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  <header>
    <title>Pre-Authenticated Roster Subscription</title>
    <abstract>This document defines a protocol and URI scheme for pre-authenticated roster links that allow a third party to automatically obtain the user's presence subscription. The goal of this is to make onboarding of new XMPP IM contacts as easy as possible.</abstract>
    &LEGALNOTICE;
    <number>0379</number>
    <status>Experimental</status>
    <type>Standards Track</type>
    <sig>Standards</sig>
    <approver>Council</approver>
    <dependencies>
      <spec>XMPP Core</spec>
      <spec>RFC 5122</spec>
      <spec>XEP-0147</spec>
    </dependencies>
    <supersedes/>
    <supersededby/>
    <shortname>pars</shortname>
    <author>
      <firstname>Georg</firstname>
      <surname>Lukas</surname>
      <email>georg@op-co.de</email>
      <jid>georg@yax.im</jid>
    </author>
    <revision>
      <version>0.1.2</version>
      <date>2017-02-16</date>
      <initials>gl</initials>
      <remark><p>Added "Usability Considerations", removed actual XMPP client, some text editing.</p></remark>
    </revision>
    <revision>
      <version>0.1.1</version>
      <date>2017-01-01</date>
      <initials>ssw</initials>
      <remark><p>Minor DTD and formatting fixes.</p></remark>
    </revision>
    <revision>
      <version>0.1.0</version>
      <date>2016-07-20</date>
      <initials>gl (XEP Editor: ssw)</initials>
      <remark><p>First draft.</p></remark>
    </revision>
  </header>
  <section1 topic='Introduction' anchor='intro'>
    <p>Romeo is an active XMPP IM (Instant Messaging) user. He convinces Juliet
      (who doesn't have an XMPP account yet) to install a client and
      register with some server. Now, Romeo only needs to create a mutual
      presence subscription with her, without yet knowing her JID.</p>
    <p>This specification allows Romeo to create an out-of-band link (URI) which,
      when opened in Juilet's (or another user's) client, will:</p>
    <ul>
      <li>Add Romeo to Juliet's roster (with a display name optionally specified by Romeo)</li>
      <li>Add Juliet to Romeo's roster (without a pre-defined display name)</li>
      <li>Establish a mutual presence subscription between Romeo and Juliet</li>
    </ul>
    <p>The perceivable effect is that with a single click, Romeo and Juliet
      become "friends" in terms of XMPP presence subscription.</p>
  </section1>
  <section1 topic='Requirements' anchor='reqs'>
    <p>This specification makes use of XMPP URIs. The basic URI scheme for XMPP
      is defined in &rfc5122; and extended in &xep0147; to support different
      actions like "roster" for roster addition and "subscribe" for presence
      subscription.
    </p>
  </section1>
  <section1 topic='Pre-Authenticated Roster Subscription' anchor='pars'>
    <p>The process of mutual roster addition and subscription involves multiple
      steps:</p>
    <ol>
      <li>Generation of invitation link</li>
      <li>Out-of-band transmission and presentation of the link</li>
      <li>Subcription request to the user by the link receiver (new contact)</li>
      <li>Approval by the user and mutual subscription request</li>
      <li>Approval by the new contact</li>
    </ol>
    <p>The general idea of the protocol and the details of the individual steps
      are outlined in the following.</p>
    <section2 topic='General Idea' anchor='general_idea'>
      <p>As Romeo doesn't know Juliet's JID, he needs to send an out-of-band
        invitation. Later, his client needs to match an incoming subscription
        request to this invitation, so it can perform a secure automatic roster
        addition and subscription approval. This matching is accomplished by
        means of an authentication token, which is generated by Romeo's client,
        added to the invitation link and then carried over into the subscription
        request eventually made by Juliet's client. Romeo's client can then
        compare the token received in a subscription request to the list of
        issued tokens, and automatically approve the subscription.</p>
<code caption='Successful PARS Protocol Flow'><![CDATA[
Romeo          mongatague.net  capulet.net     Juliet
   |              |               |               |
   | Send out-of-band invitation link             |
   | (xmpp:romeo@montague.net?preauth=token)      |
   |- - - - - - - - - - - - - - - - - - - - - - ->|
   |              |               |    roster add |
   |              |               |<--------------|
   |       presence subscription request w/ token |
   |  <presence><preauth token="..."/></presence> |
   |<---------------------------------------------|
   | (token validation check)     |               |
   | presence subscribed          |               |
   |--------------------------------------------->|
   | roster add   |               |               |
   |------------->|               |               |
   | presence subscription request|               |
   |--------------------------------------------->|
   |              |               (auto approval) |
   |              |           presence subscribed |
   |<---------------------------------------------|
]]></code>
    </section2>
    <section2 topic='Generation of Invitation Link' anchor='link_generation'>
      <p>Whenever Romeo wishes to invite somebody to his roster, his client will
        generate an invitation link that contains a new authentication token.
        This document extends the "roster" URI action defined in <cite>XEP-0147</cite> with
        a new key-value parameter named "preauth" to store the generated token.
        Romeo's client will create an <strong>xmpp:</strong> link containing Romeo's JID, the
        "roster" action, the "preauth" parameter with the token value, and
        optionally a "name" parameter with the suggested display name.
      </p>
<example caption='Invitation Link with Roster Action and Authentication Token'><![CDATA[
xmpp:romeo@montague.net?roster;preauth=1tMFqYDdKhfe2pwp;name=Romeo+Montague
]]></example>
      <p>
        If the "preauth" parameter is present, the processing client is supposed
        not only to add the user to the roster, but also to automatically send a
        subscription request containing the authentication token.
      </p>

      <p></p>

      <p><strong>Server-side implementation:</strong> it is
        possible (but out-of-scope for this document), to let the user's server
        handle generation of links as well as automatic approval of qualified
        subscription requests. This requires an additional mechanism to query the
        server for new (and possibly also for pending) invitation links.</p>

    </section2>

    <section2 topic='Out-of-band Transmission and Presentation of the Link' anchor='link_transmission'>
      <p>As Romeo doesn't know Juliet's JID in advance, he needs to use an out-of-band method (like e-mail, QR codes or NFC) to transmit the invitation link to Juliet. While these methods allow transmission of <strong>xmpp:</strong> URIs, there is no mechanism to ensure that Juliet actually has a client installed that can open the URI.</p>
      <p>One way to solve this problem is to present Juliet with a web-based landing page that contains the following elements:</p>
      <ul>
        <li>A short text that this is an XMPP invitation from Romeo.</li>
	<li>A client recommendation (based on the detected web browser/OS) with download links.</li>
        <li>A prominent button that activates the actual <strong>xmpp:</strong> link.</li>
      </ul>
      <p>There are multiple options where such a landing page could be hosted:</p>
      <ol>
        <li><strong>XSF:</strong> a central place would provide a common ground
          for a curated client list and ensure long-term availability. However,
          the operator would be able to collect meta-data and abuse authentication tokens.</li>
        <li><strong>Client developer:</strong> the developer of Romeo's client can
          provide a landing page for invitation requests created with this
          client. This is a feasible short-term solution and allows to recommend
          the same client as used by the link sender. However, it shares the
          privacy objections of the XSF solution and can not guarantee
          long-term availability of the service. If the client development shuts
          down, invitation links created with the client will cease working.</li>
        <li><strong>User's server:</strong> this is the optimal long-term
          solution, as the user's server is already entrusted with the relevant
          meta-data and will exist at least as long as the user's account on that
          server. However, this requires an additional server component to query
          for invitation URIs and a web server hosting the landing page.
          Furthermore, the server operator needs to maintain the list of
          recommended clients.</li>
      </ol>
<example caption='Developer-Hosted Landing Page Generated with Fictitious JuicyXMPP Client'><![CDATA[
https://juicyxmpp.example/i/#romeo@montague.net?preauth=1tMFqYDdKhfe2pwp;name=Romeo+Montague
]]></example>
      <p>A possible screen representation of the landing page would be:</p>
      <div class="example">
        <p><strong><em>Romeo Montague</em> has invited you to chat</strong></p>

        <p><strong><link url='xmpp:romeo@montague.net?roster;preauth=1tMFqYDdKhfe2pwp;name=Romeo%20Montague'>Add "Romeo Montague"</link></strong></p>
        <p>If this link does not work, you need to install and configure
          an XMPP client. Please visit this page again afterwards. Choose one of
          these for your <em>Tomato OS</em>:</p>
        <ul>
          <li><strong>JuicyXMPP</strong> (link to XMPP client)</li>
          <li><strong>VegetableChat</strong> (link to XMPP client)</li>
        </ul>
        <p>Check the <link url='http://xmpp.org/software/clients.html'>full list of XMPP clients</link>.</p>
        <p>No further action is required if you do not know <em>Romeo Montague</em> or do not want to chat with them.</p>
        <p>XMPP is a provider-independent form of instant messaging. That means
          you can pick from many different clients and have a free choice of
          server operators to communicate with <em>Romeo Montague</em>.</p>

      </div>

    </section2>

    <section2 topic='Subcription Request to the User by the Link Receiver (New Contact)' anchor='link_transmission'>
      <p>When Juliet opens the <strong>xmpp:</strong> URI (or the according client-supported
        landing page URI) in her client, the client should perform the usual
        roster addition action, i.e. display a dialog allowing to edit the entry
        or to cancel the process. If Juliet completes the roster addition, the
        client SHOULD also send a subscription request to Romeo. This request
        SHOULD contain a 'preauth' element containing the authentication token
        from the invitation link.
      </p>
<example caption='Subscription Request with Pre-Authenticated Roster Subscription Element'><![CDATA[
<presence to='romeo@montague.net' type='subscribe'>
  <preauth xmlns='urn:xmpp:pars:0' token='1tMFqYDdKhfe2pwp' />
</presence>
]]></example>
      <p>If Juliet's server supports <link
          url='http://xmpp.org/rfcs/rfc6121.html#sub-preapproval'>subscription
          pre-approval</link>, the client SHOULD additionally pre-approve Romeo:
      </p>
<example caption='Juliet Pre-approves Romeo'><![CDATA[
<presence to='romeo@montague.net' type='subscribed' />
]]></example>
      <p>If Juliet's server does not indicate pre-approval support, her client
        SHOULD store Romeo's JID in a local auto-approval whitelist, together
        with an appropriate expiration time.
      </p>
    </section2>

    <section2 topic='Approval by the User and Mutual Subscription Request' anchor='sub_approval'>
      <p>When Romeo's client receives a subscription request containing a
        'preauth' element, it needs to extract the authentication token and
        check if the token is a valid one and was previously issued by the client
        (see security considerations below).</p>
<example caption='Subscription Request With Tokens Received by Romeo'><![CDATA[
<presence to='romeo@montague.net' from='juliet@capulet.net' type='subscribe'>
  <preauth xmlns='urn:xmpp:pars:0' token='1tMFqYDdKhfe2pwp' />
</presence>
]]></example>
      <p>If the token is considered valid, the client SHOULD automatically approve
        the subscription request, add the sender of the subscription request to
        the roster and send a subscription request of its own.</p>
<example caption='Automatic Approval and Response Subscription Request'><![CDATA[
<presence to='juliet@capulet.net' type='subscribed' />
<presence to='juliet@capulet.net' type='subscribe' />
]]></example>

    </section2>

    <section2 topic='Approval by the New Contact' anchor='sub_mutual_approval'>
      <p>If Juliet's server supports pre-approval, it will automatically handle the
        incoming subscription request and issue a roster push. Otherwise, Juliet's
        client will receive the subscription request:</p>
<example caption='Mutual Subscription Request'><![CDATA[
<presence from='romeo@montague.net' to='juliet@capulet.net' type='subscribe' />
]]></example>
      <p>Juliet's client MUST ensure that the sender JID is contained in the
	auto-approval whitelist before automatically approving the request.
	Otherwise, it has to fallback to the normal subscription approval
	process.</p>

    </section2>
  </section1>
  <section1 topic='Business Rules' anchor='rules'>
    <section2 topic='Fallback to Manual Process' anchor='rules_fallback'>
      <p>An implementation of this protocol MUST allow for a "graceful
        degradation" to the manual subscription approval process. If a client
        receives a malformed or unknown 'preauth' token, it MUST ignore it and act
        as if no preauth token was contained.</p>
    </section2>

    <section2 topic='No Expectaion of Immediate Approval' anchor='rules_expectation'>
      <p>When sending a pre-authenticated subscription request, the contact's
        client MUST NOT expect an immediate successful approval. If the user's
        issuing client is currently offline, or if the token has expired, a manual
        approval will be performed. Therefore, the contact's client should use the
        same mechanism as before to indicate an unidirectional subscription.
      </p>
    </section2>
  </section1>
  <section1 topic='Security Considerations' anchor='security'>
    <section2 topic='Token Generation' anchor='security_token'>
      <p>As the authentication token grants automatic addition to
        Romeo's roster and automatic approval of presence subscription,
        the token SHOULD be created with a cryptographically secure random
        number generator <note>See for example <tt><link
            url='https://lwn.net/Articles/606141/'>getrandom(2)</link></tt>,
          <tt><link
            url='https://docs.oracle.com/javase/8/docs/api/java/security/SecureRandom.html'>SecureRandom</link></tt>
          or <tt>/dev/urandom</tt>. More information about the randomness
          requirements for security can be found in &rfc4086;</note> and
        provide sufficient entropy to make brute-force attacks
        infeasible. It is suggested to generate at least 80 bits of
        entropy, and to use an encoding that can be easily encoded as part
        of an URI (e.g. Base-32).</p> <p>It is possible to use a different token
        generation scheme like &saml; or JWT (&rfc7519;).
        In such a case, the issuer must ensure a comparable security level and
        limit token reuse.</p>
    </section2>
    <section2 topic='Checking Token Validity' anchor='security_validity'>
      <p>To limit the potential for abuse, the token SHOULD be limited in as follows:</p>
      <ul>
        <li><strong>Usage:</strong> in the typical scenario, each token may only
          be used once. While it is possible for a client to generate a token for
          multiple uses (like for embedding it in a contact card), the
          conventional manual roster management should be used for public
          invitation links.</li>
        <li><strong>Time:</strong> each token MUST have a limited validity time.
          As the token is transmitted out-of-band, it should provide sufficient
          reaction time, e.g. one week. This time limit also allows the issuer to
          delete expired tokens.</li>
        <li><strong>Identity:</strong> if the JID of the token receiver is known
          in advance, the token sender MUST NOT allow a different JID to redeem
          this token.</li>
      </ul>
      <p>If a token is considered invalid (due to failing any of the above
        conditions, or for other reasons), a client MUST fall back to manual
        roster addition and manual subscription approval.
      </p>
    </section2>
    <section2 topic='Interception of Links' anchor='security_intercept'>
      <p>A Monkey-in-the-Middle attacker who gains access to the invitation link
        can manipulate its fields or redeem the link themselves. However, this is
        true for all communication performed using the chosen medium and is out of
        scope for this document.</p>
      <p>Ideally, Romeo's client should highlight automatically-added roster items
        and provide an easy mechanism to remove them and cancel their
        subscription.</p>
    </section2>
    <section2 topic='Display Names' anchor='security_token'>
      <p>An attacker can lure the user by providing an invitation link with a
        'name' parameter that does not match the JID. Therefore, a client SHOULD
        always display both the JID and the proposed display name when adding a
        roster item.</p>
      <p>When the user's client automatically approves a subscription, it SHOULD
        add the new contact to the roster without a 'name' or with the 'name'
        equal to the JID, to prevent impersonation attacks.</p>
    </section2>
  </section1>
  <section1 topic='Usability Considerations' anchor='usability'>
    <section2 topic='Use of Multiple Clients' anchor='rules_multiclient'>
      <p>If a user is logged in with multiple clients, some of their clients will
        receive a subscription request with an unknown token. In this case, a client
        MAY delay the user notification for a short time, to allow another logged-in
        client to automatically handle the subscription request.</p>
    </section2>

    <section2 topic='Opening the Landing Page in an App' anchor='rules_multiclient'>
      <p>Some mobile device platforms allow an app to register itself as a
        handler for cetain URIs. This allows an XMPP client to register for <strong>xmpp:</strong>
        URIs, but also to redirect handling of cetain HTTP/HTTPS URIs. A mobile
        client SHOULD register for the associated landing page URIs and properly
        handle the contained invitations. For example, the JuicyXMPP client should
        register a handler for <strong>https://juicyxmpp.example/i/*</strong>, and present
        the "Add to roster" dialog if such a link is opened. A client MAY register
        for the landing page URIs of other providers after obtaining the
        operators' approval.
      </p>
    </section2>
    <section2 topic='Invitation Link Volatility' anchor='rules_volatile'>
      <p>By default, Romeo's client should generate personal invitation links
	that can only be redeemed once, and only for a limited time. This fact
	SHOULD be indicated by the client UI to Romeo.</p>
      <p>If a client allows customization of the validity time or the number of
	uses for a given invitation token, it SHOULD provide clear language
	to inidcate that.</p>
    </section2>
    <section2 topic='Tagging of Auto-Added Contacts' anchor='rules_group'>
      <p>When a new contact is added automatically by the client, it SHOULD
	indicate this fact to the user, and allow the user to rename / group
	the contact appropriately. One possible way to achieve this is by
	putting all auto-added contacts into a special roster group, and by
	automatically removing this group on the first manual edit of the
	contact.</p>
      <p>In this case, the roster group should be named by the client according
	to the user's locale settings. However, this approach might lead to
	different clients using different group names, resulting in multiple
	roster groups with the same goal.</p>
    </section2>
  </section1>
  <section1 topic='IANA Considerations' anchor='iana'>
    <p>This document requires no interaction with &IANA;.</p>
  </section1>
  <section1 topic='XMPP Registrar Considerations' anchor='registrar'>
    <p>Include the "urn:xmpp:pars:0" namespace in the registry of protocol
      namespaces. Include "preauth" as an additional key-value parameter to the
      roster query action.</p>
<code caption='New Roster Query Action Parameter'><![CDATA[
<querytype>
  <name>roster</name>
    ...
    <key>
      <name>preauth</name>
      <desc>the token used to obtain an automatic approval from the target JID</desc>
    </key>
</querytype>
]]></code>
  </section1>
  <section1 topic='XML Schema' anchor='schema'>
    <p>REQUIRED for protocol specifications.</p>
  </section1>
</xep>
