code> xmlns:pay='http://example.org/paymentv2'&gt;
    &lt;EncryptedData Id='ED2' xmlns='http://www.w3.org/2001/04/xmlenc#'
     Type='<a href="http://www.w3.org/2001/04/xmlenc#Element">http://www.w3.org/2001/04/xmlenc#Element</a>'&gt;
      &lt;CipherData&gt;
        &lt;CipherValue&gt;<code>new</code>EncryptedData&lt;/CipherValue&gt;
      &lt;/<code>CipherDat</code>a&gt;
    &lt;/<code>EncryptedDat</code>a&gt;
  &lt;/<code>pay:PaymentInf</code>o&gt;
</pre>

<p>where the <code>CipherValue</code> content of
'<code>newEncryptedData</code>' is the base64 encoding of the encrypted octet
sequence resulting from encrypting the <code>EncryptedData</code> element
with <code>Id='ED1'</code>.</p>

<h3>2.2 <code>EncryptedData</code> and <code>EncryptedKey</code> <a
name="sec-Usage" id="sec-Usage">Usage</a></h3>

<h4>2.2.1 <code>EncryptedData</code> with <a name="sec-eg-Symmetric-Key"
id="sec-eg-Symmetric-Key">Symmetric Key</a>  (<code>KeyName</code>)</h4>
<pre class="xml-example">  [s1] &lt;EncryptedData xmlns='http://www.w3.org/2001/04/xmlenc#'
        Type='<a href="http://www.w3.org/2001/04/xmlenc#Element">http://www.w3.org/2001/04/xmlenc#Element</a>'/&gt;
  [s2]   &lt;EncryptionMethod
          Algorithm='http://www.w3.org/2001/04/xmlenc#3des-cbc'/&gt;
  [s3]   &lt;ds:KeyInfo xmlns:ds='http://www.w3.org/2000/09/xmldsig#'&gt;
  [s4]     &lt;d<code>s:KeyName</code>&gt;John Smith&lt;/<code>ds:KeyNam</code>e&gt;
  [s5]   &lt;/<code>ds:KeyInf</code>o&gt;
  [s6]   &lt;CipherData&gt;&lt;CipherValue&gt;DEADBEEF&lt;/CipherValue&gt;&lt;/<code>CipherDat</code>a&gt;
  [s7] &lt;/<code>EncryptedDat</code>a&gt;
</pre>

<p><code>[s1]</code> The type of data encrypted may be represented as an
attribute value to aid in decryption and subsequent processing. In this case,
the data encrypted was an 'element'. Other alternatives include 'content' of
an element, or an external octet sequence which can also be identified via
the <code>MimeType</code> and <code>Encoding</code> attributes.</p>

<p><code>[s2]</code> This (3DES CBC) is a symmetric key cipher.</p>

<p><code>[s4]</code> The symmetric key has an associated name "John
Smith".</p>

<p><code>[s6]</code> <code>CipherData</code> contains a
<code>CipherValue</code>, which is a base64 encoded octet sequence.
Alternately, it could contain a <code>CipherReference</code>, which is a URI
reference along with transforms necessary to obtain the encrypted data as an
octet sequence</p>

<h4>2.2.2 <a name="sec-eg-EncryptedKey"
id="sec-eg-EncryptedKey"><code>EncryptedKey</code></a>
(<code>ReferenceList</code>, <code>ds:RetrievalMethod</code>,
<code>CarriedKeyName</code>)</h4>

<p>The following <code>EncryptedData</code> structure is very similar to the
one above, except this time the key is referenced using a
<code>ds:RetrievalMethod</code>:</p>
<pre class="xml-example">  [t01] &lt;EncryptedData Id='ED'
         xmlns='http://www.w3.org/2001/04/xmlenc#'&gt;
  [t02]   &lt;EncryptionMethod
           Algorithm='http://www.w3.org/2001/04/xmlenc#aes128-cbc'/&gt;
  [t03]   &lt;d<code>s:KeyInfo</code> xmlns:ds='http://www.w3.org/2000/09/xmldsig#'&gt;
  [t04]     &lt;ds:<code>RetrievalMethod</code> URI='#EK'
             Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey"&gt;
  [t05]     &lt;d<code>s:KeyNam</code>e&gt;Sally Doe&lt;/d<code>s:KeyNam</code>e&gt;
  [t06]   &lt;/<code>ds:KeyInf</code>o&gt;
  [t07]   &lt;CipherData&gt;&lt;CipherValue&gt;DEADBEEF&lt;/CipherValue&gt;&lt;/<code>CipherDat</code>a&gt;
  [t08] &lt;/<code>EncryptedDat</code>a&gt;
</pre>

<p><code>[t02]</code> This (AES-128-CBC) is a symmetric key cipher.</p>

<p><code>[t04]</code> <code>ds:RetrievalMethod</code> is used to indicate the
location of a key with type <code>&amp;xenc;EncryptedKey</code>. The (AES)
key is located at '#EK'.</p>

<p><code>[t05]</code> <code>ds:KeyName</code> provides an alternative method
of identifying the key needed to decrypt the <code>CipherData</code>. Either
or both the <code>ds:KeyName</code> and <code>ds:KeyRetrievalMethod</code>
could be used to identify the same key.</p>

<p>Within the same XML document, there existed an <code>EncryptedKey</code>
structure that was referenced within <code>[t04]</code>:</p>
<pre class="xml-example">  [t09] &lt;EncryptedKey Id='EK' xmlns='http://www.w3.org/2001/04/xmlenc#'&gt;
  [t10]   &lt;EncryptionMethod
           Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5"/&gt;
  [t11]   &lt;d<code>s:KeyInfo</code> xmlns:ds='http://www.w3.org/2000/09/xmldsig#'&gt;
  [t12]     &lt;d<code>s:KeyNam</code>e&gt;John Smith&lt;/<code>ds:KeyNam</code>e&gt;
  [t13]   &lt;/<code>ds:KeyInf</code>o&gt;
  [t14]   &lt;CipherData&gt;&lt;CipherValue&gt;xyzabc&lt;/CipherValue&gt;&lt;/<code>CipherDat</code>a&gt;
  [t15]   &lt;ReferenceList&gt;
  [t16]     &lt;DataReference URI='#ED'/&gt;
  [t17]   &lt;/<code>ReferenceLis</code>t&gt;
  [t18]   &lt;CarriedKeyName&gt;Sally Doe&lt;/CarriedKeyName&gt;
  [t19] &lt;/<code>EncryptedKe</code>y&gt;
</pre>

<p><code>[t09]</code> The <code>EncryptedKey</code> element is similar to the
<code>EncryptedData</code> element except that the data encrypted is always a
key value.</p>

<p><code>[t10]</code> The <code>EncryptionMethod</code> is the RSA public key
algorithm.</p>

<p><code>[t12]</code> <code>ds:KeyName</code> of "John Smith" is a property
of the key necessary for decrypting (using RSA) the
<code>CipherData</code>.</p>

<p><code>[t14]</code> The <code>CipherData</code>'s <code>CipherValue</code>
is an octet sequence that is processed (serialized, encrypted, and encoded)
by a referring encrypted object's <code>EncryptionMethod</code>. (Note, an
EncryptedKey's <code>EncryptionMethod</code> is the algorithm used to encrypt
these octets and does not speak about what type of octets they are.)</p>

<p><code>[t15-17]</code> A <code>ReferenceList</code> identifies the
encrypted objects (<code>DataReference</code> and <code>KeyReference</code>)
encrypted with this key. The <code>ReferenceList</code> contains a list of
references to data encrypted by the symmetric key carried within this
structure.</p>

<p><code>[t18]</code> The <code>CarriedKeyName</code> element is used to
identify the encrypted key value which may be referenced by the
<code>KeyName</code> element in <code>ds:KeyInfo</code>. (Since ID attribute
values must be unique to a document,<code>CarriedKeyName</code> can indicate
that several <code>EncryptedKey</code> structures contain the same key value
encrypted for different recipients.)</p>

<h1>3 <a name="sec-Encryption-Syntax" id="sec-Encryption-Syntax">Encryption
Syntax</a></h1>

<p>This section provides a detailed description of the syntax and features
for XML Encryption. Features described in this section MUST be implemented
unless otherwise noted. The syntax is defined via [<a
href="#ref-XML-schema">XML-Schema</a>] with the following XML preamble,
declaration, internal entity, and import:</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:xenc CDATA #FIXED 'http://www.w3.org/2001/04/xmlenc#'
       xmlns:ds CDATA #FIXED 'http://www.w3.org/2000/09/xmldsig#'&gt;
     &lt;!ENTITY xenc 'http://www.w3.org/2001/04/xmlenc#'&gt;
     &lt;!ENTITY % p ''&gt;
     &lt;!ENTITY % s ''&gt;
    ]&gt;

  &lt;schema xmlns='http://www.w3.org/2001/XMLSchema' version='1.0'
          xmlns:ds='http://www.w3.org/2000/09/xmldsig#'
          xmlns:xenc='http://www.w3.org/2001/04/xmlenc#'
          targetNamespace='http://www.w3.org/2001/04/xmlenc#'
          element<code>FormDefault</code>='qualified'&gt;

    &lt;import namespace='http://www.w3.org/2000/09/xmldsig#'
            schemaLocation='http://www.w3.org/TR/2001/CR-xmldsig-core-20010419/xmldsig-core-schema.xsd'/&gt;
</pre>

<h2>3.1 The <a name="sec-EncryptedType"
id="sec-EncryptedType"><code>EncryptedType</code></a></h2>

<p><code>EncryptedType</code> is the abstract type from which
<code>EncryptedData</code> and <code>EncryptedKey</code> are derived. While
these two latter element types are very similar with respect to their content
models, a syntactical distinction is useful to processing. Implementation
MUST generate laxly schema valid [<a href="#ref-XML-schema">XML-schema</a>]
<code>EncryptedData</code> or <code>EncryptedKey</code> as specified by the
subsequent schema declarations. (Note the laxly schema valid generation means
that the content permitted by <code>xsd:ANY</code> need not be valid.)
Implementations SHOULD create these XML structures
(<code>EncryptedType</code> elements and their descendents/content) in
Normalization Form C [<a href="#ref-NFC">NFC</a>, <a
href="#ref-NFC-Corrigendum">NFC-Corrigendum</a>].</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;complexType name='<code>EncryptedType</code>' abstract='true'&gt;
    &lt;sequence&gt;
      &lt;element name='<code>EncryptionMethod</code>' type='<code>xenc:EncryptionMethodType</code>'
               minOccurs='0'/&gt;
      &lt;element ref='<code>ds:KeyInfo</code>' minOccurs='0'/&gt;
      &lt;element ref='<code>xenc:CipherData</code>'/&gt;
      &lt;element ref='xenc:EncryptionProperties' minOccurs='0'/&gt;
    &lt;/sequence&gt;
    &lt;attribute name='Id' type='ID' use='optional'/&gt;
    &lt;attribute name='Type' type='anyURI' use='optional'/&gt;
    &lt;attribute name='MimeType' type='string' use='optional'/&gt;
    &lt;attribute name='Encoding' type='string' use='optional'/&gt;
   &lt;/complexType&gt;
</pre>

<p><code>EncryptionMethod</code> is an optional element that describes the
encryption algorithm applied to the cipher data. If the element is absent,
the encryption algorithm must be known by the recipient or the decryption
will fail.</p>

<p><code>ds:KeyInfo</code> is an optional element, defined by [<a
href="#ref-XML-DSIG">XML-DSIG</a>], that carries information about the key
used to encrypt the data. Subsequent sections of this specification define
new elements that may appear as children of <code>ds:KeyInfo</code>.</p>

<p><code>CipherData</code> is a mandatory element that contains the
<code>CipherValue</code> or <code>CipherReference</code> with the encrypted
data.</p>

<p><code>EncryptionProperties</code> can contain additional information
concerning the generation of the <code>EncryptedType</code> (e.g., date/time
stamp).</p>

<p><code>Id</code> is an optional attribute providing for the standard method
of assigning a string id to the element within the document context.</p>

<p><code>Type</code> is an optional attribute identifying type information
about the plaintext form of the encrypted content. While optional, this
specification takes advantage of it for mandatory processing described in <a
href="#sec-Processing-Decryption">Processing Rules: Decryption</a> (section
4.2). If the <code>EncryptedData</code> element contains data of
<code>Type</code> 'element' or element 'content', and replaces that data in
an XML document context, it is strongly recommended the <code>Type</code>
attribute be provided. Without this information, the decryptor will be unable
to automatically restore the XML document to its original cleartext form.</p>

<p><code>MimeType</code> is an optional (advisory) attribute which describes
the media type of the data which has been encrypted. The value of this
attribute is a string with values defined by [<a href="#ref-MIME">MIME</a>].
For example, if the data that is encrypted is a base64 encoded PNG, the
<code>Encoding</code> may be specified as 'base64' and the
<code>MimeType</code> as 'image/png'. This attribute is purely advisory; no
validation of the <code>MimeType</code> information is required and it does
not indicate the encryption application must do any additional processing.
Note, this information may not be necessary if it is already bound to the
identifier in the <code>Type</code> attribute. For example, the Element and
Content types defined in this specification are always UTF-8 encoded text.</p>

<h2>3.2 The <a name="sec-EncryptionMethod"
id="sec-EncryptionMethod">EncryptionMethod</a> Element</h2>

<p>EncryptionMethod is an optional element that describes the encryption
algorithm applied to the cipher data. If the element is absent, the
encryption algorithm must be known by the recipient or the decryption will
fail.</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;complexType name='EncryptionMethodType' mixed='true'&gt;
    &lt;sequence&gt;
      &lt;element name='KeySize' minOccurs='0'
               type='xenc:KeySizeType'/&gt;
      &lt;any namespace='##other' minOccurs='0'
                               maxOccurs='unbounded'/&gt;
    &lt;/sequence&gt;
    &lt;attribute name='Algorithm' type='anyURI' use='required'/&gt;
  &lt;/complexType&gt;
</pre>

<p>The permitted child elements of the <code>EncryptionMethod</code> are
determined by the specific value of the <code>Algorithm</code> attribute URI,
and the <code>KeySize</code> child element is always permitted. For example,
the <a href="#sec-RSA-OAEP">RSA-OAEP algorithm</a> (section 5.4.2) uses the
<code>ds:DigestMethod</code> and <code>OAEPparams</code> elements. (We rely
upon the <code>ANY</code> schema construct because it is not possible to
specify element content based on the value of an attribute.)</p>

<p>The presence of any child element under <code>EncryptionMethod</code>
which is not permitted by the algorithm or the presence of a
<code>KeySize</code> child inconsistent with the algorithm MUST be treated as
an error. (All algorithm URIs specified in this document imply a key size but
this is not true in general. Most popular stream cipher algorithms take
variable size keys.)</p>

<h2>3.3 The <a name="sec-CipherData"
id="sec-CipherData"><code>CipherData</code></a> Element</h2>

<p>The <code>CipherData</code> is a mandatory element that provides the
encrypted data. It must either contain the encrypted octet sequence as base64
encoded text of the <code>CipherValue</code> element, or provide a reference
to an external location containing the encrypted octet sequence via the
<code>CipherReference</code> element.</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;element name='<code>CipherData</code>' type='<code>xenc:CipherDataType</code>'/&gt;
  &lt;complexType name='<code>CipherDataType</code>'&gt;
     &lt;choice&gt;
       &lt;element name='<code>CipherValue</code>' type='base64Binary'/&gt;
       &lt;element ref='<code>xenc:CipherReference</code>'/&gt;
     &lt;/choice&gt;
   &lt;/complexType&gt;
</pre>

<h3>3.3.1 The <a name="sec-CipherReference"
id="sec-CipherReference"><code>CipherReference</code></a> Element</h3>

<p>If <code>CipherValue</code> is not supplied directly, the
<code>CipherReference</code> identifies a source which, when processed,
yields the encrypted octet sequence.</p>

<p>The actual value is obtained as follows. The <code>CipherReference</code>
<code>URI</code> contains an identifier that is dereferenced. Should the
<code>CipherReference</code> element contain an OPTIONAL sequence of
<code>Transform</code>s, the data resulting from dereferencing the URI is
transformed as specified so as to yield the intended cipher value. For
example, if the value is base64 encoded within an XML document; the
transforms could specify an XPath expression followed by a base64 decoding so
as to extract the octets.</p>

<p>The syntax of the <code>URI</code> and <code>Transforms</code> is similar
to that of [<a href="#ref-XML-DSIG">XML-DSIG</a>]. However, there is a
difference between signature and encryption processing. In [<a
href="#ref-XML-DSIG">XML-DSIG</a>] both generation and validation processing
start with the same source data and perform that transform in the same order.
In encryption, the decryptor has only the cipher data and the specified
transforms are enumerated for the decryptor, in the order necessary to obtain
the octets. Consequently, because it has different semantics
<code>Transforms</code> is in the <code>&amp;xenc;</code> namespace.</p>

<p>For example, if the relevant cipher value is captured within a
<code>CipherValue</code> element within a different XML document, the
<code>CipherReference</code> might look as follows:</p>
<pre class="xml-example">  &lt;CipherReference URI="http://www.example.com/CipherValues.xml"&gt;
    &lt;Transforms&gt;
      &lt;ds:Transform
       Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116"&gt;
          &lt;ds:XPath xmlns:rep="http://www.example.org/repository"&gt;
            self::text()[parent::rep:CipherValue[@Id="example1"]]
          &lt;/ds:XPath&gt;
      &lt;/ds:Transform&gt;
      &lt;ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#base64"/&gt;
    &lt;/Transforms&gt;
  &lt;/CipherReference&gt;
</pre>

<p>Implementations MUST support the <code>CipherReference</code> feature and
the same URI encoding, dereferencing, scheme, and HTTP response codes as that
of [<a href="#ref-XML-DSIG">XML-DSIG</a>]. The <code>Transform</code> feature
and particular transform algorithms are OPTIONAL.</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;element name='<code>CipherReference</code>' type='<code>xenc:CipherReferenceType</code>'/&gt;
   &lt;complexType name='<code>CipherReferenceType</code>'&gt;
       &lt;sequence&gt;
         &lt;element name='Transforms' type='xenc:TransformsType' minOccurs='0'/&gt;
       &lt;/sequence&gt;
       &lt;attribute name='URI' type='anyURI' use='required'/&gt;
   &lt;/complexType&gt;

    &lt;complexType name='TransformsType'&gt;
       &lt;sequence&gt;
         &lt;element ref='ds:Transform' maxOccurs='unbounded'/&gt;
       &lt;/sequence&gt;
     &lt;/complexType&gt;
</pre>

<h2>3.4 The <a name="sec-EncryptedData"
id="sec-EncryptedData"><code>EncryptedData</code></a> Element</h2>

<p>The <code>EncryptedData</code> element is the core element in the syntax.
Not only does its <code>CipherData</code> child contain the encrypted data,
but it's also the element that replaces the encrypted element, or serves as
the new document root.</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;element name='<code>EncryptedData</code>' type='<code>xenc:EncryptedDataType</code>'/&gt;
  &lt;complexType name='<code>EncryptedDataType</code>'&gt;
    &lt;complexContent&gt;
     &lt;extension base='<code>xenc:EncryptedType</code>'&gt;
     &lt;/extension&gt;
    &lt;/complexContent&gt;
  &lt;/complexType&gt;
</pre>

<h2>3.5 <a name="sec-Extensions-to-KeyInfo"
id="sec-Extensions-to-KeyInfo">Extensions to <code>ds:KeyInfo</code></a>
Element</h2>

<p>There are three ways that the keying material needed to decrypt
<code>CipherData</code> can be provided:</p>
<ol>
  <li>The <code>EncryptedData</code> or <code>EncryptedKey</code> element
    specify the associated keying material via a child of
    <code>ds:KeyInfo</code>. All of the child elements of
    ds:<code>KeyInfo</code> specified in [<a
    href="#ref-XML-DSIG">XML-DSIG</a>] MAY be used as qualified:
    <ol>
      <li>Using the literal, unprotected, <code>ds:KeyValue</code> to
        transport the encryption key is obviously NOT RECOMMENDED.</li>
      <li>Support of <code>ds:KeyName</code> to refer to an
        <code>EncryptedKey</code> <code>CarriedKeyName</code> is
      RECOMMENDED.</li>
      <li>Support for same document <code>ds:RetrievalMethod</code> is
        REQUIRED.</li>
    </ol>
    <p>In addition, we provide two additional child elements: applications
    MUST support <code><a href="#sec-EncryptedKey">EncryptedKey</a></code>
    (section 3.5.1) and MAY support <code><a
    href="#sec-Alg-KeyAgreement">AgreementMethod</a></code> (section 5.5).</p>
  </li>
  <li>A detached (not inside <code>ds:KeyInfo</code>)
    <code>EncryptedKey</code> element can specify the
    <code>EncryptedData</code> or <code>EncryptedKey</code> to which its
    decrypted key will apply via a <code><a
    href="#sec-ReferenceList">DataReference</a></code> or <a
    href="#sec-ReferenceList"><code>KeyReference</code></a> (section
  3.6).</li>
  <li>The keying material can be determined by the recipient by application
    context and thus need not be explicitly mentioned in the transmitted
  XML.</li>
</ol>

<h3>3.5.1 The <a name="sec-EncryptedKey"
id="sec-EncryptedKey"><code>EncryptedKey</code></a> Element</h3>
<dl>
  <dt>Identifer</dt>
    <dd><code><a name="EncryptedKey"
      id="EncryptedKey">Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey"</a></code>
      <p>(This can be used within a <code>ds:RetrievalMethod</code> element
      to identify the referent's type.)</p>
    </dd>
</dl>

<p>The <code>EncryptedKey</code> element is used to transport encryption keys
from the originator to a known recipient(s). It may be used as a stand-alone
XML document, be placed within an application document, or appear inside an
<code>EncryptedData</code> element as a child of a <code>ds:KeyInfo</code>
element. The key value is always encrypted to the recipient(s). When
<code>EncryptedKey</code> is decrypted the resulting octets are made
available to the <code>EncryptionMethod</code> algorithm without any
additional processing.</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;element name='<code>EncryptedKey</code>' type='<code>xenc:EncryptedKeyType</code>'/&gt;
  &lt;complexType name='<code>EncryptedKeyType</code>'&gt;
    &lt;complexContent&gt;
      &lt;extension base='<code>xenc:EncryptedType</code>'&gt;
        &lt;sequence&gt;
          &lt;element ref='<code>xenc:ReferenceList</code>' minOccurs='0'/&gt;
          &lt;element name='<code>CarriedKeyName</code>' type='string' minOccurs='0'/&gt;
        &lt;/sequence&gt;
        &lt;attribute name='Recipient' type='string' use='optional'/&gt;
      &lt;/extension&gt;
    &lt;/complexContent&gt;
  &lt;/complexType&gt;
</pre>

<p><code>ReferenceList</code> is an optional element containing pointers to
data and keys encrypted using this key. The reference list may contain
multiple references to <code>EncryptedKey</code> and
<code>EncryptedData</code> elements. This is done using
<code>KeyReference</code> and <code>DataReference</code> elements
respectively. These are defined below.</p>

<p><code>CarriedKeyName</code> is an optional element for associating a user
readable name with the key value. This may then be used to reference the key
using the <code>ds:KeyName</code> element within <code>ds:KeyInfo</code>. The
same <code>CarriedKeyName</code> label, unlike an ID type, may occur multiple
times within a single document. The value of the key is to be the same in all
<code>EncryptedKey</code> elements identified with the same
<code>CarriedKeyName</code> label within a single XML document. Note that
because whitespace is significant in the value of the <code>ds:KeyName</code>
element, whitespace is also significant in the value of the
<code>CarriedKeyName</code> element.</p>

<p><code>Recipient</code> is an optional attribute that contains a hint as to
which recipient this encrypted key value is intended for. Its contents are
application dependent.</p>

<p>The <code>Type</code> attribute inheritted from <code>EncryptedType</code>
can be used to further specify the type of the encrypted key if the
<code>EncryptionMethod</code> <code>Algorithm</code> does not define a
unambiguous encoding/representation. (Note, all the algorithms in this
specification have an unambigous representation for their associated key
structures.)</p>

<h3>3.5.2 The <a name="sec-ds:RetrievalMethod"
id="sec-ds:RetrievalMethod"><code>ds:RetrievalMethod</code></a> Element</h3>

<p>The <code>ds:RetrievalMethod</code> <code>[<a
href="#ref-XML-DSIG">XML-DSIG</a>]</code>with a <code>Type</code> of
'<code>http://www.w3.org/2001/04/xmlenc#EncryptedKey</code>' provides a way
to express a link to an <code>EncryptedKey</code> element containing the key
needed to decrypt the <code>CipherData</code> associated with an
<code>EncryptedData</code> or <code>EncryptedKey</code> element. The
<code>ds:RetrievalMethod</code> with this type is always a child of the
<code>ds:KeyInfo</code> element and may appear multiple times. If there is
more than one instance of a <code>ds:RetrievalMethod</code> in a
<code>ds:KeyInfo</code> of this type, then the <code>EncryptedKey</code>
objects referred to must contain the same key value, possibly encrypted in
different ways or for different recipients.</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;!--
      &lt;attribute name='Type' type='anyURI' use='optional'
      fixed='http://www.w3.org/2001/04/xmlenc#<code>EncryptedKey</code>' /&gt;
  --&gt;
</pre>

<h3>3.6 The <a name="sec-ReferenceList"
id="sec-ReferenceList"><code>ReferenceList</code></a> Element</h3>

<p><code>ReferenceList</code> is an element that contains pointers from a key
value of an <code>EncryptedKey</code> to items encrypted by that key value
(<code>EncryptedData</code> or <code>EncryptedKey</code> elements).</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;element name='<code>ReferenceList</code>'&gt;
    &lt;complexType&gt;
      &lt;choice&gt;
        &lt;element name='<code>DataReference</code>' type='<code>xenc:ReferenceType</code>'
         minOccurs='0' maxOccurs='unbounded'/&gt;
        &lt;element name='<code>KeyReference</code>' type='<code>xenc:ReferenceType</code>'
         minOccurs='0' maxOccurs='unbounded'/&gt;
      &lt;/choice&gt;
    &lt;/complexType&gt;
  &lt;/element&gt;

  &lt;complexType name='<code>ReferenceType</code>'&gt;
    &lt;sequence&gt;
      &lt;any namespace='##other' minOccurs='0' maxOccurs='unbounded'/&gt;
    &lt;/sequence&gt;
    &lt;attribute name='URI' type='anyURI' use='required'/&gt;
  &lt;/complexType&gt;
</pre>

<p><code>DataReference</code> elements are used to refer to
<code>EncryptedData</code> elements that were encrypted using the key defined
in the enclosing <code>EncryptedKey</code> element. Multiple
<code>DataReference</code> elements can occur if multiple
<code>EncryptedData</code> elements exist that are encrypted by the same
key.</p>

<p><code>KeyReference</code> elements are used to refer to
<code>EncryptedKey</code> elements that were encrypted using the key defined
in the enclosing <code>EncryptedKey</code> element. Multiple
<code>KeyReference</code> elements can occur if multiple
<code>EncryptedKey</code> elements exist that are encrypted by the same
key.</p>

<p>For both types of references one may optionally specify child elements to
aid the recipient in retrieving the <code>EncryptedKey</code> and/or
<code>EncryptedData</code> elements. These could include information such as
XPath transforms, decompression transforms, or information on how to retrieve
the elements from a document storage facility. For example:</p>
<pre class="xml-example">  &lt;ReferenceList&gt;
    &lt;DataReference URI="#invoice34"&gt;
      &lt;ds:Transforms&gt;
        &lt;ds:Transform Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116"&gt;
          &lt;ds:XPath xmlns:xenc="http://www.w3.org/2001/04/xmlenc#"&gt;
              self::xenc:EncryptedData[@Id="example1"]
          &lt;/ds:XPath&gt;
        &lt;/ds:Transform&gt;
      &lt;/ds:Transforms&gt;
    &lt;/DataReference&gt;
  &lt;/ReferenceList&gt;
</pre>

<h3>3.7 The <a name="sec-EncryptionProperties"
id="sec-EncryptionProperties"><code>EncryptionProperties</code></a>
Element</h3>
<dl>
  <dt>Identifier</dt>
    <dd><code><a name="EncryptionProperties"
      id="EncryptionProperties">Type="http://www.w3.org/2001/04/xmlenc#EncryptionProperties"</a></code>
      <p>(This can be used within a <code>ds:Reference</code> element to
      identify the referent's type.)</p>
    </dd>
</dl>

<p>Additional information items concerning the generation of the
<code>EncryptedData</code> or <code>EncryptedKey</code> can be placed in an
<code>EncryptionProperty</code> element (e.g., date/time stamp or the serial
number of cryptographic hardware used during encryption). The
<code>Target</code> attribute identifies the <code>EncryptedType</code>
structure being described. <code>anyAttribute</code> permits the inclusion of
attributes from the XML namespace to be included (i.e.,
<code>xml:space</code>, <code>xml:lang</code>, and <code>xml:base</code>).</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;element name='EncryptionProperties' type='xenc:EncryptionPropertiesType'/&gt;
  &lt;complexType name='EncryptionPropertiesType'&gt;
    &lt;sequence&gt;
      &lt;element ref='xenc:EncryptionProperty' maxOccurs='unbounded'/&gt;
    &lt;/sequence&gt;
    &lt;attribute name='Id' type='ID' use='optional'/&gt;
  &lt;/complexType&gt;

    &lt;element name='EncryptionProperty' type='xenc:EncryptionPropertyType'/&gt;
    &lt;complexType name='EncryptionPropertyType' mixed='true'&gt;
      &lt;choice maxOccurs='unbounded'&gt;
        &lt;any namespace='##other' processContents='lax'/&gt;
      &lt;/choice&gt;
      &lt;attribute name='Target' type='anyURI' use='optional'/&gt;
      &lt;attribute name='Id' type='ID' use='optional'/&gt;
      &lt;anyAttribute namespace="http://www.w3.org/XML/1998/namespace"/&gt;
    &lt;/complexType&gt;
</pre>

<h2><a name="sec-Processing" id="sec-Processing">4 Processing Rules</a></h2>

<p>This section describes the operations to be performed as part of
encryption and decryption processing by implementations of this
specification. The conformance requirements are specified over the following
roles:</p>
<dl>
  <dt>Application</dt>
    <dd>The application which makes request of an XML Encryption
      implementation via the provision of data and parameters necessary for
      its processing.</dd>
  <dt>Encryptor</dt>
    <dd>An XML Encryption implementation with the role of encrypting
    data.</dd>
  <dt>Decryptor</dt>
    <dd>An XML Encryption implementation with the role of decrypting
    data.</dd>
</dl>

<h3><a name="sec-Processing-Encryption" id="sec-Processing-Encryption">4.1
Encryption</a></h3>

<p>For each data item to be encrypted as an <code>EncryptedData</code> or
<code>EncryptedKey</code> (elements derived from <code>EncryptedType</code>),
the <strong>encryptor</strong> must:</p>
<ol>
  <li>Select the algorithm (and parameters) to be used in encrypting this
    data.</li>
  <li>Obtain and (optionally) represent the key.
    <ol>
      <li>If the key is to be identified (via naming, URI, or included in a
        child element), construct the <code>ds:KeyInfo</code> as approriate
        (e.g., <code>ds:KeyName</code>, <code>ds:KeyValue</code>,
        <code>ds:RetrievalMethod</code>, etc.)</li>
      <li>If the key itself is to be encrypted, construct an
        <code>EncryptedKey</code> element by recursively applying this
        encryption process. The result may then be a child of
        <code>ds:KeyInfo</code>, or it may exist elsewhere and may be
        identified in the preceding step.</li>
    </ol>
  </li>
  <li> Encrypt the data
    <ol>
      <li>If the data is an '<a
        href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-element">element</a>'
        [<a href="#ref-XML">XML</a>, section 3] or element '<a
        href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">content</a>'
        [<a href="#ref-XML">XML</a>, section 3.1], obtain the octets by
        serializing the data in UTF-8 as specified in [<a
        href="#ref-XML">XML</a>]. ([<a href="#ref-NFC">NFC</a>] MUST be
        applied when this involves conversion from a legacy (i.e.
        non-Unicode) encoding.) Serialization MAY be done by the
        <strong>encryptor</strong>. If the encryptor does not serialize, then
        the <strong>application</strong> MUST perform the serialization.</li>
      <li>If the data is of any other type that is not already octets, the
        <strong>application</strong> MUST serialize it as octets.</li>
      <li>Encrypt the octets using the algorithm and key from steps 1 and
      2.</li>
      <li>Unless the <strong>decryptor</strong> will implicitly know the type
        of the encrypted data, the <strong>encryptor</strong> SHOULD provide
        the type for representation.
        <p>The definition of this type as bound to an identifier specifies
        how to obtain and interpret the plaintext octets after decryption.
        For example, the idenifier could indicate that the data is an
        instance of another application (e.g., some XML compression
        application) that must be further processed. Or, if the data is a
        simple octet sequence it MAY be described with the
        <code>MimeType</code> and <code>Encoding</code> attributes. For
        example, the data might be an XML document
        (<code>MimeType="text/xml"</code>), sequence of characters
        (<code>MimeType="text/plain"</code>), or binary image data
        (<code>MimeType="image/png</code>").</p>
      </li>
    </ol>
  </li>
  <li>Build the <code>EncryptedType</code> (<code>EncryptedData</code> or
    <code>EncryptedKey</code>) structure:
    <p>An <code>EncryptedType</code> structure represents all of the
    information previously discussed including the type of the encrypted
    data, encryption algorithm, parameters, key, type of the encrypted data,
    etc.</p>
    <ol>
      <li>If the encrypted octet sequence obtained in step 3 is to be stored
        in the <code>CipherData</code> element within the
        <code>EncryptedType</code>, then the encrypted octet sequence is
        base64 encoded and inserted as the content of a
        <code>CipherValue</code> element.</li>
      <li>If the encrypted octet sequence is to be stored externally to the
        <code>EncryptedType</code> structure, then store or return the
        encrypted octet sequence, and represent the URI and transforms (if
        any) required for the decryptor to retrieve the encrypted octet
        sequence within a <code>CipherReference</code> element.</li>
    </ol>
  </li>
  <li>Process EncryptedData
    <ol>
      <li>If the <code>Type</code> of the encrypted data is '<a
        href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-element">element</a>'
        or element '<a
        href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">content</a>',
        then the <strong>encryptor</strong> MUST be able to return the
        <code>EncryptedData</code> element to the
        <strong>application</strong>. The <strong>application</strong> MAY
        use this as the top-level element in a new XML document or insert it
        into another XML document, which may require a re-encoding.
        <p>The <strong>encryptor</strong> SHOULD be able to replace the
        unencrypted 'element' or 'content' with the EncryptedData element.
        When an <strong>application</strong> requires an XML element or
        content to be replaced, it supplies the XML document context in
        addition to identifying the element or content to be replaced. The
        <strong>encryptor</strong> removes the identified element or content
        and inserts the <code>EncryptedData</code> element in its place.</p>
        <p>(Note: If the <code>Type</code> is "content" the document
        resulting from decryption will not be well-formed if (a) the original
        plaintext was not well-formed (e.g., PCDATA by itself is not
        well-formed) and (b) the <code>EncryptedData</code> element was
        previously the root element of the document)</p>
      </li>
      <li>If the <code>Type</code> of the encrypted data <em>is not</em> '<a
        href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-element">element</a>'
        or element '<a
        href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">content</a>',
        then the <strong>encryptor</strong> MUST always return the
        <code>EncryptedData</code> element to the
        <strong>application</strong>. The <strong>application</strong> MAY
        use this as the top-level element in a new XML document or insert it
        into another XML document, which may require a re-encoding.</li>
    </ol>
  </li>
</ol>

<h3><a name="sec-Processing-Decryption" id="sec-Processing-Decryption">4.2
Decryption</a></h3>

<p>For each <code>EncryptedType</code> derived element, (i.e.,
<code>EncryptedData</code> or <code>EncryptedKey</code>), to be decrypted,
the <strong>decryptor</strong> must:</p>
<ol>
  <li>Process the element to determine the algorithm, parameters and
    <code>ds:KeyInfo</code> element to be used. If some information is
    omitted, the <strong>application</strong> MUST supply it.</li>
  <li>Locate the data encryption key according to the <code>ds:KeyInfo</code>
    element, which may contain one or more children elements. These children
    have no implied processing order. If the data encryption key is
    encrypted, locate the corresponding key to decrypt it. (This may be a
    recursive step as the key-encryption key may itself be encrypted.) Or,
    one might retrieve the data encryption key from a local store using the
    provided attributes or implicit binding.</li>
  <li>Decrypt the data contained in the <code>CipherData</code> element.
    <ol>
      <li>If a <code>CipherValue</code> child element is present, then the
        associated text value is retrieved and base64 decoded so as to obtain
        the encrypted octet sequence.</li>
      <li>If a <code>CipherReference</code> child element is present, the URI
        and transforms (if any) are used to retrieve the encrypted octet
        sequence.</li>
      <li>The encrypted octet sequence is decrypted using the
        algorithm/parameters and key value already determined from steps 1
        and 2.</li>
    </ol>
  </li>
  <li>Process decrypted data of <code>Type</code> '<a
    href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-element">element</a>'
    or element '<a
    href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">content</a>'.
    <ol>
      <li>The cleartext octet sequence obtained in step 3 is interpreted as
        UTF-8 encoded character data.</li>
      <li>The <strong>decryptor</strong> MUST be able to return the value of
        <code>Type</code> and the UTF-8 encoded XML character data. The
        <strong>decryptor</strong> is NOT REQUIRED to perform validation on
        the serialized XML.</li>
      <li>The <strong>decryptor</strong> SHOULD support the ability to
        replace the <code>EncryptedData</code> element with the decrypted '<a
        href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-element">element</a>'
        or element '<a
        href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">content</a>'
        represented by the UTF-8 encoded characters. The
        <strong>decryptor</strong> is NOT REQUIRED to perform validation on
        the result of this replacement operation.
        <p>The application supplies the XML document context and identifies
        the <code>EncryptedData</code> element being replaced. If the
        document into which the replacement is occurring is not UTF-8, the
        <strong>decryptor</strong> MUST transcode the UTF-8 encoded
        characters into the target encoding.</p>
      </li>
    </ol>
  </li>
  <li>Process decrypted data if <code>Type</code> is unspecified or is
    <em>not</em> '<a
    href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-element">element</a>'
    or element '<a
    href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">content</a>'.
    <ol>
      <li>The cleartext octet sequence obtained in <strong>Step 3</strong>
        MUST be returned to the <strong>application</strong> for further
        processing along with the <code>Type</code>, <code>MimeType</code>,
        and <code>Encoding</code> attribute values when specified.
        <code>MimeType</code> and <code>Encoding</code> are advisory. The
        <code>Type</code> value is normative as it may contain information
        necessary for the processing or interpration of the data by the
        application.</li>
      <li>Note, this step includes processing data decrypted from an
        <code>EncryptedKey</code>. The cleartext octet sequence represents a
        key value and is used by the application in decrypting other
        <code>EncryptedType</code> element(s).</li>
    </ol>
  </li>
</ol>

<h3>4.3 <a name="sec-Processing-XML" id="sec-Processing-XML">XML
Encryption</a></h3>

<p>Encryption and decryption operations are transforms on octets. The
<strong>application</strong> is responsible for the marshalling XML such that
it can be serialized into an octet sequence, encrypted, decrypted, and be of
use to the recipient.</p>

<p>For example, if the application wishes to canonicalize its data or
encode/compress the data in an XML packaging format, the application needs to
marshal the XML accordingly and identify the resulting type via the
<code>EncryptedData</code> <code>Type</code> attribute. The likelihood of
successful decryption and subsequent processing will be dependent on the
recipient's support for the given type. Also, if the data is intended to be
processed both before encryption and after decryption (e.g., XML Signature
[<a href="#ref-XML-DSIG">XML-DSIG</a>] validation or an XSLT transform) the
encrypting application must be careful to preserve information necessary for
that process's success.</p>

<p>For interoperability purposes, the following types MUST be implemented.</p>
<dl>
  <dt><a id="Element" name="Element">element</a> '<a
  href="http://www.w3.org/2001/04/xmlenc#Element">http://www.w3.org/2001/04/xmlenc#Element</a>'</dt>
    <dd>"[39]  <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-element">element</a>
      ::= <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-EmptyElemTag"><code>EmptyElemTag</code></a>
      | <a href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-STag">STag</a>
      <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">content</a>
      <a href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-ETag">ETag</a>"
      [<a href="#ref-XML">XML</a>]</dd>
  <dt><a id="Content" name="Content">content</a> '<a
  href="http://www.w3.org/2001/04/xmlenc#Content">http://www.w3.org/2001/04/xmlenc#Content</a>'</dt>
    <dd>"[43] <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-content">content</a>
      ::= <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-CharData"><code>CharData</code></a>?
      ((<a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-element">element</a>
      | <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-Reference">Reference</a>
      | <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-CDSect">CDSect</a>
      | <a href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-PI">PI</a> |
      <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-Comment">Comment</a>)
      <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006#NT-CharData"><code>CharData</code></a>?)*"
      [<a href="#ref-XML">XML</a>]</dd>
</dl>

<h2>5. <a id="sec-Algorithms" name="sec-Algorithms">Algorithms</a></h2>

<p>This section discusses algorithms used with the XML Encryption
specification. Entries contain the identifier to be used as the value of the
<code>Algorithm</code> attribute of the <code>EncryptionMethod</code> element
or other element representing the role of the algorithm, a reference to the
formal specification, definitions for the representation of keys and the
results of cryptographic operations where applicable, and general
applicability comments.</p>

<h3>5.1 <a id="sec-AlgID" name="sec-AlgID">Algorithm Identifiers and
Implementation Requirements</a></h3>

<p>All algorithms listed below have implicit parameters depending on their
role. For example, the data to be encrypted or decrypted, keying material,
and direction of operation (encrypting or decrypting) for encryption
algorithms. Any explicit additional parameters to an algorithm appear as
content elements within the element. Such parameter child elements have
descriptive element names, which are frequently algorithm specific, and
SHOULD be in the same namespace as this XML Encryption specification, the XML
Signature specification, or in an algorithm specific namespace. An example of
such an explicit parameter could be a nonce (unique quantity) provided to a
key agreement algorithm.</p>

<p>This specification defines a set of algorithms, their URIs, and
requirements for implementation. Levels of requirement specified, such as
"REQUIRED" or "OPTIONAL", refere to implementation, not use. Furthermore, the
mechanism is extensible, and alternative algorithms may be used.</p>

<h4><a id="sec-Table-of-Algorithms" name="sec-Table-of-Algorithms">Table of
Algorithms</a></h4>

<p>The table below lists the categories of algorithms. Within each category,
a brief name, the level of implementation requirement, and an identifying URI
are given for each algorithm.</p>
<dl>
  <dt>Block Encryption</dt>
    <dd><ol>
        <li>REQUIRED TRIPLEDES<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#tripledes-cbc">http://www.w3.org/2001/04/xmlenc#tripledes-cbc</a></li>
        <li>REQUIRED AES-128<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#aes128-cbc">http://www.w3.org/2001/04/xmlenc#aes128-cbc</a></li>
        <li>REQUIRED AES-256<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#aes256-cbc">http://www.w3.org/2001/04/xmlenc#aes256-cbc</a></li>
        <li>OPTIONAL AES-192<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#aes192-cbc">http://www.w3.org/2001/04/xmlenc#aes192-cbc</a></li>
      </ol>
    </dd>
  <dt>Stream Encryption</dt>
    <dd><ol>
        <li>none<br />
          Syntax and recommendations are given below to support user
          specified algorithms.</li>
      </ol>
    </dd>
  <dt>Key Transport</dt>
    <dd><ol>
        <li>REQUIRED RSA-v1.5<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#rsa-1_5">http://www.w3.org/2001/04/xmlenc#rsa-1_5</a></li>
        <li>REQUIRED RSA-OAEP<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p">http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p</a></li>
      </ol>
    </dd>
  <dt>Key Agreement</dt>
    <dd><ol>
        <li>OPTIONAL Diffie-Hellman<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#dh">http://www.w3.org/2001/04/xmlenc#dh</a></li>
      </ol>
    </dd>
  <dt>Symmetric Key Wrap</dt>
    <dd><ol>
        <li>REQUIRED TRIPLEDES KeyWrap<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#kw-tripledes">http://www.w3.org/2001/04/xmlenc#kw-tripledes</a></li>
        <li>REQUIRED AES-128 KeyWrap<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#kw-aes128">http://www.w3.org/2001/04/xmlenc#kw-aes128</a></li>
        <li>REQUIRED AES-256 KeyWrap<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#kw-aes256">http://www.w3.org/2001/04/xmlenc#kw-aes256</a></li>
        <li>OPTIONAL AES-192 KeyWrap<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#kw-aes192">http://www.w3.org/2001/04/xmlenc#kw-aes192</a></li>
      </ol>
    </dd>
  <dt>Message Digest</dt>
    <dd><ol>
        <li>REQUIRED SHA1<br />
          <a
          href="http://www.w3.org/2000/09/xmldsig#sha1">http://www.w3.org/2000/09/xmldsig#sha1</a></li>
        <li>RECOMMENDED SHA256<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#sha256">http://www.w3.org/2001/04/xmlenc#sha256</a></li>
        <li>OPTIONAL SHA512<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#sha512">http://www.w3.org/2001/04/xmlenc#sha512</a></li>
        <li>OPTIONAL RIPEMD-160<br />
          <a
          href="http://www.w3.org/2001/04/xmlenc#ripemd160">http://www.w3.org/2001/04/xmlenc#ripemd160</a></li>
      </ol>
    </dd>
  <dt>Message Authentication</dt>
    <dd><ol>
        <li>RECOMMENDED XML Digital Signature<br />
          <a
          href="http://www.w3.org/TR/2001/CR-xmldsig-core-20010419/">http://www.w3.org/TR/2001/CR-xmldsig-core-20010419/</a></li>
      </ol>
    </dd>
  <dt>Canonicalization</dt>
    <dd><ol>
        <li>OPTIONAL Canonical XML (omits comments)<br />
          <a
          href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315">http://www.w3.org/TR/2001/REC-xml-c14n-20010315</a></li>
        <li>OPTIONAL Canonical XML with Comments<br />
          <a
          href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments">http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments</a></li>
        <li>OPTIONAL Exclusive XML Canonicalization (omits comments)<br />
          <a
          href="http://www.w3.org/2001/10/xml-exc-c14n#">http://www.w3.org/2001/10/xml-exc-c14n#</a></li>
        <li>OPTIONAL Exclusive XML Canonicalization with Comments<br />
          <a
          href="http://www.w3.org/2001/10/xml-exc-c14n#WithComments">http://www.w3.org/2001/10/xml-exc-c14n#WithComments</a></li>
      </ol>
    </dd>
  <dt>Encoding</dt>
    <dd><ol>
        <li>REQUIRED base64<br />
          <a
          href="http://www.w3.org/2000/09/xmldsig#base64">http://www.w3.org/2000/09/xmldsig#base64</a></li>
      </ol>
    </dd>
</dl>

<h3>5.2 <a name="sec-Alg-Block" id="sec-Alg-Block">Block Encryption
Algorithms</a></h3>

<p>Block encryption algorithms are designed for encrypting and decrypting
data in fixed size, multiple octet blocks. Their identifiers appear as the
value of the <code>Algorithm</code> attributes of
<code>EncryptionMethod</code> elements that are children of
<code>EncryptedData</code>.</p>

<p>Block encryption algorithms take, as implicit arguments, the data to be
encrypted or decrypted, the keying material, and their direction of
operation. For all of these algorithms specified below, an initialization
vector (IV) is required that is encoded with the cipher text. For user
specified block encryption algorithms, the IV, if any, could be specified as
being with the cipher data, as an algorithm content element, or elsewhere.</p>

<p>The IV is encoded with and before the cipher text for the algorithms below
for ease of availability to the decryption code and to emphasize its
association with the cipher text. Good cryptographic practice requires that a
different IV be used for every encryption.</p>

<h4><a id="sec-Padding" name="sec-Padding">Padding</a></h4>

<p>Since the data being encrypted is an arbitrary number of octets, it may
not be a multiple of the block size. This is solved by padding the plain text
up to the block size before encryption and unpadding after decrytion. The
padding algorithm is to calculate the smallest non-zero number of octets, say
<code>N</code>, that must be suffixed to the plain text to bring it up to a
multiple of the block size. We will assume the block size is <code>B</code>
octets so <code>N</code> is in the range of 1 to <code>B</code>. Pad by
suffixing the plain text with <code>N-1</code> arbitrary pad bytes and a
final byte whose value is <code>N</code>. On decryption, just take the last
byte and, after sanity checking it, strip that many bytes from the end of the
decrypted cipher text.</p>

<p>For example, assume an 8 byte block size and plain text of
<code>0x616263</code>. The padded plain text would then be
<code>0x616263????????05</code> where the "??" bytes can be any value.
Similarly, plain text of <code>0x2122232425262728</code> would be padded to
<code>0x2122232425262728??????????????08</code>.</p>

<h4>5.2.1 <a name="sec-tripledes-cbc" id="sec-tripledes-cbc">Triple
DES</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a id="tripledes-cbc" name="tripledes-cbc"
      href="http://www.w3.org/2001/04/xmlenc#tripledes-cbc">http://www.w3.org/2001/04/xmlenc#tripledes-cbc</a>
      (REQUIRED)</dd>
</dl>

<p>ANSI X9.52 [<a href="#ref-TRIPLEDES">TRIPLEDES</a>] specifies three
sequential FIPS 46-3 [<a href="#ref-DES">DES</a>] operations. The XML
Encryption TRIPLEDES consists of a DES encrypt, a DES decrypt, and a DES
encrypt used in the Cipher Block Chaining (CBC) mode with 192 bits of key and
a 64 bit Initialization Vector (IV). Of the key bits, the first 64 are used
in the first DES operation, the second 64 bits in the middle DES operation,
and the third 64 bits in the last DES operation.</p>

<p>Note: Each of these 64 bits of key contain 56 effective bits and 8 parity
bits. Thus there are only 168 operational bits out of the 192 being
transported for a TRIPLEDES key. (Depending on the criterion used for
analysis, the effective strength of the key may be thought to be 112 bits
(due to meet in the middle attacks) or even less.)</p>

<p>The resulting cipher text is prefixed by the IV. If included in XML
output, it is then base64 encoded. An example TRIPLEDES EncryptionMethod is
as follows:</p>
<pre class="xml-example">  &lt;EncryptionMethod
   Algorithm="http://www.w3.org/2001/04/xmlenc#tripledes-cbc"/&gt;
</pre>

<h4>5.2.2 <a name="sec-AES" id="sec-AES">AES</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a id="aes128-cbc" href="http://www.w3.org/2001/04/xmlenc#aes128-cbc"
      name="aes128-cbc">http://www.w3.org/2001/04/xmlenc#aes128-cbc</a>
      (REQUIRED)</dd>
    <dd><a id="aes192-cbc" href="http://www.w3.org/2001/04/xmlenc#aes192-cbc"
      name="aes192-cbc">http://www.w3.org/2001/04/xmlenc#aes192-cbc</a>
      (OPTIONAL)</dd>
    <dd><a id="aes256-cbc" href="http://www.w3.org/2001/04/xmlenc#aes256-cbc"
      name="aes256-cbc">http://www.w3.org/2001/04/xmlenc#aes256-cbc</a>
      (REQUIRED)</dd>
</dl>

<p>[<a href="#ref-AES">AES</a>] is used in the Cipher Block Chaining (CBC)
mode with a 128 bit initialization vector (IV). The resulting cipher text is
prefixed by the IV. If included in XML output, it is then base64 encoded. An
example AES EncryptionMethod is as follows:</p>
<pre class="xml-example">  &lt;EncryptionMethod
   Algorithm="http://www.w3.org/2001/04/xmlenc#aes128-cbc"/&gt;
</pre>

<h3>5.3 <a name="sec-Alg-Stream" id="sec-Alg-Stream">Stream Encryption
Algorithms</a></h3>

<p>Simple stream encryption algorithms generate, based on the key, a stream
of bytes which are XORed with the plain text data bytes to produce the cipher
text on encryption and with the cipher text bytes to produce plain text on
decryption. They are normally used for the encryption of data and are
specified by the value of the <code>Algorithm</code> attribute of the
<code>EncryptionMethod</code> child of an <code>EncryptedData</code>
element.</p>

<p>NOTE: It is critical that each simple stream encryption key (or key and
initialization vector (IV) if an IV is also used) be used once only. If the
same key (or key and IV) is ever used on two messages then, by XORing the two
cipher texts, you can obtain the XOR of the two plain texts. This is usually
very compromising.</p>

<p>No specific steam encryption algorithms are specified herein but this
section is included to provide general guidelines.</p>

<p>Stream algorithms typically use the optional <code>KeySize</code> explicit
parameter. In cases where the key size is not apparent from the algorithm URI
or key source, as in the use of key agreement methods, this parameter sets
the key size. If the size of the key to be used is apparent and disagrees
with the <code>KeySize</code> parameter, an error MUST be returned.
Implementation of any stream algorithms is optional. The schema for the
KeySize parameter is as follows:</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;simpleType name='KeySizeType'&gt;
    &lt;restriction base="integer"/&gt;
  &lt;/simpleType&gt;
</pre>

<h3>5.4 <a name="sec-Alg-KeyTransport" id="sec-Alg-KeyTransport">Key
Transport</a></h3>

<p>Key Transport algorithms are public key encryption algorithms especially
specified for encrypting and decrypting keys. Their identifiers appear as
<code>Algorithm</code> attributes to <code>EncryptionMethod</code> elements
that are children of <code>EncryptedKey</code>. <code>EncryptedKey</code> is
in turn the child of a <code>ds:KeyInfo</code> element. The type of key being
transported, that is to say the algorithm in which it is planned to use the
transported key, is given by the <code>Algorithm</code> attribute of the
<code>EncryptionMethod</code> child of the <code>EncryptedData</code> or
<code>EncryptedKey</code> parent of this <code>ds:KeyInfo</code> element.</p>

<p>(Key Transport algorithms may optionally be used to encrypt data in which
case they appear directly as the <code>Algorithm</code> attriubte of an
<code>EncryptionMethod</code> child of an <code>EncryptedData</code> element.
Because they use public key algorithms directly, Key Transport algorithms are
not efficient for the transport of any amounts of data significantly larger
than symmetric keys.)</p>

<p>The Key Transport algorithms given below are those used in conjunction
with the Cryptographic Message Syntax (CMS) of S/MIME [<a
href="#ref-CMS-Algorithms">CMS-Algorithms</a>, <a
href="#ref-CMS-AES">CMS-AES</a>].</p>

<h4>5.4.1 <a name="sec-RSA-1_5" id="sec-RSA-1_5">RSA Version 1.5</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a id="rsa-1_5" href="http://www.w3.org/2001/04/xmlenc#rsa-1_5"
      name="rsa-1_5">http://www.w3.org/2001/04/xmlenc#rsa-1_5</a>
    (REQUIRED)</dd>
</dl>

<p>The RSAES-PKCS1-v1_5 algorithm, specified in RFC 2437 [<a
href="#ref-PKCS1">PKCS1</a>], takes no explicit parameters. An example of an
RSA Version 1.5 <code>EncryptionMethod</code> element is:</p>
<pre class="xml-example">  &lt;EncryptionMethod
   Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5"/&gt;
</pre>

<p>The <code>CipherValue</code> for such an encrypted key is the base64 [<a
href="#ref-MIME">MIME</a>] encoding of the octet string computed as per RFC
2437 [<a href="#ref-PKCS1">PKCS1</a>, section 7.2.1: Encryption operation].
As specified in the EME-PKCS1-v1_5 function RFC 2437 [<a
href="#ref-PKCS1">PKCS1</a>, section 9.1.2.1], the value input to the key
transport function is as follows:</p>
<pre class="xml-example">   CRYPT ( PAD ( KEY ))
</pre>

<p>where the padding is of the following special form:</p>
<pre class="xml-example">   02 | PS* | 00 | key
</pre>

<p>where "|" is concatenation, "02" and "00" are fixed octets of the
corresponding hexadecimal value, PS is a string of strong pseudo-random
octets [<a href="#ref-RANDOM">RANDOM</a>] at least eight octets long,
containing no zero octets, and long enough that the value of the quantity
being CRYPTed is one octet shorter than the RSA modulus, and "key" is the key
being transported. The key is 192 bits for TRIPLEDES and 128, 192, or 256
bits for AES. Support of this key transport algorithm for transporting 192
bit keys is mandatory to implement. Support of this algorithm for
transporting other keys is optional. RAS-OAEP is recommended for the
transport of AES keys.</p>

<p>The resulting base64 [<a href="#ref-MIME">MIME</a>] string is the value of
the child text node of the <code>CipherData</code> element, e.g.</p>
<pre class="xml-example">  &lt;CipherData&gt; IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4
     t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw=
  &lt;/CipherData&gt;
</pre>

<h4>5.4.2 <a name="sec-RSA-OAEP" id="sec-RSA-OAEP">RSA-OAEP</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a id="rsa-oaep-mgf1p"
      href="http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p"
      name="rsa-oaep-mgf1p">http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p</a>
      (REQUIRED)</dd>
</dl>

<p>THE RSAES-OAEP-ENCRYPT, as specified in RFC 2437 [<a
href="#ref-PKCS1">PKCS1</a>], algorithm takes as explicit parameters a
message digest function and an optional octet string <code>OAEPparams</code>.
The message digest function is indicated by the <code>Algorithm</code>
attribute of a child <code>ds:DigestMethod</code> element and the octet
string is the base64 decoding of the content of an optional
<code>OAEPparams</code> child element.</p>
<pre class="xml-dtd">Schema Definition:

&lt;element ref='ds:DigestMethod' minOccurs='0'/&gt;
&lt;element name='OAEPparams' minOccurs='0'
         type='base64Binary'/&gt;
</pre>

<p>An example of an RSA-OAEP element is:</p>
<pre class="xml-example">  &lt;EncryptionMethod
     Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p"&gt;
     &lt;ds:DigestMethod
        Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/&gt;
     &lt;OAEPparams&gt; 9lWu3Q== &lt;/OAEPparams&gt;
  &lt;EncryptionMethod&gt;
</pre>

<p>The <code>CipherValue</code> for an RSA-OAEP encrypted key is the base64
[<a href="#ref-MIME">MIME</a>] encoding of the octet string computed as per
RFC 2437 [<a href="#ref-PKCS1">PKCS1</a>, section 7.1.1: Encryption
operation]. As described in the EME-OAEP-ENCODE function RFC 2437 [<a
href="#ref-PKCS1">PKCS1</a>, section 9.1.1.1], the value input to the key
transport function is calculated using the message digest function and string
specified in the <code>DigestMethod</code> and <code>OAEPparams</code>
elements and using the mask generator function MGF1 specified in RFC 2437.
The desired output length for EME-OAEP-ENCODE is one byte shorter than the
RSA modulus.</p>

<p>The transported key size is 192 bits for TRIPLEDES and 128, 192, or 256
bits for AES. Implementations MUST implement RSA-OAEP for the transport of
128 and 256 bit keys. They MAY implement RSA-OAEP for the transport of other
keys.</p>

<h3>5.5 <a name="sec-Alg-KeyAgreement" id="sec-Alg-KeyAgreement">Key
Agreement</a></h3>

<p>A Key Agreement algorithm provides for the derivation of a shared secret
key based on a shared secret computed from certain types of compatible public
keys from both the sender and the recipient. Information from the originator
to determine the secret is indicated by an optional
<code>OriginatorKeyInfo</code> parameter child of an
<code>AgreementMethod</code> element while that associated with the recipient
is indicated by an optional <code>RecipientKeyInfo</code>. A shared key is
derived from this shared secret by a method determined by the Key Agreement
algorithm.</p>

<p>Note: XML Encryption does not provide an on-line key agreement negotiation
protocol. The <code>AgreementMethod</code> element can be used by the
originator to identify the keys and computational procedure that were used to
obtain a shared encryption key. The method used to obtain or select the keys
or algorithm used for the agreement computation is beyond the scope of this
specification.</p>

<p>The <code>AgreementMethod</code> element appears as the content of a
<code>ds:KeyInfo</code> since, like other <code>ds:KeyInfo</code> children,
it yields a key. This <code>ds:KeyInfo</code> is in turn a child of an
<code>EncryptedData</code> or <code>EncryptedKey</code> element. The
<code>Algorithm</code> attribute and <code>KeySize</code> child of the
<code>EncryptionMethod</code> element under this <code>EncryptedData</code>
or <code>EncryptedKey</code> element are implicit parameters to the key
agreement computation. In cases where this <code>EncryptionMethod</code>
algorithm URI is insufficient to determine the key length, a
<code>KeySize</code> MUST have been included. In addition, the sender may
place a <code>KA-Nonce</code> element under <code>AgreementMethod</code> to
assure that different keying material is generated even for repeated
agreements using the same sender and recipient public keys. For example:</p>
<pre class="xml-example"> &lt;EncryptedData&gt;
   &lt;EncryptionMethod Algorithm="Example:Block/Alg"
     &lt;KeySize&gt;80&lt;/KeySize&gt;
   &lt;/EncryptionMethod&gt;
   &lt;ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#"&gt;
     &lt;AgreementMethod Algorithm="Example:Agreement/Algorithm"&gt;
       &lt;KA-Nonce&gt;Zm9v&lt;/KA-Nonce&gt;
       &lt;DigestMethod
        Algorithm="http://www.w3.org/2001/04/xmlenc#sha1"&gt;
       &lt;OriginatorKeyInfo&gt;
         &lt;ds:KeyValue&gt;...originator...&lt;/ds:KeyValue&gt;
       &lt;/OriginatorKeyInfo&gt;
       &lt;RecipientKeyInfo&gt;
         &lt;ds:KeyValue&gt;...recipient...&lt;/ds:KeyValue&gt;
       &lt;/RecipientKeyInfo&gt;
     &lt;/AgreementMethod&gt;
   &lt;/ds:KeyInfo&gt;
   &lt;CipherData&gt;...&lt;/CipherData&gt;
&lt;/EncryptedData&gt;
</pre>

<p>If the agreed key is being used to wrap a key, rather than data as above,
then <code>AgreementMethod</code> would appear inside a
<code>ds:KeyInfo</code> inside an <code>EncryptedKey</code> element.</p>

<p>The Schema for <code>AgreementMethod</code> is as follows:</p>
<pre class="xml-dtd">  Schema Definition:

  &lt;element name="AgreementMethod"
           type="xenc:AgreementMethodType"/&gt;
  &lt;complexType name="AgreementMethodType" mixed="true"&gt;
    &lt;sequence&gt;
      &lt;element name="KA-Nonce" minOccurs="0" type="base64Binary"/&gt;
      &lt;element ref="ds:DigestMethod" minOccurs="0"/&gt;
      &lt;element name="OriginatorKeyInfo" minOccurs="0"
               type="ds:KeyInfoType"/&gt;
      &lt;element name="RecipientKeyInfo" minOccurs="0"
               type="ds:KeyInfoType"/&gt;
      &lt;any namespace="##other" minOccurs="0"
                               maxOccurs="unbounded"/&gt;
     &lt;/sequence&gt;
   &lt;attribute name="Algorithm" type="anyURI" use="required"/&gt;
   &lt;/complexType&gt;
</pre>

<h4>5.5.1 <a name="sec-DHKeyValue" id="sec-DHKeyValue">Diffie-Hellman Key
Values</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a id="DHKeyValue" href="http://www.w3.org/2001/04/xmlenc#DHKeyValue"
      name="DHKeyValue">http://www.w3.org/2001/04/xmlenc#DHKeyValue</a>
      (OPTIONAL)</dd>
</dl>

<p>Diffie-Hellman keys can appear directly within <code>KeyValue</code>
elements or be obtained by <code>ds:RetrievalMethod</code> fetches as well as
appearing in certificates and the like. The above identifier can be used as
the value of the <code>Type</code> attribute of <code>Reference</code> or
<code>ds:RetrievalMethod</code> elements.</p>

<p>As specified in [<a href="#ref-ESDH">ESDH</a>], a DH public key consists
of up to six quantities, two large primes p and q, a "generator" g, the
public key, and validation parameters "seed" and "pgenCounter". These relate
as follows: The public key = ( g**x mod p ) where x is the corresponding
private key; p = j*q + 1 where j &gt;= 2. "seed" and "pgenCounter" are
optional and can be used to determine if the Diffie-Hellman key has been
generated in conformance with the algorithm specified in [<a
href="#ref-ESDH">ESDH</a>]. Because the primes and generator can be safely
shared over many DH keys, they may be known from the application environment
and are optional. The schema for a <code>DHKeyValue</code> is as follows:</p>
<pre class="xml-dtd">   <code>Schema:

  &lt;element name="DHKeyValue" type="xenc:DHKeyValueType"/&gt;
  &lt;complexType name="DHKeyValueType"&gt;
     &lt;sequence&gt;
        &lt;sequence minOccurs="0"&gt;
           &lt;element name="P" type="ds:CryptoBinary"/&gt;
           &lt;element name="Q" type="ds:CryptoBinary"/&gt;
           &lt;element name="Generator"type="ds:CryptoBinary"/&gt;
        &lt;/sequence&gt;
        &lt;element name="Public" type="ds:CryptoBinary"/&gt;
        &lt;sequence minOccurs="0"&gt;
           &lt;element name="seed" type="ds:CryptoBinary"/&gt;
           &lt;element name="pgenCounter" type="ds:CryptoBinary"/&gt;
        &lt;/sequence&amp;gy;
     &lt;/sequence&gt;
  &lt;/complexType&gt;</code></pre>

<h4>5.5.2 <a name="sec-DHKeyAgreement" id="sec-DHKeyAgreement">Diffie-Hellman
Key Agreement</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a name="dh" id="dh"
      href="http://www.w3.org/2001/04/xmlenc#dh">http://www.w3.org/2001/04/xmlenc#dh</a>
      (OPTIONAL)</dd>
</dl>

<p>The Diffie-Hellman (DH) key agreement protocol [<a
href="#ref-ESDH">ESDH</a>] involves the derivation of shared secret
information based on compatible DH keys from the sender and recipient. Two DH
public keys are compatible if they have the same prime and generator. If, for
the second one, <code>Y = g**y mod p</code>, then the two parties can
calculate the shared secret <code>ZZ = ( g**(x*y) mod p )</code> even though
each knows only their own private key and the other party's public key.
Leading zero bytes MUST be maintained in <code>ZZ</code> so it will be the
same length, in bytes, as <code>p</code>. The size of <code>p</code> MUST be
at least 512 bits and <code>g</code> at least 160 bits. There are numerous
other complex security considerations in the selection of <code>g</code>,
<code>p</code>, and a random <code>x</code> as described in [<a
href="#ref-ESDH">ESDH</a>].</p>

<p>Diffie-Hellman key agreement is optional to implement. An example of a DH
<code>AgreementMethod</code> element is as follows:</p>
<pre class="xml-example">  &lt;AgreementMethod
      Algorithm="http://www.w3.org/2001/04/xmlenc#dh"
      ds:xmlns="http://www.w3.org/2000/09/xmldsig#"&gt;
    &lt;KA-Nonce&gt;Zm9v&lt;/KA-Nonce&gt;
    &lt;ds:DigestMethod
     Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/&gt;
    &lt;OriginatorKeyInfo&gt;
      &lt;ds:X509Data&gt;&lt;ds:X509Certificate&gt;
        ...
      &lt;/ds:X509Certificate&gt;&lt;/ds:X509Data&gt;
    &lt;/OriginatorKeyInfo&gt;
    &lt;RecipientKeyInfo&gt;&lt;ds:KeyValue&gt;
      ...
    &lt;/ds:KeyValue&gt;&lt;/RecipientKeyInfo&gt;
  &lt;/AgreementMethod&gt;
</pre>

<p>Assume the Diffie-Hellman shared secret is the octet sequence
<code>ZZ</code>. The shared keying material needed will then be calculated as
follows:</p>
<pre class="xml-example">  Keying Material = KM(1) | KM(2) | ...
</pre>

<p>where "|" is byte stream concatenation and</p>
<pre class="xml-example">  KM(counter) = DigestAlg ( ZZ | counter | EncryptionAlg |
                KA-Nonce | KeySize )
</pre>
<dl>
  <dt><code>DigestAlg</code></dt>
    <dd>The message digest algorithm specified by the
      <code>DigestMethod</code> child of <code>AgreementMethod</code>.</dd>
  <dt><code>EncryptionAlg</code></dt>
    <dd>The URI of the encryption algorithm, including possible key wrap
      algorithms, in which the derived keying material is to be used
      ("Example:Block/Alg" in the example above), not the URI of the
      agreement algorithm. This is the value of the <code>Algorithm</code>
      attribute of the <code>EncryptionMethod</code> child of the
      <code>EncryptedData</code> or <code>EncryptedKey</code> grandparent of
      <code>AgreementMethod</code>.</dd>
  <dt><code>KA-Nonce</code></dt>
    <dd>The base64 decoding the content of the <code>KA-Nonce</code> child of
      <code>AgreementMethod</code>, if present. If the <code>KA-Nonce</code>
      element is absent, it is null.</dd>
  <dt><code>Counter</code></dt>
    <dd>A one byte counter starting at one and incrementing by one. It is
      expressed as two hex digits where letters A through F are in upper
    case.</dd>
  <dt><code>KeySize</code></dt>
    <dd>The size in bits of the key to be derived from the shared secret as
      the UTF-8 string for the corresponding decimal integer with only digits
      in the string and no leading zeros. For some algorithms the key size is
      inherent in the URI. For others, such as most stream ciphers, it must
      be explicitly provided.</dd>
</dl>

<p>For example, the initial <code>(KM(1))</code> calculation for the
<code>EncryptionMethod</code> of the <a href="#sec-Alg-KeyAgreement">Key
Agreement</a> example (section 5.5) would be as follows, where the binary one
byte counter value of 1 is represented by the two character UTF-8 sequence
<code>01</code>, <code>ZZ</code> is the shared secret, and "<code>foo</code>"
is the base64 decoding of "<code>Zm9v</code>".</p>
<pre class="xml-example">  SHA-1 ( ZZ01Example:Block/Algfoo80 )
</pre>

<p>Assuming that <code>ZZ</code> is <code>0xDEADBEEF</code>, that would be</p>
<pre class="xml-example">  SHA-1 ( 0xDEADBEEF4578616D706C652F416C673031666F6F3830 )
</pre>

<p>whose value is</p>
<pre class="xml-example">  0x2D8CD81DB07FB45231524E339FFACF8079E5162C
</pre>

<p>Each application of <code>DigestAlg</code> for successive values of
<code>Counter</code> will produce some additional number of bytes of keying
material. From the concatenated string of one or more <code>KM</code>'s,
enough leading bytes are taken to meet the need for an actual key and the
remainder discarded. For example, if <code>DigestAlg</code> is SHA1 which
produces 20 octets of hash, then for 128 bit AES the first 16 bytes from
<code>KM(1)</code> would be taken and the remaining 4 bytes discarded. For
256 bit AES, all of <code>KM(1)</code> suffixed with the first 12 bytes of
KM(2) would be taken and the remaining 8 bytes of <code>KM(2)</code>
discarded.</p>

<h3>5.6 <a name="sec-Alg-SymmetricKeyWrap"
id="sec-Alg-SymmetricKeyWrap">Symmetric Key Wrap</a></h3>

<p>Symmetric Key Wrap algorithms are shared secret key encryption algorithms
especially specified for encrypting and decrypting symmetric keys. Their
identifiers appear as <code>Algorithm</code> attribute values to
<code>EncryptionMethod</code> elements that are children of
<code>EncryptedKey</code> which is in turn a child of <code>ds:KeyInfo</code>
which is in turn a child of <code>EncryptedData</code> or another
<code>EncryptedKey</code>. The type of the key being wrapped is indicated by
the <code>Algorithm</code> attribute of <code>EncryptionMethod</code> child
of the parent of the <code>ds:KeyInfo</code> grandparent of the
<code>EncryptionMethod</code> specifying the symmetric key wrap algorithm.</p>

<h4>5.6.1 <a name="sec-CMSKeyChecksum" id="sec-CMSKeyChecksum">CMS Key
Checksum</a></h4>

<p>Some key wrap algorithms make use of the key checksum defined in CMS [<a
href="#ref-CMS-Wrap">CMS-Wrap</a>]. This is used to provide an integrity
check value for the key being wrapped. The algorithm is</p>
<ol>
  <li>Compute the 20 octet SHA-1 hash on the key being wrapped.</li>
  <li>Use the first 8 octets of this hash as the checksum value.</li>
</ol>

<h4>5.6.2 <a name="sec-kw-tripledes" id="sec-kw-tripledes">CMS Triple DES Key
Wrap</a></h4>
<dl>
  <dt>Identifiers and Requirements:</dt>
    <dd><a id="kw-tripledes" name="kw-tripledes"
      href="http://www.w3.org/2001/04/xmlenc#kw-tripledes">http://www.w3.org/2001/04/xmlenc#kw-tripledes</a>
      (REQUIRED)</dd>
</dl>

<p>XML Encryption implementations MUST support TRIPLEDES wrapping of 168 bit
keys and may optionally support TRIPLEDES wrapping of other keys.</p>

<p>An example of a TRIPLEDES Key Wrap <code>EncryptionMethod</code> element
is a as follows:</p>
<pre class="xml-example">  <code>&lt;EncryptionMethod
   Algorithm="http://www.w3.org/2001/04/xmlenc#kw-tripledes"/&gt;</code></pre>

<p>The following algorithm wraps (encrypts) a key (the wrapped key, WK) under
a TRIPLEDES key-encryption-key (KEK) as specified in [<a
href="#ref-CMS-Algorithms">CMS-Algorithms</a>]:</p>
<ol>
  <li>Represent the key being wrapped as an octet sequence. If it is a
    TRIPLEDES key, this is 24 octets (192 bits) with odd parity bit as the
    bottom bit of each octet.</li>
  <li>Compute the <a href="#sec-CMSKeyChecksum">CMS key checksum</a> (section
    5.6.1) call this CKS.</li>
  <li>Let <code>WKCKS = WK || CKS</code>, where || is concatenation.</li>
  <li>Generate 8 random octets [<a href="#ref-RANDOM">RANDOM</a>] and call
    this IV.</li>
  <li>Encrypt WKCKS in CBC mode using KEK as the key and IV as the
    initialization vector. Call the results TEMP1.</li>
  <li>Left <code>TEMP2 = IV || TEMP1</code>.</li>
  <li>Reverse the order of the octets in <code>TEMP2</code> and call the
    result <code>TEMP3</code>.</li>
  <li>Encrypt <code>TEMP3</code> in CBC mode using the <code>KEK</code> and
    an initialization vector of <code>0x4adda22c79e82105</code>. The
    resulting cipher text is the desired result. It is 40 octets long if a
    168 bit key is being wrapped.</li>
</ol>

<p>The following algorithm unwraps (decrypts) a key as specified in [<a
href="#ref-CMS-Algorithms">CMS-Algorithms</a>]:</p>
<ol>
  <li>Check if the length of the cipher text is reasonable given the key
    type. It must be 40 bytes for a 168 bit key and either 32, 40, or 48
    bytes for a 128, 192, or 256 bit key. If the length is not supported or
    inconsistent with the algorithm for which the key is intended, return
    error.</li>
  <li>Decrypt the cipher text with TRIPLEDES in CBC mode using the
    <code>KEK</code> and an initialization vector (IV) of
    <code>0x4adda22c79e82105</code>. Call the output <code>TEMP3</code>.</li>
  <li>Reverse the order of the octets in TEMP3 and call the result
    <code>TEMP2</code>.</li>
  <li>Decompose TEMP2 into IV, the first 8 octets, and <code>TEMP1</code>,
    the remaining octets.</li>
  <li>Decrypt TEMP1 using TRIPLEDES in CBC mode using the <code>KEK</code>
    and the IV found in the previous step. Call the result
  <code>WKCKS</code>.</li>
  <li>Decompose <code>WKCKS</code>. <code>CKS</code> is the last 8 octets and
    <code>WK</code>, the wrapped key, are those octets before the
    <code>CKS</code>.</li>
  <li>Calculate a <a href="#sec-CMSKeyChecksum">CMS key checksum</a> (section
    5.6.1) over the <code>WK</code> and compare with the <code>CKS</code>
    extracted in the above step. If they are not equal, return error.</li>
  <li><code>WK</code> is the wrapped key, now extracted for use in data
    decryption.</li>
</ol>

<h4>5.6.3 <a name="sec-kw-aes" id="sec-kw-aes">AES KeyWrap</a></h4>
<dl>
  <dt>Identifiers and Requirements:</dt>
    <dd><a id="kw-aes128" href="http://www.w3.org/2001/04/xmlenc#kw-aes128"
      name="kw-aes128">http://www.w3.org/2001/04/xmlenc#kw-aes128</a>
      (REQUIRED)</dd>
    <dd><a id="kw-aes192" href="http://www.w3.org/2001/04/xmlenc#kw-aes192"
      name="kw-aes192">http://www.w3.org/2001/04/xmlenc#kw-aes192</a>
      (OPTIONAL)</dd>
    <dd><a id="kw-aes256" href="http://www.w3.org/2001/04/xmlenc#kw-aes256"
      name="kw-aes256">http://www.w3.org/2001/04/xmlenc#kw-aes256</a>
      (REQUIRED)</dd>
</dl>

<p>Implementation of AES key wrap is described below, as suggested by NIST.
It provides for confidentiality and integrity. This algorithm is defined only
for inputs which are a multiple of 64 bits. The information wrapped need not
actually be a key. The algorithm is the same whatever the size of the AES key
used in wrapping, called the key encrypting key or <code>KEK</code>. The
implementation requirements are indicated below.</p>
<dl>
  <dt>128 bit AES Key Encrypting Key</dt>
    <dd>Implementation of wrapping 128 bit keys REQUIRED.<br />
      Wrapping of other key sizes OPTIONAL.</dd>
  <dt>192 bit AES Key Encrypting Key</dt>
    <dd>All support OPTIONAL.</dd>
  <dt>256 bit AES Key Encrypting Key</dt>
    <dd>Implementation of wrapping 256 bit keys REQUIRED.<br />
      Wrapping of other key sizes OPTIONAL.</dd>
</dl>

<p>Assume tha the data to be wrapped consists of <code>N</code> 64-bit data
blocks denoted <code>P(1)</code>, <code>P(2)</code>, <code>P(3)</code> ...
<code>P(N)</code>. The result of wrapping will be <code>N+1</code> 64-bit
blocks denoted <code>C(0)</code>, <code>C(1)</code>, <code>C(2)</code>, ...
<code>C(N)</code>. They key encrypting key is represented by <code>K</code>.
Assume integers <code>i</code>, <code>j</code>, and <code>t</code> and
intermediate 64-bit register <code>A</code>, 128-bit register <code>B</code>,
and array of 64-bit quantities <code>R(1)</code> through
<code>R(N)</code>.</p>

<p>"|" represents concatentation so <code>x|y</code>, where <code>x</code>
and <code>y</code> and 64-bit quantities, is the 128-bit quantity with
<code>x</code> in the most significant bits and <code>y</code> in the least
significant bits. <code>AES(K)enc(x)</code> is the operation of AES
encrypting the 128-bit quantity <code>x</code> under the key <code>K</code>.
<code>AES(K)dec(x)</code> is the corresponding decryption opteration.
<code>XOR(x,y)</code> is the bitwise exclusive or of <code>x</code> and
<code>y</code>. <code>MSB(x)</code> and <code>LSB(y)</code> are the most
significant 64 bits and least significant 64 bits of x and y respectively.</p>

<p>If <code>N</code> is 1, a single AES operation is performed for wrap or
unwrap. If <code>N&gt;1</code>, then <code>6*N</code> AES operations are
performed for warp or unwrap.</p>

<p>The key wrap algorithm is as follows:</p>
<ol>
  <li>If <code>N</code> is <code>1</code>:
    <ul>
      <li><code>B=AES(K)enc(0xA6A6A6A6A6A6A6A6|P(1)</code>)</li>
      <li><code>C(0)=MSB(B)</code></li>
      <li><code>C(1)=LSB(B)</code></li>
    </ul>
    If <code>N&gt;1</code>, perform the following steps:</li>
  <li>Initialize variables:
    <ul>
      <li>Set <code>A</code> to <code>0xA6A6A6A6A6A6A6A6</code></li>
      <li>For<code>i=1</code> to <code>N</code>,<br />
        <code>R(i)=P(i)</code></li>
    </ul>
  </li>
  <li>Calculate intermediate values:
    <ul>
      <li>For<code>j=0</code> to <code>5</code>,
        <ul>
          <li>For <code>i=1</code> to <code>N</code>,<br />
            <code>t= i + j*N</code><br />
            <code>B=AES(K)enc(A|R(i))</code><br />
            <code>A=XOR(t,MSB(B))</code><br />
            <code>R(i)=LSB(B)</code></li>
        </ul>
      </li>
    </ul>
  </li>
  <li>Output the results:
    <ul>
      <li>Set <code>C(0)=A</code></li>
      <li>For <code>i=1</code> to <code>N</code>,<br />
        <code>C(i)=R(i)</code></li>
    </ul>
  </li>
</ol>

<p>The key unwrap algorithm is as follows:</p>
<ol>
  <li>If <code>N</code> is <code>1</code>:
    <ul>
      <li><code>B=AES(K)dec(C(0)|C(1))</code></li>
      <li><code>P(1)=LSB(B)</code></li>
      <li>If <code>MSB(B)</code> is <code>0xA6A6A6A6A6A6A6A6</code>, return
        success. Otherwise, return an integrity check failure error.</li>
    </ul>
    If <code>N</code>&gt;1, perform the following steps:</li>
  <li>Initialize the variables:
    <ul>
      <li><code>A=C(0)</code></li>
      <li>For <code>i=1</code> to <code>N</code>,<br />
        <code>R(i)=C(i)</code></li>
    </ul>
  </li>
  <li>Calculate intermediate values:
    <ul>
      <li>For <code>j=5</code> to <code>0</code>,
        <ul>
          <li>For <code>i=N</code> to <code>1</code>,<br />
            <code>t= i + j*N</code><br />
            <code>B=AES(K)dec(XOR(t,A)|R(i))</code><br />
            <code>A=MSB(B)</code><br />
            <code>R(i)=LSB(B)</code></li>
        </ul>
      </li>
    </ul>
  </li>
  <li>Output the results:
    <ul>
      <li>For <code>i=1</code> to <code>N</code>,<br />
        <code>P(i)=R(i)</code></li>
      <li>If <code>A</code> is <code>0xA6A6A6A6A6A6A6A6</code>, return
        success. Otherwise, return an integrity check failure error.</li>
    </ul>
  </li>
</ol>

<p>For example, wrapping the data
<code>0x00112233445566778899AABBCCDDEEFF</code> with the <code>KEK
0x000102030405060708090A0B0C0D0E0F</code> produces the ciphertext of
<code>0x1FA68B0A8112B447</code>, <code>0xAEF34BD8FB5A7B82</code>,
<code>0x9D3E862371D2CFE5</code>.</p>

<h3>5.7 <a name="sec-Alg-MessageDigest" id="sec-Alg-MessageDigest">Message
Digest</a></h3>

<p>Message digest algorithms can be used in <code>AgreementMethod</code> as
part of the key derivation, within RSA-OAEP encryption as a hash function,
and in connection with the HMAC message authentication code method as
described in [<a href="#ref-XML-DSIG">XML-DSIG</a>].)</p>

<h4>5.7.1 <a name="sec-SHA1" id="sec-SHA1">SHA1</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a
      href="http://www.w3.org/2000/09/xmldsig#sha1">http://www.w3.org/2000/09/xmldsig#sha1</a>
      (REQUIRED)</dd>
</dl>

<p>The SHA-1 algorithm [<a href="#ref-SHA">SHA</a>] takes no explicit
parameters. XML encryption implementations MUST implement SHA-1. An example
of an SHA-1 <code>DigestMethod</code> element is:</p>
<pre class="xml-example">   &lt;DigestMethod
    Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/&gt;
</pre>

<p>A SHA-1 digest is a 160-bit string. The content of the
<code>DigestValue</code> element shall be the base64 encoding of this bit
string viewed as a 20-octet octet stream. For example, the
<code>DigestValue</code> element for the message digest:</p>
<pre class="xml-example">   <code>A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D</code></pre>

<p>from Appendix A of the SHA-1 standard would be:</p>
<pre class="xml-example">   <code>&lt;DigestValue&gt;qZk+NkcGgWq6PiVxeFDCbJzQ2J0=&lt;/DigestValue&gt;</code></pre>

<h4>5.7.2 <a name="sec-SHA256" id="sec-SHA256">SHA256</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a id="sha256" href="http://www.w3.org/2001/04/xmlenc#sha256"
      name="sha256">http://www.w3.org/2001/04/xmlenc#sha256</a>
    (RECOMMENDED)</dd>
</dl>

<p>The SHA-256 algorithm [<a href="#ref-SHA">SHA</a>] takes no explicit
parameters. It is RECOMMENDED that XML encryption implementations implement
SHA-256. An example of an SHA-256 <code>DigestMethod</code> element is:</p>
<pre class="xml-example">   <code>&lt;DigestMethod
    Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/&gt;</code></pre>

<p>A SHA-256 digest is a 256-bit string. The content of the
<code>DigestValue</code> element shall be the base64 encoding of this bit
string viewed as a 32-octet octet stream.</p>

<h4>5.7.3 <a name="sec-SHA512" id="sec-SHA512">SHA512</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a id="sha512" href="http://www.w3.org/2001/04/xmlenc#sha512"
      name="sha512">http://www.w3.org/2001/04/xmlenc#sha512</a>
    (OPTIONAL)</dd>
</dl>

<p>The SHA-512 algorithm [<a href="#ref-SHA">SHA</a>] takes no explicit
parameters. An example of an SHA-512 <code>DigestMethod</code> element is:</p>
<pre class="xml-example">   &lt;DigestMethod
    Algorithm="http://www.w3.org/2001/04/xmlenc#sha512"/&gt;
</pre>

<p>A SHA-512 digest is a 512-bit string. The content of the
<code>DigestValue</code> element shall be the base64 encoding of this bit
string viewed as a 64-octet octet stream.</p>

<h4>5.7.4 <a name="sec-RIPEMD-160" id="sec-RIPEMD-160">RIPEMD-160</a></h4>
<dl>
  <dt>Identifier:</dt>
    <dd><a id="ripemd160" href="http://www.w3.org/2001/04/xmlenc#ripemd160"
      name="ripemd160">http://www.w3.org/2001/04/xmlenc#ripemd160</a>
      (OPTIONAL)</dd>
</dl>

<p>The RIPEMD-160 algorithm [<a href="#ref-RIPEMD-160">RIPEMD-160</a>] takes
no explicit parameters. An example of an RIPEMD-160 <code>DigestMethod</code>
element is:</p>
<pre class="xml-example">   &lt;DigestMethod
    Algorithm="http://www.w3.org/2001/04/xmlenc#ripemd160"/&gt;
</pre>

<p>A RIPEMD-160 digest is a 160-bit string. The content of the
<code>DigestValue</code> element shall be the base64 encoding of this bit
string viewed as a 20-octet octet stream.</p>

<h3>5.8 <a name="sec-Alg-MessageAuthentication"
id="sec-Alg-MessageAuthentication">Message Authentication</a></h3>
<dl>
  <dt>Identifier:</dt>
    <dd><a
      href="http://www.w3.org/2000/09/xmldsig#">http://www.w3.org/2000/09/xmldsig#</a>
      (RECOMMENDED)</dd>
</dl>

<p>XML Signature [<a href="#ref-XML-DSIG">XML-DSIG</a>] is optional to
implement for XML encryption applications. It is the recommended way to
provide key based authentication.</p>

<h3>5.9 <a name="sec-Alg-Canonicalition"
id="sec-Alg-Canonicalition">Canonicalization</a></h3>

<p>A Canonicalization of XML is a method of consistently serializing XML into
an octet stream as is necessary prior to encrypting XML.</p>

<h4><a id="sec-Inclusive-Canonicalization"
name="sec-Inclusive-Canonicalization">5.9.1 Inclusive
Canonicalization</a></h4>
<dl>
  <dt>Identifiers:</dt>
    <dd><a
      href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315">http://www.w3.org/TR/2001/REC-xml-c14n-20010315</a>
      (OPTIONAL)</dd>
    <dd><a
      href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments">http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments</a>
      (OPTIONAL)</dd>
</dl>

<p>Canonical XML [<a href="#ref-XML-C14N">Canon</a>] is a method of
serializing XML which includes the in scope namespace and xml namespace
attribute context from ancestors of the XML being serialized.</p>

<p>If XML is to be encrypted and then later decrypted into a different
environment and it is desired to preserve namespace prefix bindings and the
value of attributes in the "xml" namespace of its original environment, then
the canonical XML with comments version of the XML should be the
serialization that is encrypted.</p>

<h4><a id="sec-Exclusive-Canonicalization"
name="sec-Exclusive-Canonicalization">5.9.2 Exclusive
Canonicalization</a></h4>
<dl>
  <dt>Identifiers:</dt>
    <dd><a
      href="http://www.w3.org/2001/10/xml-exc-c14n#">http://www.w3.org/2001/10/xml-exc-c14n#</a>
      (OPTIONAL)</dd>
    <dd><a
      href="http://www.w3.org/2001/10/xml-exc-c14n#WithComments">http://www.w3.org/2001/10/xml-exc-c14n#WithComments</a>
      (OPTIONAL)</dd>
</dl>

<p>Exclusive XML Canonicalization [<a href="#ref-EXC-C14N">Exclusive</a>]
serializes XML in such a way as to include to the minimum extent practical
the namespace prefix binding and xml namespace attribute context inherited
from ancestor elements.</p>

<p>It is the recommended method where the outer context of a fragment which
was signed and then encrypted may be changed. Otherwise the validation of the
signature over the fragment may fail because the canonicalization by
signature validation may include unnecessary namespaces into the fragment.</p>

<h2>6 <a name="sec-Security" id="sec-Security">Security
Considerations</a></h2>

<h3>6.1 <a name="sec-Sign-with-Encrypt"
id="sec-Sign-with-Encrypt">Relationship to XML Digital Signatures</a></h3>

<p>The application of both encryption and digital signatures over portions of
an XML document can make subsequent decryption and signature verification
difficult. In particular, when verifying a signature one must know whether
the signature was computed over the encrypted or unencrypted form of
elements.</p>

<p>A separate, but important, issue is introducing cryptographic
vulnerabilities when combining digital signatures and encryption over a
common XML element. Hal Finney has suggested that encrypting digitally signed
data, while leaving the digital signature in the clear, may allow plaintext
guessing attacks. This vulnerability can be mitigated by using secure hashes
and the nonces in the text being processed.</p>

<p>In accordance with the requirements document [<a
href="#ref-EncReq">EncReq</a>] the interaction of encryption and signing is
an application issue and out of scope of the specification. However, we make
the following recommendations:</p>
<ol type="1">
  <li>When data is encrypted, any digest or signature over that data should
    be encrypted. This satisfies the first issue in that only those
    signatures that can be seen can be validated. It also addresses the
    possibility of a plaintext guessing vulnerability, though it may not be
    possible to identify (or even know of) all the signatures over a given
    piece of data.</li>
  <li>Employ the "decrypt-except" signature transform [<a
    href="#ref-XML-DSIG-Decrypt">XML-DSIG-Decrypt]</a>. It works as follows:
    during signature transform processing, if you encounter a decrypt
    transform, decrypt all encrypted content in the document except for those
    excepted by an enumerated set of references. This specification will also
    need to address vulnerabilities arising from plaintext guessing attacks
    in a similar way.</li>
</ol>

<p>Additionally, while the following warnings pertain to incorrect inferences
by the user about the authenticity of information encrypted, applications
should discourage user misapprehension by communicating clearly which
information has integrity, or is authenticated, confidential, or
non-repudiable when multiple processes (e.g., signature and encryption) and
algorithms (e.g., symmetric and asymmetric) are used:</p>
<ol>
  <li>When an encrypted envelope contains a signature, the signature does not
    necessarily protect the authenticity or integrity of the ciphertext [<a
    href="#ref-Davis">Davis</a>] .</li>
  <li>While the signature secures plaintext it only covers that which is
    signed, recipients of encrypted messages must not infer integrity or
    authenticity of other unsigned information (e.g., headers) within the
    encrypted envelope, see [<a href="#ref-XML-DSIG">XML-DSIG</a>, <a
    href="http://www.w3.org/TR/xmldsig-core/#sec-Secure">8.1.1 Only What is
    Signed is Secure</a>].</li>
</ol>

<h3>6.2 <a name="sec-InformationRevealed"
id="sec-InformationRevealed">Information Revealed</a></h3>

<p>Where a symmetric key is shared amongst multiple recipients, that
symmetric key should <em>only</em> be used for the data intended for
<em>all</em> recipients; even if one recipient is not directed to information
intended (exclusively) for another in the same symmetric key, the information
might be discovered and decrypted.</p>

<p>Additionally, application designers should be careful not to reveal any
information in parameters or algorithm identifiers (e.g., information in a
URI) that weakens the encryption.</p>

<h3>6.3 <a name="sec-Nonce" id="sec-Nonce">Nonce</a> and IV (Initializaiton
Value or Vector)</h3>

<p>An undesirable characteristic of many encryption algorithms and/or their
modes is that the same plaintext when encrypted with the same key has the
same resulting ciphertext. While this is unsurprising, it invites various
attacks which are mitigated by including an arbitrary and non-repeating
(under a given key) data with the plaintext prior to encryption. In
encryption chaining modes this data is the first to be encrypted and is
consequently called the IV (initalization value or vector).</p>

<p>Different algorithms and modes have further requirements on the
characteristic of this information (e.g., randomness and secrecy) that affect
the features (e.g., confidentiality and integrity) and their resistence to
attack.</p>

<p>Given that XML data is redundant (e.g., Unicode encodings and repeated
tags ) and that attackers may know the data's structure (e.g., DTDs and
schemas) encryption algorithms must be carefully implemented and used in this
regard.</p>

<p>For the Cipher Block Chaining (CBC) mode used by this specification, the
IV must not be reused for any key and should be random, but it need not be
secret. Additionally, under this mode an adversary modifying the IV can make
a known change in the plain text after decryption. This attack can be avoided
by securing the integrity of the plain text data, for example by signing
it.</p>

<h2>7 <a name="sec-Schema" id="sec-Schema">Schema</a> and Valid Examples</h2>
<dl>
  <dt>Schema</dt>
    <dd><a href="xenc-schema.xsd">xenc-schema.xsd</a></dd>
  <dt>Example</dt>
    <dd><a href="enc-example.xml">enc-example.xml</a></dd>
</dl>

<h2>8 <a id="sec-References" name="sec-References">References</a></h2>
<dl>
  <dt><a name="ref-TRIPLEDES" id="ref-TRIPLEDES">TRIPLEDES</a></dt>
    <dd>ANSI X9.52: Triple Data Encryption Algorithm Modes of Operation.
    1998.</dd>
  <dt><a name="ref-AES" id="ref-AES">AES</a></dt>
    <dd><a
      href="http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf">NIST
      FIPS 197: Advanced Encryption Standard (AES)</a>. November 2001.</dd>
    <dd><a
      href="http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf">http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf</a></dd>
  <dt><a name="ref-CMS-AES" id="ref-CMS-AES">CMS-AES</a></dt>
    <dd><a
      href="http://www.ietf.org/internet-drafts/draft-ietf-smime-aes-alg-04.txt">Use
      of the Advanced Encryption Algorithm in CMS</a>. Internet-Draft. J.
      Schaad, R. Housley. July 2001.</dd>
    <dd><a
      href="http://www.ietf.org/internet-drafts/draft-ietf-smime-aes-alg-04.txt">http://www.ietf.org/internet-drafts/draft-ietf-smime-aes-alg-04.txt</a></dd>
  <dt><a name="ref-CMS-Algorithms"
  id="ref-CMS-Algorithms">CMS-Algorithms</a></dt>
    <dd><a
      href="http://www.ietf.org/internet-drafts/draft-ietf-smime-cmsalg-07.txt">Cryptographic
      Message Syntax (CMS) Algorithms</a>. Internet-Draft. R. Housley.
      September 2001.</dd>
    <dd><a
      href="http://www.ietf.org/internet-drafts/draft-ietf-smime-cmsalg-07.txt">http://www.ietf.org/internet-drafts/draft-ietf-smime-cmsalg-07.txt</a></dd>
  <dt><a name="ref-CMS-Wrap" id="ref-CMS-Wrap">CMS-Wrap</a></dt>
    <dd><a
      href="http://www.ietf.org/internet-drafts/draft-ietf-smime-key-wrap-01.txt">Triple-DES
      and RC2 Key Wrapping</a>. Internet Draft. R. Housely. September
    2001.</dd>
    <dd><a
      href="http://www.ietf.org/internet-drafts/draft-ietf-smime-key-wrap-01.txt">http://www.ietf.org/internet-drafts/draft-ietf-smime-key-wrap-01.txt</a></dd>
  <dt><a name="ref-Davis" id="ref-Davis">Davis</a></dt>
    <dd><a
      href="http://www.usenix.org/publications/library/proceedings/usenix01/davis.html">Defective
      Sign &amp; Encrypt in S/MIME, PKCS#7, MOSS, PEM, PGP, and XML.</a> D.
      Davis. USENIX Annual Technical Conference. 2001.</dd>
    <dd><a
      href="http://www.usenix.org/publications/library/proceedings/usenix01/davis.html">http://www.usenix.org/publications/library/proceedings/usenix01/davis.html</a></dd>
  <dt><a name="ref-DES" id="ref-DES">DES</a></dt>
    <dd><a
      href="http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf">NIST
      FIPS 46-3: Data Encryption Standard</a> (DES). October 1999.</dd>
    <dd><a
      href="http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf">http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf</a></dd>
  <dt><a id="ref-DOM" name="ref-DOM">DOM</a></dt>
    <dd><a
      href="http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/">Document
      Object Model (DOM) Level 1 Specification.</a> Recommendation. V.
      Apparao, S. Byrne, M. Champion, S. Isaacs, I. Jacobs, A. Le Hors, G.
      Nicol, J. Robie, R. Sutor, C. Wilson, L. Wood. W3C Recommendation,
      October 1998.</dd>
    <dd><a
      href="http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/">http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/</a></dd>
  <dt><a name="ref-EncReq" id="ref-EncReq">EncReq</a></dt>
    <dd><a
      href="http://www.w3.org/TR/2001/WD-xml-encryption-req-20010420">XML
      Encryption Requirements</a>. W3C Note. J. Reagle. W3C, March 2002.</dd>
    <dd><a
      href="http://www.w3.org/TR/2002/NOTE-xml-encryption-req-20020304">http://www.w3.org/TR/2002/NOTE-xml-encryption-req-20020304</a></dd>
  <dt><a name="ref-ESDH" id="ref-ESDH">ESDH</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2631.txt">RFC 2631:
      Diffie-Hellman Key Agreement Method.</a> Standards Track. E. Rescorla.
      1999.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2631.txt">http://www.ietf.org/rfc/rfc2631.txt</a></dd>
  <dt><a name="ref-EXC-C14N" id="ref-EXC-C14N">Exclusive</a></dt>
    <dd><a
      href="http://www.w3.org/TR/2002/CR-xml-exc-c14n-20020212">Exclusive
      Canonicalization</a>. W3C Candidate Recommendation. February 2002.</dd>
    <dd><a
      href="http://www.w3.org/TR/2002/CR-xml-exc-c14n-20020212">http://www.w3.org/TR/2002/CR-xml-exc-c14n-20020212</a></dd>
  <dt><a name="ref-Glossary" id="ref-Glossary">Glossary</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2828.txt">RFC 2828. Internet
      Security Glossary</a>. Informational. R Shirey. 2000</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2828.txt">http://www.ietf.org/rfc/rfc2828.txt</a></dd>
  <dt><a id="ref-HMAC" name="ref-HMAC">HMAC</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2104.txt">RFC 2104: HMAC:
      Keyed-Hashing for Message Authentication</a>. Informational. H.
      Krawczyk, M. Bellare, R. Canetti. February 1997.H. Krawczyk, M.
      Bellare, R. Canetti. February 1997.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2104.txt">http://www.ietf.org/rfc/rfc2104.txt</a></dd>
  <dt><a id="ref-HTTP" name="ref-HTTP">HTTP</a></dt>
    <dd><a href="http://www.w3.org/Protocols/rfc2616/rfc2616.html">RFC 2616:
      Hypertext Transfer Protocol -- HTTP/1.1.</a> Standards Track. J.
      Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee.
      June 1999.J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T.
      Berners-Lee. June 1999.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2616.txt">http://www.ietf.org/rfc/rfc2616.txt</a></dd>
  <dt><a id="ref-InfoSet" name="ref-InfoSet">InfoSet</a></dt>
    <dd><a href="http://www.w3.org/TR/xml-infoset/">XML Information Set</a>.
      J. Cowan and R. Tobin. W3C Recommendation, October 2001.</dd>
  <dt><a id="ref-KEYWORDS" name="ref-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> Best Current Practice.
      S. Bradner. March 1997.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a></dd>
  <dt><a id="ref-MD5" name="ref-MD5">MD5</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc1321.txt">RFC 1321: The MD5
      Message-Digest Algorithm.</a> Informational. R. Rivest. April 1992.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc1321.txt">http://www.ietf.org/rfc/rfc1321.txt</a></dd>
  <dt><a id="ref-MIME" name="ref-MIME">MIME</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2045.txt">RFC 2045: Multipurpose
      Internet Mail Extensions (MIME) Part One: Format of Internet Message
      Bodies</a>. Standards Track. N. Freed &amp; N. Borenstein. November
      1996.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2045.txt">http://www.ietf.org/rfc/rfc2045.txt</a></dd>
  <dt><a id="ref-NFC" name="ref-NFC">NFC</a></dt>
    <dd><em>TR15, Unicode Normalization Forms.</em> M. Davis, M. Dürst.
      Revision 18: November 1999. <a
      href="http://www.unicode.org/unicode/reports/tr15/tr15-18.html">http://www.unicode.org/unicode/reports/tr15/tr15-18.html</a>.</dd>
  <dt><a id="ref-NFC-Corrigendum"
  name="ref-NFC-Corrigendum">NFC-Corrigendum</a></dt>
    <dd><em>Normalization Corrigendum</em> . The Unicode Consortium. <a
      href="http://www.unicode.org/unicode/uni2errata/Normalization_Corrigendum.html">http://www.unicode.org/unicode/uni2errata/Normalization_Corrigendum.html</a>.</dd>
  <dt><a name="ref-prop1" id="ref-prop1">prop1</a></dt>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Aug/0001.html">XML
      Encryption strawman proposal</a>. E. Simon and B. LaMacchia. Aug 09
      2000.</dd>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Aug/0001.html">http://lists.w3.org/Archives/Public/xml-encryption/2000Aug/0001.html</a></dd>
  <dt><a name="ref-prop2" id="ref-prop2">prop2</a></dt>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Aug/0005.html">Another
      proposal of XML Encryption</a>. T.i Imamura. Aug 14 2000.</dd>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Aug/0005.html">http://lists.w3.org/Archives/Public/xml-encryption/2000Aug/0005.html</a></dd>
  <dt><a name="ref-prop3" id="ref-prop3">prop3</a></dt>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Dec/att-0024/01-XMLEncryption_v01.html">XML
      Encryption Syntax and Processing</a> . B. Dillaway, B. Fox, T. Imamura,
      B. LaMacchia, H. Maruyama, J. Schaad, E. Simon. December 2000.</dd>
    <dd><a
      href="http://lists.w3.org/Archives/Public/xml-encryption/2000Dec/att-0024/01-XMLEncryption_v01.html"></a>http://lists.w3.org/Archives/Public/xml-encryption/2000Dec/att-0024/01-XMLEncryption_v01.html</dd>
  <dt><a id="ref-PKCS1" name="ref-PKCS1">PKCS1</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2437.txt">RFC 2437: PKCS #1: RSA
      Cryptography Specifications Version 2.0.</a> Informational. B. Kaliski,
      J. Staddon. October 1998.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2437.txt">http://www.ietf.org/rfc/rfc2437.txt</a></dd>
  <dt><a name="ref-RANDOM" id="ref-RANDOM">RANDOM</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc1750.txt">RFC 1750: Randomness
      Recommendations for Security</a>. Informational. D. Eastlake, S.
      Crocker, J. Schiller. December 1994.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc1750.txt">http://www.ietf.org/rfc/rfc1750.txt</a></dd>
  <dt><a name="ref-RIPEMD-160" id="ref-RIPEMD-160">RIPEMD-160</a></dt>
    <dd>CryptoBytes, Volume 3, Number 2. <a
      href="ftp://ftp.rsasecurity.com/pub/cryptobytes/crypto3n2.pdf">The
      Cryptographic Hash Function RIPEMD-160</a>. RSA Laboratories. Autumn
      1997.</dd>
    <dd><a
      href="ftp://ftp.rsasecurity.com/pub/cryptobytes/crypto3n2.pdf">ftp://ftp.rsasecurity.com/pub/cryptobytes/crypto3n2.pdf</a></dd>
    <dd><a
      href="http://www.esat.kuleuven.ac.be/~cosicart/pdf/AB-9601/AB-9601.pdf">http://www.esat.kuleuven.ac.be/~cosicart/pdf/AB-9601/AB-9601.pdf</a></dd>
  <dt><a name="ref-SHA" id="ref-SHA">SHA</a></dt>
    <dd><a href="http://www.itl.nist.gov/fipspubs/fip180-1.htm">Secure Hash
      Standard</a>. NIST FIPS 180-1. April 1995.</dd>
    <dd><a
      href="http://www.itl.nist.gov/fipspubs/fip180-1.htm">http://www.itl.nist.gov/fipspubs/fip180-1.htm</a></dd>
    <dd>(Being extended to cover SHA-256 and SHA-512. See <a
      href="http://csrc.nist.gov/encryption/shs/dfips-180-2.pdf">Draft FIPS
      180-2</a>.)</dd>
    <dd>(<a
      href="http://csrc.nist.gov/encryption/shs/dfips-180-2.pdf">http://csrc.nist.gov/encryption/shs/dfips-180-2.pdf</a></dd>
  <dt><a name="ref-UTF-16" id="ref-UTF-16">UTF-16</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2781.txt">RFC 2781: UTF-16, an
      encoding of ISO 10646.</a> Informational. P. Hoffman , F. Yergeau.
      February 2000.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2781.txt">http://www.ietf.org/rfc/rfc2781.txt</a></dd>
  <dt><a id="ref-UTF-8" name="ref-UTF-8">UTF-8</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2279.txt">RFC 2279: UTF-8, a
      transformation format of ISO 10646F.</a> Standards Track. Yergeau.
      January 1998.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2279.txt">http://www.ietf.org/rfc/rfc2279.txt</a></dd>
  <dt><a id="ref-URI" name="ref-URI">URI</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc2396.txt">RFC 2396: Uniform
      Resource Identifiers (URI): Generic Syntax</a>. Standards Track. T.
      Berners-Lee, R. Fielding, L. Masinter. August 1998.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2396.txt">http://www.ietf.org/rfc/rfc2396.txt</a></dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc1738.txt">http://www.ietf.org/rfc/rfc1738.txt</a></dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2141.txt">http://www.ietf.org/rfc/rfc2141.txt</a></dd>
    <dd><a href="http://www.ietf.org/rfc/rfc2611.txt">RFC 2611: URN Namespace
      Definition Mechanisms.</a> Best Current Practices. Daigle, D. van
      Gulik, R. Iannella, P. Falstrom. June 1999.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2611.txt">http://www.ietf.org/rfc/rfc2611.txt</a></dd>
  <dt><a name="ref-X509v3" id="ref-X509v3">X509v3</a></dt>
    <dd>ITU-T Recommendation X.509 version 3 (1997). "Information Technology
      - Open Systems Interconnection - The Directory Authentication
      Framework"  ISO/IEC 9594-8:1997.</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, E. Maler. W3C Recommendation, October 2000.</dd>
  <dt><a id="ref-XML-C14N" name="ref-XML-C14N">XML-C14N</a></dt>
    <dd><a href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315">Canonical
      XML.</a> J. Boyer. W3C Recommendation, March 2001.</dd>
    <dd><a
      href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315">http://www.w3.org/TR/2001/REC-xml-c14n-20010315</a></dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc3076.txt">http://www.ietf.org/rfc/rfc3076.txt</a></dd>
  <dt><a name="ref-XML-DSIG" id="ref-XML-DSIG">XML-DSIG</a></dt>
    <dd><a
      href="http://www.w3.org/TR/2001/PR-xmldsig-core-20010820/">XML-Signature
      Syntax and Processing</a>. D. Eastlake, J. Reagle, and D. Solo. W3C
      Recommendation, February 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 name="ref-XML-DSIG-Decrypt"
  id="ref-XML-DSIG-Decrypt">XML-DSIG-Decrypt</a></dt>
    <dd><a
      href="http://www.w3.org/TR/2001/WD-xmlenc-decrypt-20010626">Decryption
      Transform for XML Signature</a>. T. Imamura and H. Maruyama. W3C
      Candidate Recommendation, March 2002.</dd>
    <dd><a
      href="http://www.w3.org/TR/2002/CR-xmlenc-decrypt-20020304">http://www.w3.org/TR/2002/CR-xmlenc-decrypt-20020304</a></dd>
  <dt><a name="ref-XML-MT" id="ref-XML-MT">XML-MT</a></dt>
    <dd><a href="http://www.ietf.org/rfc/rfc3023.txt">RFC 3023: XML Media
      Types.</a> Informational. M. Murata, S. St.Laurent, D Kohn. January
      2001.</dd>
    <dd><a
      href="http://www.ietf.org/rfc/rfc2376.txt">http://www.ietf.org/rfc/rfc2376.txt</a></dd>
  <dt><a id="ref-XML-NS" name="ref-XML-NS">XML-NS</a></dt>
    <dd><a
      href="http://www.w3.org/TR/1999/REC-xml-names-19990114/">Namespaces in
      XML</a>. T. Bray, D. Hollander, A. Layman. W3C Recommendation, January
      1999.</dd>
    <dd><a
      href="http://www.w3.org/TR/1999/REC-xml-names-19990114/">http://www.w3.org/TR/1999/REC-xml-names-19990114</a></dd>
  <dt><a id="ref-XML-schema" name="ref-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> D. Beech, M. Maloney, N. Mendelsohn. W3C
      Recommendation, May 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/2000/CR-xmlschema-2-20001024/">XML
      Schema Part 2: Datatypes</a>. P. Biron, A. Malhotra. W3C
      Recommendation, May 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>
</dl>
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