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    <header>
        <title>Encapsulated Digital Signatures in XMPP</title>
        <abstract>This document provides a technical specification for Encapsulated Digital
            Signatures in the Extensible Messaging and Presence Protocol (XMPP).</abstract>
        &LEGALNOTICE;
        <number>0290</number>
        <status>Deferred</status>
        <type>Standards Track</type>
        <sig>Standards</sig>
        <approver>Council</approver>
        <dependencies>
            <spec>XMPP Core</spec>
            <spec>XEP-0001</spec>
        </dependencies>
        <supersedes/>
        <supersededby/>
        <shortname>N/A</shortname>
        &kdz;
        <revision>
            <version>0.2</version>
            <date>2011-01-28</date>
            <initials>kdz</initials>
            <remark>
                <p>Merge manifest and schema-desc objects.</p>
            </remark>
        </revision>
        <revision>
            <version>0.1</version>
            <date>2011-01-26</date>
            <initials>psa</initials>
            <remark>
                <p>Initial published version.</p>
            </remark>
        </revision>
        <revision>
            <version>0.0.1</version>
            <date>2010-11-29</date>
            <initials>kdz</initials>
            <remark>
                <p>Proto-XEP draft.</p>
            </remark>
        </revision>
    </header>

    <section1 topic="Introduction" anchor="intro">
        <p class="box"><em>This document is one of two proposals for digital signatures in XMPP. It
                is expected that only one of these proposals be progressed beyond Experimental on
                the Standards Track.</em></p>

        <p>This document provides a technical specification for Encapsulated Digital Signatures in
            Extensible Messaging and Presence Protocol (&xmpp;).</p>
        <p>XMPP Digital Signatures may be used to provide signer authentication, data integrity,
            non-repudiation, and other security services.</p>
        <p>This extension is intended to be highly flexible, supporting a wide range of
            applications. The extension not only supports signing by the originator, but by other
            entities which handle XMPP stanzas. Multiple entities may independently sign a
            stanza.</p>
        <p>A signed manifest approach is used to allow selective signing (only select elements may
            be included in the manifest) and to allow flexibility in handling verification
            errors.</p>
        <p>This extension is intended to support <em>optimistic signing</em>.</p>
        <p>This document offers an encapsulated signature approach based upon &w3xmlsig; (XMLDSIG).
            Implementations of this extension not required to fully implement the XML DSIG
            specification, they may implement only the minimal subset necessary to support this
            extension.</p>
        <p>It is noted that a number of object-level XMPP digital signature extensions have been
            specified over the years. These include &rfc3923; (XMPP E2E), &xep0116; (XMPP PGP), and
            &xep0285;. The limited applicability of encapsulating signature approaches in XMPP is
            discussed in &xep0274;.</p>
    </section1>
    <section1 topic="Signing XMPP Stanzas" anchor="stanza">
        <p>The process that a sending agent follows for securing stanzas is very similar regardless
            of the form of stanza (i.e., &lt;iq/&gt;, &lt;message/&gt;, or &lt;presence/&gt;).</p>

        <p>The signer begins with the cleartext version of the &lt;message/&gt; stanza "S":</p>
        <example><![CDATA[
<message from='juliet@capulet.net/balcony'
         id='183ef129'
         to='romeo@montague.net'>
    <thread>8996aef0-061d-012d-347a-549a200771aa</thread>
    <body>Wherefore art thou, Romeo?</body>
</message>
          ]]></example>

        <p>This is modified prior to signing as follows:</p>
        <ul>
            <li>Default attribute values are added. (Namespace declarations are not modified.)</li>
            <li>Each child element of the stanza is augmented by a <tt>id</tt> attribute qualified
                in the <tt>urn:xmpp:dsig:0</tt> namespace. As these attributes are used to identify
                the element within a manifest, they must be sufficient unique.</li>
        </ul>
        <example><![CDATA[
<message from='juliet@capulet.net'
         id='183ef129'
         to='romeo@montague.net'
         type='chat'>
    <thread xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-1">8996aef0-061d-012d-347a-549a200771aa</thread>
    <body xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-2">Wherefore art thou, Romeo?</body>
</message>
          ]]></example>

        <p>The signer builds a stanza description object containing a signer element, the stanza
            element, and a timestamp element. The signer element value is the bare JID of the
            signing entity. When the signing entity is a service entity, the JID may only contain
            service domain. The stanza-desc element is the stanza prepared above with each of its
            child elements replaced by an reference element. The reference element references the
            child element via a "urn:xmpp:dsig:ref:0" URI with an anchor of the value of child
            element's <tt>d:id</tt>. The value of the reference element is the digest value produced
            by the digest method after the specified transforms are applied.</p>
        <example><![CDATA[
<stanza-desc id="stanza-desc" xmlns="urn:xmpp:dsig:0">
    <signer>juliet@capulet.net</signer>
    <Transforms><Transform Algorithm="urn:xmpp:dsig:transform:0"/></Transform>
    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    <message from='juliet@capulet.net'
            id='183ef129'
            to='romeo@montague.net'
            type='chat'>            
        <reference URI='urn:xmpp:dsig:ref:0#xxxx-1'>...</reference>
        <reference URI='urn:xmpp:dsig:ref:0#xxxx-2'>...</reference>
    </message>
    <timestamp>2010-11-11T13:33:00.123Z</timestamp>
</stanza-desc>
]]></example>

        <p>The signer then builds a SignedInfo element.</p>
        <example><![CDATA[
<SignedInfo>
    <CanonicalizationMethod Algorithm="urn:xmpp:dsig:transform:0"/>
    <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/
    <Reference URI="#stanza-desc">
        <Transforms>
            <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
        </Transform>
        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
        <DigestValue>...</DigestValue>
    </Reference>
</SignedInfo>
]]></example>

        <p>And then produces a Signature element:</p>
        <example><![CDATA[
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
    <SignedInfo>
        <CanonicalizationMethod Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
        <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/
        <Reference URI="#stanza-desc">
            <Transforms>
                <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
            </Transform>
            <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
            <DigestValue>...</DigestValue>
        </Reference>
    </SignedInfo>
    <SignatureValue/>
    <Object>
        <stanza-desc id="stanza-desc" xmlns="urn:xmpp:dsig:0">
            <signer>juliet@capulet.net</signer>
            <Transforms><Transform Algorithm="urn:xmpp:dsig:transform:0"/></Transform>
            <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
            <message from='juliet@capulet.net'
                    id='183ef129'
                    to='romeo@montague.net'
                    type='chat'>            
                <reference URI='urn:xmpp:dsig:ref:0#xxxx-1'>...</reference>
                <reference URI='urn:xmpp:dsig:ref:0#xxxx-2'>...</reference>
            </message>
            <timestamp>2010-11-11T13:33:00.123Z</timestamp>
        </stanza-desc>
    </Object>
</Signature>
          ]]></example>

        <p>The signer than computes the SignatureValue element, processing the Signature element as
            a detached signature, and replaces the empty Signature element with it. Finally, the
            signer inserts the Signature element into stanza and forwards the stanza as it normally
            would.</p>
        <example><![CDATA[ 
<message from='juliet@capulet.net'
         id='183ef129'
         to='romeo@montague.net'
         type='chat'>
    <thread xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-1">8996aef0-061d-012d-347a-549a200771aa</thread>
    <body xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-2">Wherefore art thou, Romeo?</body>
    <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
        <SignedInfo>
            <CanonicalizationMethod Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
            <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/
            <Reference URI="#stanza-desc">
                <Transforms>
                    <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/>
                </Transform>
                <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                <DigestValue>...</DigestValue>
            </Reference>
        </SignedInfo>
        <SignatureValue>...</SignatureValue>
        <Object>
            <stanza-desc id="stanza-desc" xmlns="urn:xmpp:dsig:0">
                <signer>juliet@capulet.net</signer>
                <Transforms><Transform Algorithm="urn:xmpp:dsig:transform:0"/></Transform>
                <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                <message from='juliet@capulet.net'
                    id='183ef129'
                    to='romeo@montague.net'
                    type='chat'>            
                    <reference URI='urn:xmpp:dsig:ref:0#xxxx-1'>...</reference>
                    <reference URI='urn:xmpp:dsig:ref:0#xxxx-2'>...</reference>
                </message>
                <timestamp>2010-11-11T13:33:00.123Z</timestamp>
           </stanza-desc>
        </Object>
    </Signature>
 </message>
 ]]></example>
    </section1>

    <section1 topic="Transformation Algorithm" anchor="transform">
        <p>For referenced elements of a stanza, the referenced element is normalized (omitting
            comments), as detailed in &w3canon;, as a document subset of a document containing a
            stream element containing the appropriate jabber:client stanza element containing the
            stanza with the referenced elements. Note that jabber:client stanzas are used when
            constructing this document even when a server is signing a stanza to be sent to another
            server.</p>
        <p>For the stanza in Example 2, the input document would be:</p>
        <example><![CDATA[
<stream:stream xmlns='jabber:client" xmlns:stream='http://etherx.jabber.org/streams'>
    <message from='juliet@capulet.net'
             id='183ef129'
             to='romeo@montague.net'
             type='chat'>
        <thread xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-1">8996aef0-061d-012d-347a-549a200771aa</thread>
        <body xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-2">Wherefore art thou, Romeo?</body>
    </message>
</stream:stream>
          ]]></example>

        <p>The canonical form of element referenced by <tt>urn:xmpp:dsig:ref:0#xxx-1</tt> would
            be:</p>
        <example><![CDATA[<thread xmlns="jabber:client" xmlns:d="urn:xmpp:dsig:0" d:id="xxxx-1">8996aef0-061d-012d-347a-549a200771aa</thread>]]></example>
    </section1>

    <section1 topic="Stanza Element References" anchor="references">
        <p>The URI 'uri:xmpp:dsig:ref:0#xxxx" refers to the child element of the stanza which
            contains the 'uri:xmpp:dsig:0' 'id' attribute with the value "xxxx".</p>
    </section1>

    <section1 topic="Inclusion and Checking of Timestamps" anchor="timestamps">
        <p>Timestamps are included to help prevent replay attacks. All timestamps MUST conform to
            &rfc3339; and be presented as UTC with no offset, always including the seconds and
            fractions of a second to three digits (resulting in a datetime 24 characters in length).
            Absent a local adjustment to the sending agent's perceived time or the underlying clock
            time, the sending agent MUST ensure that the timestamps it sends to the receiver
            increase monotonically (if necessary by incrementing the seconds fraction in the
            timestamp if the clock returns the same time for multiple requests). The following rules
            apply to the receiving application:</p>

        <ul style="symbols">
            <li>It MUST verify that the timestamp received is within five minutes of the current
                time, except as described below for offline messages.</li>
            <li>If the foregoing check fails, the timestamp SHOULD be presented to the receiving
                entity (human or application) marked with descriptive text indicating "old
                timestamp" or "future timestamp" and the receiving entity MAY return a stanza error
                to the sender (except as precluded in the protocol).</li>
        </ul>

        <p>The foregoing timestamp checks assume that the recipient is online when the message is
            received. However, if the recipient is offline then the server will probably store the
            message for delivery when the recipient is next online (offline storage does not apply
            to &lt;iq/&gt; or &lt;presence/&gt; stanzas, only &lt;message/&gt; stanzas). As
            described in &xep0160;, when sending an offline message to the recipient, the server
            SHOULD include delayed delivery data as specified in &xep0203; so that the recipient
            knows that this is an offline message and also knows the original time of receipt at the
            server. In this case, the recipient SHOULD verify that the timestamp received in the
            encrypted message is within five minutes of the time stamped by the recipient's server
            in the &lt;delay/&gt; element.</p>
    </section1>

    <section1 topic="Mandatory-to-Implement Cryptographic Algorithms" anchor="mti">
        <p>All implementations MUST support the following algorithms. Implementations MAY support
            other algorithms as well.</p>
        <ul>
            <li>TBD</li>
        </ul>
    </section1>

    <section1 topic="Certificates" anchor="certs">
        <p>To participate in end-to-end signing using the methods defined in this document, a client
            needs to possess an X.509 certificate. It is expected that many clients will generate
            their own (self-signed) certificates rather than obtain a certificate issued by a
            certification authority (CA). In any case the certificate MUST include an XMPP address
            that is represented using the ASN.1 Object Identifier "id-on-xmppAddr" as specified in
            Section 5.1.1 of RFC 3920bis.</p>
    </section1>

    <section1 topic="Security Considerations" anchor="security">
        <p>TBD.</p>
    </section1>

    <section1 topic="XMPP Registrar Considerations" anchor="reg">
        <section2 topic="XML Namespace Name for Signed Data in XMPP" anchor="ns">
            <p>A URN sub-namespace of signed content for the Extensible Messaging and Presence
                Protocol (XMPP) is defined as follows.</p>
            <dl>
                <di>
                    <dt>URI:</dt>
                    <dd>urn:xmpp:dsig</dd>
                </di>
                <di>
                    <dt>Specification:</dt>
                    <dd>ProtoXEP</dd>
                </di>
                <di>
                    <dt>Description:</dt>
                    <dd>This is an XML namespace name of signed content for the Extensible Messaging
                        and Presence Protocol as defined by ProtoXEP.</dd>
                </di>
                <di>
                    <dt>Registrant Contact:</dt>
                    <dd>XSF</dd>
                </di>
            </dl>
        </section2>
    </section1>
</xep>
