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428 lines
22 KiB
XML
428 lines
22 KiB
XML
<?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>Pre-Authenticated Roster Subscription</title>
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<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>
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&LEGALNOTICE;
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<number>0379</number>
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<status>Proposed</status>
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<lastcall>2021-10-20</lastcall>
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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>RFC 5122</spec>
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<spec>XEP-0147</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>pars</shortname>
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<author>
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<firstname>Georg</firstname>
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<surname>Lukas</surname>
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<email>georg@op-co.de</email>
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<jid>georg@yax.im</jid>
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</author>
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<revision>
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<version>0.3.3</version>
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<date>2021-03-04</date>
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<initials>mw</initials>
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<remark><p>Cross-document editorial adjustments for inclusive language.</p></remark>
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</revision>
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<revision>
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<version>0.3.2</version>
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<date>2020-01-14</date>
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<initials>licaon-kter</initials>
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<remark>Fix a bunch of typos.</remark>
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</revision>
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<revision>
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<version>0.3.1</version>
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<date>2018-11-03</date>
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<initials>pep</initials>
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<remark>Fix a bunch of typos, batch-style.</remark>
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</revision>
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<revision>
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<version>0.3.0</version>
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<date>2018-10-01</date>
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<initials>XEP Editor (jsc)</initials>
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<remark>Defer due to lack of activity.</remark>
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</revision>
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<revision>
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<version>0.2.0</version>
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<date>2017-03-06</date>
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<initials>gl</initials>
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<remark><p>Change URI format, editing, add reference to XEP-0401.</p></remark>
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</revision>
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<revision>
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<version>0.1.2</version>
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<date>2017-02-16</date>
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<initials>gl</initials>
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<remark><p>Added "Usability Considerations", removed actual XMPP client, some text editing.</p></remark>
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</revision>
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<revision>
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<version>0.1.1</version>
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<date>2017-01-01</date>
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<initials>ssw</initials>
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<remark><p>Minor DTD and formatting fixes.</p></remark>
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</revision>
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<revision>
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<version>0.1.0</version>
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<date>2016-07-20</date>
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<initials>gl (XEP Editor: ssw)</initials>
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<remark><p>First draft.</p></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>Romeo is an active XMPP IM (Instant Messaging) user. He convinces Juliet
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(who doesn't have an XMPP account yet) to install a client and
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register with some server. Now, Romeo only needs to create a mutual
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presence subscription with her, without yet knowing her JID.</p>
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<p>This specification allows Romeo to create an out-of-band link (URI) which,
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when opened in Juliet's (or another user's) client, will:</p>
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<ul>
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<li>Add Romeo to Juliet's roster (with a display name optionally specified by Romeo)</li>
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<li>Add Juliet to Romeo's roster (without a predefined display name)</li>
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<li>Establish a mutual presence subscription between Romeo and Juliet</li>
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</ul>
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<p>The perceivable effect is that with a single click, Romeo and Juliet
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become "friends" in terms of XMPP presence subscription.</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<p>This specification makes use of XMPP URIs. The basic URI scheme for XMPP
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is defined in &rfc5122; and extended in &xep0147; to support different
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actions like "roster" for roster addition and "subscribe" for presence
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subscription.
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</p>
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</section1>
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<section1 topic='Pre-Authenticated Roster Subscription' anchor='pars'>
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<p>The process of mutual roster addition and subscription involves multiple
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steps:</p>
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<ol>
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<li>Generation of invitation link</li>
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<li>Out-of-band transmission and presentation of the link</li>
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<li>Subscription request to the user by the link receiver (new contact)</li>
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<li>Approval by the user and mutual subscription request</li>
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<li>Approval by the new contact</li>
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</ol>
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<p>The general idea of the protocol and the details of the individual steps
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are outlined in the following.</p>
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<section2 topic='General Idea' anchor='general_idea'>
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<p>As Romeo doesn't know Juliet's JID, he needs to send an out-of-band
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invitation. Later, his client needs to match an incoming subscription
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request to this invitation, so it can perform a secure automatic roster
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addition and subscription approval. This matching is accomplished by
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means of an authentication token, which is generated by Romeo's client,
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added to the invitation link and then carried over into the subscription
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request eventually made by Juliet's client. Romeo's client can then
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compare the token received in a subscription request to the list of
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issued tokens, and automatically approve the subscription.</p>
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<code caption='Successful PARS Protocol Flow'><![CDATA[
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Romeo mongatague.net capulet.net Juliet
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| | | |
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| Send out-of-band invitation link |
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| xmpp:romeo@montague.net?roster;preauth=token |
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|- - - - - - - - - - - - - - - - - - - - - - ->|
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| | | roster add |
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| | |<--------------|
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| presence subscription request w/ token |
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| <presence><preauth token="..."/></presence> |
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|<---------------------------------------------|
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| (token validation check) | |
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| presence subscribed | |
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|--------------------------------------------->|
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| roster add | | |
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|------------->| | |
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| presence subscription request| |
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|--------------------------------------------->|
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| | (auto approval) |
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| | presence subscribed |
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|<---------------------------------------------|
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]]></code>
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</section2>
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<section2 topic='Generation of Invitation Link' anchor='link_generation'>
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<p>Whenever Romeo wishes to invite somebody to his roster, his client will
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generate an invitation link that contains a new authentication token.
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This document extends the "roster" URI action defined in <cite>XEP-0147</cite> with
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a new key-value parameter named "preauth" to store the generated token.
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Romeo's client will create an <strong>xmpp:</strong> link containing Romeo's JID, the
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"roster" action, the "preauth" parameter with the token value, and
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optionally a "name" parameter with the suggested display name.
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</p>
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<example caption='Invitation Link with Roster Action and Authentication Token'><![CDATA[
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xmpp:romeo@montague.net?roster;preauth=1tMFqYDdKhfe2pwp;name=Romeo+Montague
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]]></example>
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<p>
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If the "preauth" parameter is present, the processing client is supposed
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not only to add the user to the roster, but also to automatically send a
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subscription request containing the authentication token.
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</p>
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<p></p>
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<p><strong>Server-side implementation:</strong> it is
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possible (but out-of-scope for this document), to let the user's server
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handle generation of links as well as automatic approval of qualified
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subscription requests. One such approach is documented in &xep0401;.</p>
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</section2>
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<section2 topic='Out-of-band Transmission and Presentation of the Link' anchor='link_transmission'>
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<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>
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<p>One way to solve this problem is to present Juliet with a web-based landing page that contains the following elements:</p>
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<ul>
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<li>A short text that this is an XMPP invitation from Romeo.</li>
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<li>A client recommendation (based on the detected web browser/OS) with download links.</li>
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<li>A prominent button that activates the actual <strong>xmpp:</strong> link.</li>
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</ul>
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<p>There are multiple options where such a landing page could be hosted:</p>
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<ol>
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<li><strong>XSF:</strong> a central place would provide a common ground
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for a curated client list and ensure long-term availability. However,
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the operator would be able to collect metadata and abuse authentication tokens.</li>
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<li><strong>Client developer:</strong> the developer of Romeo's client can
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provide a landing page for invitation requests created with this
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client. This is a feasible short-term solution and allows to recommend
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the same client as used by the link sender. However, it shares the
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privacy objections of the XSF solution and can not guarantee
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long-term availability of the service. If the client development shuts
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down, invitation links created with the client will cease working.</li>
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<li><strong>User's server:</strong> this is the optimal long-term
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solution, as the user's server is already entrusted with the relevant
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metadata and will exist at least as long as the user's account on that
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server. However, this requires an additional server component to query
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for invitation URIs and a web server hosting the landing page.
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Furthermore, the server operator needs to maintain the list of
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recommended clients.</li>
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</ol>
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<example caption='Developer-Hosted Landing Page Generated with Fictitious JuicyXMPP Client'><![CDATA[
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https://juicyxmpp.example/i/#romeo@montague.net?roster;preauth=1tMFqYDdKhfe2pwp;name=Romeo+Montague
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]]></example>
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<p>A possible screen representation of the landing page would be:</p>
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<div class="example">
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<p><strong><em>Romeo Montague</em> has invited you to chat</strong></p>
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<p><strong><link url='xmpp:romeo@montague.net?roster;preauth=1tMFqYDdKhfe2pwp;name=Romeo%20Montague'>Add "Romeo Montague"</link></strong></p>
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<p>If this link does not work, you need to install and configure
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an XMPP client. Please visit this page again afterwards. Choose one of
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these for your <em>Tomato OS</em>:</p>
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<ul>
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<li><strong>JuicyXMPP</strong> (link to XMPP client)</li>
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<li><strong>VegetableChat</strong> (link to XMPP client)</li>
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</ul>
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<p>Check the <link url='http://xmpp.org/software/clients.html'>full list of XMPP clients</link>.</p>
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<p>No further action is required if you do not know <em>Romeo Montague</em> or do not want to chat with them.</p>
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<p>XMPP is a provider-independent form of instant messaging. That means
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you can pick from many different clients and have a free choice of
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server operators to communicate with <em>Romeo Montague</em>.</p>
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</div>
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</section2>
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<section2 topic='Subcription Request to the User by the Link Receiver (New Contact)' anchor='link_transmission'>
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<p>When Juliet opens the <strong>xmpp:</strong> URI (or the according client-supported
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landing page URI) in her client, the client should perform the usual
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roster addition action, i.e. display a dialog allowing to edit the entry
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or to cancel the process. If Juliet completes the roster addition, the
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client SHOULD also send a subscription request to Romeo. This request
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SHOULD contain a 'preauth' element containing the authentication token
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from the invitation link.
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</p>
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<example caption='Subscription Request with Pre-Authenticated Roster Subscription Element'><![CDATA[
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<presence to='romeo@montague.net' type='subscribe'>
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<preauth xmlns='urn:xmpp:pars:0' token='1tMFqYDdKhfe2pwp' />
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</presence>
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]]></example>
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<p>If Juliet's server supports <link
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url='http://xmpp.org/rfcs/rfc6121.html#sub-preapproval'>subscription
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pre-approval</link>, the client SHOULD additionally pre-approve Romeo:
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</p>
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<example caption='Juliet Pre-approves Romeo'><![CDATA[
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<presence to='romeo@montague.net' type='subscribed' />
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]]></example>
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<p>If Juliet's server does not indicate pre-approval support, her client
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SHOULD store Romeo's JID in a local auto-approval list, together
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with an appropriate expiration time.
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</p>
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</section2>
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<section2 topic='Approval by the User and Mutual Subscription Request' anchor='sub_approval'>
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<p>When Romeo's client receives a subscription request containing a
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'preauth' element, it needs to extract the authentication token and
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check if the token is a valid one and was previously issued by the client
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(see security considerations below).</p>
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<example caption='Subscription Request With Tokens Received by Romeo'><![CDATA[
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<presence to='romeo@montague.net' from='juliet@capulet.net' type='subscribe'>
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<preauth xmlns='urn:xmpp:pars:0' token='1tMFqYDdKhfe2pwp' />
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</presence>
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]]></example>
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<p>If the token is considered valid, the client SHOULD automatically approve
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the subscription request, add the sender of the subscription request to
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the roster and send a subscription request of its own.</p>
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<example caption='Automatic Approval and Response Subscription Request'><![CDATA[
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<presence to='juliet@capulet.net' type='subscribed' />
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<presence to='juliet@capulet.net' type='subscribe' />
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]]></example>
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</section2>
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<section2 topic='Approval by the New Contact' anchor='sub_mutual_approval'>
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<p>If Juliet's server supports pre-approval, it will automatically handle the
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incoming subscription request and issue a roster push. Otherwise, Juliet's
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client will receive the subscription request:</p>
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<example caption='Mutual Subscription Request'><![CDATA[
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<presence from='romeo@montague.net' to='juliet@capulet.net' type='subscribe' />
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]]></example>
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<p>Juliet's client MUST ensure that the sender JID is contained in the
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auto-approval list before automatically approving the request.
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Otherwise, it has to fallback to the normal subscription approval
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process.</p>
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</section2>
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</section1>
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<section1 topic='Business Rules' anchor='rules'>
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<section2 topic='Fallback to Manual Process' anchor='rules_fallback'>
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<p>An implementation of this protocol MUST allow for a "graceful
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degradation" to the manual subscription approval process. If a client
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receives a malformed or unknown 'preauth' token, it MUST ignore it and act
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as if no preauth token was contained.</p>
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</section2>
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<section2 topic='No Expectation of Immediate Approval' anchor='rules_expectation'>
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<p>When sending a pre-authenticated subscription request, the contact's
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client MUST NOT expect an immediate successful approval. If the user's
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issuing client is currently offline, or if the token has expired, a manual
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approval will be performed. Therefore, the contact's client should use the
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same mechanism as before to indicate an unidirectional subscription.
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</p>
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</section2>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<section2 topic='Token Generation' anchor='security_token'>
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<p>As the authentication token grants automatic addition to
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Romeo's roster and automatic approval of presence subscription,
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the token SHOULD be created with a cryptographically secure random
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number generator <note>See for example <tt><link
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url='https://lwn.net/Articles/606141/'>getrandom(2)</link></tt>,
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<tt><link
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url='https://docs.oracle.com/javase/8/docs/api/java/security/SecureRandom.html'>SecureRandom</link></tt>
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or <tt>/dev/urandom</tt>. More information about the randomness
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requirements for security can be found in &rfc4086;</note> and
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provide sufficient entropy to make brute-force attacks
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infeasible. It is suggested to generate at least 80 bits of
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entropy, and to use an encoding that can be easily encoded as part
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of an URI (e.g. Base-32).</p> <p>It is possible to use a different token
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generation scheme like &saml; or JWT (&rfc7519;).
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In such a case, the issuer must ensure a comparable security level and
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limit token reuse.</p>
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</section2>
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<section2 topic='Checking Token Validity' anchor='security_validity'>
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<p>To limit the potential for abuse, the token SHOULD be limited in as follows:</p>
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<ul>
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<li><strong>Usage:</strong> in the typical scenario, each token may only
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be used once. While it is possible for a client to generate a token for
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multiple uses (like for embedding it in a contact card), the
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conventional manual roster management should be used for public
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invitation links.</li>
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<li><strong>Time:</strong> each token MUST have a limited validity time.
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As the token is transmitted out-of-band, it should provide sufficient
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reaction time, e.g. one week. This time limit also allows the issuer to
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delete expired tokens.</li>
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<li><strong>Identity:</strong> if the JID of the token receiver is known
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in advance, the token sender MUST NOT allow a different JID to redeem
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this token.</li>
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</ul>
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<p>If a token is considered invalid (due to failing any of the above
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conditions, or for other reasons), a client MUST fall back to manual
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roster addition and manual subscription approval.
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</p>
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</section2>
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<section2 topic='Interception of Links' anchor='security_intercept'>
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<p>A Monkey-in-the-Middle attacker who gains access to the invitation link
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can manipulate its fields or redeem the link themselves. However, this is
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true for all communication performed using the chosen medium and is out of
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scope for this document.</p>
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<p>Ideally, Romeo's client should highlight automatically-added roster items
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and provide an easy mechanism to remove them and cancel their
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subscription.</p>
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</section2>
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<section2 topic='Display Names' anchor='security_display'>
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<p>An attacker can lure the user by providing an invitation link with a
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'name' parameter that does not match the JID. Therefore, a client SHOULD
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always display both the JID and the proposed display name when adding a
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roster item.</p>
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<p>When the user's client automatically approves a subscription, it SHOULD
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add the new contact to the roster without a 'name' or with the 'name'
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equal to the JID, to prevent impersonation attacks.</p>
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</section2>
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</section1>
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<section1 topic='Usability Considerations' anchor='usability'>
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<section2 topic='Use of Multiple Clients' anchor='rules_multiclient'>
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<p>If a user is logged in with multiple clients, some of their clients will
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receive a subscription request with an unknown token. In this case, a client
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MAY delay the user notification for a short time, to allow another logged-in
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client to automatically handle the subscription request.</p>
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</section2>
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<section2 topic='Opening the Landing Page in an App' anchor='rules_multiclient'>
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<p>Some mobile device platforms allow an app to register itself as a
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handler for certain URIs. This allows an XMPP client to register for <strong>xmpp:</strong>
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URIs, but also to redirect handling of certain HTTP/HTTPS URIs. A mobile
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client SHOULD register for the associated landing page URIs and properly
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handle the contained invitations. For example, the JuicyXMPP client should
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register a handler for <strong>https://juicyxmpp.example/i/*</strong>, and present
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the "Add to roster" dialog if such a link is opened. A client MAY register
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for the landing page URIs of other providers after obtaining the
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operators' approval.
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</p>
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</section2>
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<section2 topic='Invitation Link Volatility' anchor='rules_volatile'>
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<p>By default, Romeo's client should generate personal invitation links
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that can only be redeemed once, and only for a limited time. This fact
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SHOULD be indicated by the client UI to Romeo.</p>
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<p>If a client allows customization of the validity time or the number of
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uses for a given invitation token, it SHOULD provide clear language
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to indicate that.</p>
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</section2>
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<section2 topic='Tagging of Auto-Added Contacts' anchor='rules_group'>
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<p>When a new contact is added automatically by the client, it SHOULD
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indicate this fact to the user, and allow the user to rename / group
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the contact appropriately. One possible way to achieve this is by
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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>
|