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<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>Encapsulated Digital Signatures in XMPP</title>
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<abstract>This document provides a technical specification for Encapsulated Digital
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Signatures in the Extensible Messaging and Presence Protocol (XMPP).</abstract>
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&LEGALNOTICE;
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<number>0290</number>
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<status>Deferred</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies>
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<spec>XMPP Core</spec>
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<spec>XEP-0001</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>N/A</shortname>
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&kdz;
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<revision>
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<version>0.2</version>
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<date>2011-01-28</date>
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<initials>kdz</initials>
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<remark>
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<p>Merge manifest and schema-desc objects.</p>
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</remark>
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</revision>
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<revision>
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<version>0.1</version>
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<date>2011-01-26</date>
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<initials>psa</initials>
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<remark>
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<p>Initial published version.</p>
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</remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2010-11-29</date>
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<initials>kdz</initials>
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<remark>
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<p>Proto-XEP draft.</p>
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</remark>
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</revision>
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</header>
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<section1 topic="Introduction" anchor="intro">
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<p class="box"><em>This document is one of two proposals for digital signatures in XMPP. It
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is expected that only one of these proposals be progressed beyond Experimental on
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the Standards Track.</em></p>
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2011-01-26 20:52:00 -05:00
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<p>This document provides a technical specification for Encapsulated Digital Signatures in
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Extensible Messaging and Presence Protocol (&xmpp;).</p>
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<p>XMPP Digital Signatures may be used to provide signer authentication, data integrity,
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non-repudiation, and other security services.</p>
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<p>This extension is intended to be highly flexible, supporting a wide range of
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applications. The extension not only supports signing by the originator, but by other
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entities which handle XMPP stanzas. Multiple entities may independently sign a
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stanza.</p>
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<p>A signed manifest approach is used to allow selective signing (only select elements may
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be included in the manifest) and to allow flexibility in handling verification
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errors.</p>
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<p>This extension is intended to support <em>optimistic signing</em>.</p>
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<p>This document offers an encapsulated signature approach based upon &w3xmlsig; (XMLDSIG).
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Implementations of this extension not required to fully implement the XML DSIG
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specification, they may implement only the minimal subset necessary to support this
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extension.</p>
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<p>It is noted that a number of object-level XMPP digital signature extensions have been
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specified over the years. These include &rfc3923; (XMPP E2E), &xep0116; (XMPP PGP), and
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&xep0285;. The limited applicability of encapsulating signature approaches in XMPP is
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discussed in &xep0274;.</p>
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</section1>
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<section1 topic="Signing XMPP Stanzas" anchor="stanza">
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<p>The process that a sending agent follows for securing stanzas is very similar regardless
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of the form of stanza (i.e., <iq/>, <message/>, or <presence/>).</p>
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<p>The signer begins with the cleartext version of the <message/> stanza "S":</p>
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<example><![CDATA[
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<message from='juliet@capulet.net/balcony'
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id='183ef129'
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to='romeo@montague.net'>
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<thread>8996aef0-061d-012d-347a-549a200771aa</thread>
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<body>Wherefore art thou, Romeo?</body>
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</message>
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]]></example>
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<p>This is modified prior to signing as follows:</p>
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<ul>
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<li>Default attribute values are added. (Namespace declarations are not modified.)</li>
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<li>Each child element of the stanza is augmented by a <tt>id</tt> attribute qualified
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in the <tt>urn:xmpp:dsig:0</tt> namespace. As these attributes are used to identify
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the element within a manifest, they must be sufficient unique.</li>
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</ul>
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<example><![CDATA[
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<message from='juliet@capulet.net'
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id='183ef129'
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to='romeo@montague.net'
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type='chat'>
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<thread xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-1">8996aef0-061d-012d-347a-549a200771aa</thread>
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<body xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-2">Wherefore art thou, Romeo?</body>
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</message>
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]]></example>
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<p>The signer builds a stanza description object containing a signer element, the stanza
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element, and a timestamp element. The signer element value is the bare JID of the
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signing entity. When the signing entity is a service entity, the JID may only contain
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service domain. The stanza-desc element is the stanza prepared above with each of its
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child elements replaced by an reference element. The reference element references the
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child element via a "urn:xmpp:dsig:ref:0" URI with an anchor of the value of child
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element's <tt>d:id</tt>. The value of the reference element is the digest value produced
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by the digest method after the specified transforms are applied.</p>
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<example><![CDATA[
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<stanza-desc id="stanza-desc" xmlns="urn:xmpp:dsig:0">
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<signer>juliet@capulet.net</signer>
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<Transforms><Transform Algorithm="urn:xmpp:dsig:transform:0"/></Transform>
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<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
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<message from='juliet@capulet.net'
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id='183ef129'
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to='romeo@montague.net'
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type='chat'>
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<reference URI='urn:xmpp:dsig:ref:0#xxxx-1'>...</reference>
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<reference URI='urn:xmpp:dsig:ref:0#xxxx-2'>...</reference>
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</message>
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<timestamp>2010-11-11T13:33:00.123Z</timestamp>
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</stanza-desc>
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]]></example>
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<p>The signer then builds a SignedInfo element.</p>
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<example><![CDATA[
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<SignedInfo>
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<CanonicalizationMethod Algorithm="urn:xmpp:dsig:transform:0"/>
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<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/
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<Reference URI="#stanza-desc">
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<Transforms>
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<Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
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</Transform>
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<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
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<DigestValue>...</DigestValue>
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</Reference>
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</SignedInfo>
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]]></example>
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<p>And then produces a Signature element:</p>
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<example><![CDATA[
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<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
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<SignedInfo>
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<CanonicalizationMethod Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
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<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/
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<Reference URI="#stanza-desc">
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<Transforms>
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<Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
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</Transform>
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<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
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<DigestValue>...</DigestValue>
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</Reference>
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</SignedInfo>
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<SignatureValue/>
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<Object>
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<stanza-desc id="stanza-desc" xmlns="urn:xmpp:dsig:0">
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<signer>juliet@capulet.net</signer>
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<Transforms><Transform Algorithm="urn:xmpp:dsig:transform:0"/></Transform>
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<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
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<message from='juliet@capulet.net'
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id='183ef129'
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to='romeo@montague.net'
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type='chat'>
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<reference URI='urn:xmpp:dsig:ref:0#xxxx-1'>...</reference>
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<reference URI='urn:xmpp:dsig:ref:0#xxxx-2'>...</reference>
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</message>
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<timestamp>2010-11-11T13:33:00.123Z</timestamp>
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</stanza-desc>
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</Object>
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</Signature>
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]]></example>
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<p>The signer than computes the SignatureValue element, processing the Signature element as
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a detached signature, and replaces the empty Signature element with it. Finally, the
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signer inserts the Signature element into stanza and forwards the stanza as it normally
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would.</p>
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<example><![CDATA[
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<message from='juliet@capulet.net'
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id='183ef129'
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to='romeo@montague.net'
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type='chat'>
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<thread xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-1">8996aef0-061d-012d-347a-549a200771aa</thread>
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<body xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-2">Wherefore art thou, Romeo?</body>
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<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
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<SignedInfo>
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<CanonicalizationMethod Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
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<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/
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<Reference URI="#stanza-desc">
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<Transforms>
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<Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
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</Transform>
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<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
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<DigestValue>...</DigestValue>
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</Reference>
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</SignedInfo>
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<SignatureValue>...</SignatureValue>
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<Object>
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<stanza-desc id="stanza-desc" xmlns="urn:xmpp:dsig:0">
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<signer>juliet@capulet.net</signer>
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<Transforms><Transform Algorithm="urn:xmpp:dsig:transform:0"/></Transform>
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<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
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<message from='juliet@capulet.net'
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id='183ef129'
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to='romeo@montague.net'
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type='chat'>
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<reference URI='urn:xmpp:dsig:ref:0#xxxx-1'>...</reference>
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<reference URI='urn:xmpp:dsig:ref:0#xxxx-2'>...</reference>
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</message>
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<timestamp>2010-11-11T13:33:00.123Z</timestamp>
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</stanza-desc>
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</Object>
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</Signature>
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</message>
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]]></example>
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</section1>
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<section1 topic="Transformation Algorithm" anchor="transform">
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<p>For referenced elements of a stanza, the referenced element is normalized (omitting
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comments), as detailed in &w3canon;, as a document subset of a document containing a
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stream element containing the appropriate jabber:client stanza element containing the
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stanza with the referenced elements. Note that jabber:client stanzas are used when
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constructing this document even when a server is signing a stanza to be sent to another
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server.</p>
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<p>For the stanza in Example 2, the input document would be:</p>
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<example><![CDATA[
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<stream:stream xmlns='jabber:client" xmlns:stream='http://etherx.jabber.org/streams'>
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<message from='juliet@capulet.net'
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id='183ef129'
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to='romeo@montague.net'
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type='chat'>
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<thread xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-1">8996aef0-061d-012d-347a-549a200771aa</thread>
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<body xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-2">Wherefore art thou, Romeo?</body>
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</message>
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</stream:stream>
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]]></example>
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<p>The canonical form of element referenced by <tt>urn:xmpp:dsig:ref:0#xxx-1</tt> would
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be:</p>
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<example><![CDATA[<thread xmlns="jabber:client" xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-1">8996aef0-061d-012d-347a-549a200771aa</thread>]]></example>
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</section1>
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<section1 topic="Stanza Element References" anchor="references">
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<p>The URI 'uri:xmpp:dsig:ref:0#xxxx" refers to the child element of the stanza which
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contains the 'uri:xmpp:dsig:0' 'id' attribute with the value "xxxx".</p>
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</section1>
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<section1 topic="Inclusion and Checking of Timestamps" anchor="timestamps">
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<p>Timestamps are included to help prevent replay attacks. All timestamps MUST conform to
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&rfc3339; and be presented as UTC with no offset, always including the seconds and
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fractions of a second to three digits (resulting in a datetime 24 characters in length).
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Absent a local adjustment to the sending agent's perceived time or the underlying clock
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time, the sending agent MUST ensure that the timestamps it sends to the receiver
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increase monotonically (if necessary by incrementing the seconds fraction in the
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timestamp if the clock returns the same time for multiple requests). The following rules
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apply to the receiving application:</p>
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<ul style="symbols">
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<li>It MUST verify that the timestamp received is within five minutes of the current
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time, except as described below for offline messages.</li>
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<li>If the foregoing check fails, the timestamp SHOULD be presented to the receiving
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entity (human or application) marked with descriptive text indicating "old
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timestamp" or "future timestamp" and the receiving entity MAY return a stanza error
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to the sender (except as precluded in the protocol).</li>
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</ul>
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<p>The foregoing timestamp checks assume that the recipient is online when the message is
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received. However, if the recipient is offline then the server will probably store the
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message for delivery when the recipient is next online (offline storage does not apply
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to <iq/> or <presence/> stanzas, only <message/> stanzas). As
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described in &xep0160;, when sending an offline message to the recipient, the server
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SHOULD include delayed delivery data as specified in &xep0203; so that the recipient
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knows that this is an offline message and also knows the original time of receipt at the
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server. In this case, the recipient SHOULD verify that the timestamp received in the
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encrypted message is within five minutes of the time stamped by the recipient's server
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in the <delay/> element.</p>
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</section1>
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<section1 topic="Mandatory-to-Implement Cryptographic Algorithms" anchor="mti">
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<p>All implementations MUST support the following algorithms. Implementations MAY support
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other algorithms as well.</p>
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<ul>
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<li>TBD</li>
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</ul>
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</section1>
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<section1 topic="Certificates" anchor="certs">
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<p>To participate in end-to-end signing using the methods defined in this document, a client
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needs to possess an X.509 certificate. It is expected that many clients will generate
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their own (self-signed) certificates rather than obtain a certificate issued by a
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certification authority (CA). In any case the certificate MUST include an XMPP address
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that is represented using the ASN.1 Object Identifier "id-on-xmppAddr" as specified in
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Section 5.1.1 of RFC 3920bis.</p>
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</section1>
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<section1 topic="Security Considerations" anchor="security">
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<p>TBD.</p>
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</section1>
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<section1 topic="XMPP Registrar Considerations" anchor="reg">
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<section2 topic="XML Namespace Name for Signed Data in XMPP" anchor="ns">
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<p>A URN sub-namespace of signed content for the Extensible Messaging and Presence
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|
Protocol (XMPP) is defined as follows.</p>
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<dl>
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<di>
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|
<dt>URI:</dt>
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|
<dd>urn:xmpp:dsig</dd>
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</di>
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<di>
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<dt>Specification:</dt>
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|
<dd>ProtoXEP</dd>
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|
</di>
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<di>
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<dt>Description:</dt>
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|
<dd>This is an XML namespace name of signed content for the Extensible Messaging
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|
and Presence Protocol as defined by ProtoXEP.</dd>
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|
</di>
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<di>
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|
<dt>Registrant Contact:</dt>
|
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|
|
<dd>XSF</dd>
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|
</di>
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|
</dl>
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</section2>
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</section1>
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</xep>
|
