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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>Explicit Message Encryption</title>
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<abstract>This specification provides a way to mark encrypted messages so the
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recipient can discover how to decrypt it.</abstract>
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&LEGALNOTICE;
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<number>0380</number>
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<status>Experimental</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>XMPP IM</spec>
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<spec>XEP-0030</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>EME</shortname>
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&linkmauve;
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<revision>
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<version>0.3.0</version>
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<date>2019-04-28</date>
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<initials>lnj</initials>
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<remark>
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<p>Added OMEMO encryption namespace.</p>
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<p>Made XEP experimental again.</p>
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</remark>
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</revision>
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2018-01-25 07:45:08 -05:00
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<revision>
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<version>0.2.0</version>
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<date>2018-01-25</date>
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<initials>XEP Editor (jwi)</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>0.1</version>
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<date>2016-10-26</date>
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<initials>fs</initials>
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<remark>
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<p>Initial published version approved by the XMPP Council.</p>
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</remark>
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</revision>
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2016-08-28 11:05:02 -04:00
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<revision>
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<version>0.0.2</version>
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<date>2016-08-28</date>
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<initials>egp</initials>
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<remark><ul>
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<li>Made the 'name' attribute optional for existing mechanisms.</li>
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<li>Added a remark about the possibility to hide encrypted messages
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following user input.</li>
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<li>Made explicit that this protocol affects any encryption mechanism,
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present or future, not only those listed here.</li>
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<li>Display the namespace of the encryption mechanism in the default
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messages.</li>
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</ul></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>2016-08-14</date>
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<initials>egp</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>In the past few years we have seen a strong interest in end to end
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encryption, with multiple competing mechanisms being defined on top of
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XMPP (e.g., &xep0027;, &xep0364; or &xep0373;). This specification
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addresses the lack of proper discoverability of most of these solutions by
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adding a machine-readable explanation of how a specific message has been
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encrypted.</p>
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<p>In a federated network where no central entity can mandate a particular
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encryption mechanism, it becomes important to allow end users to know that
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a message could not be decrypted (e.g., due to a missing plugin), and to
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never fail to display that a message has been received due to that.</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<p>This document addresses the following requirements:</p>
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<ol>
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<li>Enable a client to mark a message as encrypted.</li>
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<li>Enable a client to determine whether a message was encrypted, no matter
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the encryption mechanism used.</li>
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<li>Enable a client to offer the user a possibility to decrypt a received
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message (depending on the encryption method).</li>
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<li>Enable a client to offer the user a possibility to decrypt subsequently
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received messages.</li>
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</ol>
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<p>This document DOES NOT address the non-message usecases, encrypted
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presence and iq have very different requirements than those defined
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here.</p>
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</section1>
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<section1 topic='Use Cases' anchor='usecases'>
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<section2 topic='Basic Flow' anchor='flow'>
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<p>Romeo, wanting to get Juliet’s attention but not wanting to reveal his
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intentions to the montague.lit nor to the capulet.lit servers, sends an
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encrypted message tagged as OTR, as follows:</p>
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<example caption='Example of tagged message encrypted with OTR'><![CDATA[
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<message to='juliet@capulet.lit/balcony'
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from='romeo@montague.lit/orchard'
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id='secret1'>
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<body>?OTR?v23?...</body>
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<encryption xmlns='urn:xmpp:eme:0'
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namespace='urn:xmpp:otr:0'/>
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</message>
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]]></example>
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<p>Juliet’s client, noticing it does not have any OTR capability, will
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display that the message was encrypted but that it is not able to decrypt
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it instead of displaying the body, for example:</p>
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<div class='example'>
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<p>This message was encrypted with OTR (urn:xmpp:otr:0) and could not be
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decrypted.</p>
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</div>
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<p>Juliet may then communicate to Romeo that she was unable to receive his
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message, through an error, or maybe out of band.</p>
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<p>Romeo, standing firm in his belief that they should not communicate
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without encryption in their world where anyone could be a malicious
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listener, then discovers that one of Juliet’s clients support &xep0373; and
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subsequently starts an encrypted session using that protocol.</p>
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<example caption='Example of tagged message encrypted with OX'><![CDATA[
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<message to='juliet@capulet.lit/balcony'
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from='romeo@montague.lit/orchard'
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id='secret2'>
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<openpgp xmlns='urn:xmpp:openpgp:0'>
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...
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</openpgp>
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<body>This message is encrypted with OpenPGP for XMPP.</body>
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<encryption xmlns='urn:xmpp:eme:0'
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namespace='urn:xmpp:openpgp:0'/>
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</message>
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]]></example>
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<p>Upon receiving this message, Juliet’s current client prompts her to enable
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a plugin, or even do it on its own, possible representations include:</p>
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<div class='example'>
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<p>This message was encrypted with OpenPGP for XMPP
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(urn:xmpp:openpgp:0), <link url="#">click here</link> to enable this
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plugin.</p>
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</div>
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</section2>
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<section2 topic='Protocols Supported' anchor='protocols'>
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<p>Any encryption mechanism using message as a transport is a candidate, and
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MAY have a 'name' attribute to help the receiving client display it to the
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user, in case this client doesn’t understand its namespace yet. A 'name'
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attribute SHOULD NOT be included for the protocols listed herein, and
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SHOULD be ignored by a receiving client:</p>
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2016-08-16 20:41:11 -04:00
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<table>
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<tr>
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<th>Name</th>
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<th>Namespace</th>
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<th>Specification</th>
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</tr>
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<tr>
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<td>OTR</td>
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<td>urn:xmpp:otr:0</td>
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<td>&xep0364;</td>
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</tr>
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<tr>
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<td>Legacy OpenPGP</td>
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<td>jabber:x:encrypted</td>
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<td>&xep0027;</td>
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</tr>
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<tr>
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<td>OpenPGP for XMPP</td>
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<td>urn:xmpp:openpgp:0</td>
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<td>&xep0373;</td>
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</tr>
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<tr>
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<td>OMEMO</td>
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<td>eu.siacs.conversations.axolotl</td>
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<td>&xep0384;</td>
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</tr>
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</table>
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</section2>
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<section2 topic='Determining Support' anchor='disco'>
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<p>If an entity supports the Encrypted Message Extension protocol, it MUST
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report that by including a &xep0030; feature of "urn:xmpp:eme:0" in
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response to disco#info requests:</p>
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<example caption='Client queries for entity features'><![CDATA[
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<iq type='get'
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id='disco1'
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to='juliet@capulet.lit/balcony'
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from='romeo@montague.lit/orchard'>
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<query xmlns='http://jabber.org/protocol/disco#info'/>
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</iq>
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]]></example>
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<example caption='Entity responds with features'><![CDATA[
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<iq type='result'
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id='disco1'
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to='romeo@montague.lit/orchard'
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from='juliet@capulet.lit/balcony'>
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<query xmlns='http://jabber.org/protocol/disco#info'>
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...
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<feature var='urn:xmpp:eme:0'/>
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...
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</query>
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</iq>
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]]></example>
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<p>Support can also be determined via &xep0115;, a.k.a. "caps".</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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<p>Entities MUST report a failure to the user if they cannot decrypt an
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incoming message for any reason, and MAY prompt the user to install or
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enable a plugin to decrypt it.</p>
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<p>Entities SHOULD include a non-encrypted body as possible, since older
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clients not supporting this protocol might otherwise ignore messages sent
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with an unknown encryption, making both the sender frustrated that their
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message did not get an answer, and the recipient frustrated that they never
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saw any message.</p>
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<p>A sender entity MAY include the <encryption/> element even if the
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recipient doesn’t advertise support for it in their disco, or isn’t
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currently connected, since the recipient may be using multiple clients with
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different capabilities.</p>
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<p>A sender entity MAY include a 'name' attribute for any encryption
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mechanism not listed in this specification, to help the receiving entity
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present it to the user, but SHOULD NOT include one for the ones listed
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here.</p>
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<p>A receiving entity MUST NOT use the 'name' attribute if it is present and
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they already have a name associated with it.</p>
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<p>A receiving entity MAY not display anything in case an encrypted message
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has been received, if the user agreed to that behaviour.</p>
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</section1>
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<section1 topic='Internationalization Considerations' anchor='i18n'>
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<p>When a message is marked with an encryption tag and can not be decrypted,
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the body can safely be ignored and a localized message displayed
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instead.</p>
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<p>If an entity includes a 'name' attribute, it should attempt to localise it
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to the best of its abilities for the receiving client.</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>A malicious entity could try to mimick the style of a client’s failure
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message, maybe including a link to a compromised plugin, so a client should
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not make those missing plugin messages look like normal messages.</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document requires no interaction with the Internet Assigned Numbers
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Authority (IANA).</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<section2 topic='Protocol Namespaces' anchor='registrar-ns'>
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<p>This specification defines the following XML namespace:</p>
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<ul>
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<li>'urn:xmpp:eme:0'</li>
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</ul>
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</section2>
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</section1>
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<section1 topic='XML Schema' anchor='schema'>
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<code><![CDATA[
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<?xml version='1.0' encoding='UTF-8'?>
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<xs:schema attributeFormDefault="unqualified"
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elementFormDefault="qualified"
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targetNamespace="urn:xmpp:eme:0"
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xmlns:xs="http://www.w3.org/2001/XMLSchema">
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<xs:annotation>
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<xs:documentation>
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The protocol documented by this schema is defined in
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XEP-xxxx: http://xmpp.org/extensions/xep-xxxx.html
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</xs:documentation>
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</xs:annotation>
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<xs:element name="encryption">
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<xs:complexType>
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<xs:attribute type="xs:string" use="required" name="namespace"/>
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<xs:attribute type="xs:string" use="optional" name="name"/>
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</xs:complexType>
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</xs:element>
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</xs:schema>
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]]></code>
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</section1>
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<section1 topic='Acknowledgements' anchor='ack'>
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<p>Thanks to Mathieu Pasquet for his feedback.</p>
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</section1>
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</xep>
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