xeps/xep-0396.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY secret "&lt;secret/&gt;">
  <!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>Jingle Encrypted Transports - OMEMO</title>
  <abstract>Extension for JET introducing OMEMO End-to-End Encrypted Jingle Transports.</abstract>
  &LEGALNOTICE;
  <number>0396</number>
  <status>Experimental</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XEP-0391</spec>
    <spec>XEP-0234</spec>
    <spec>XEP-0384</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>jet-omemo</shortname>
  <schemaloc>
    <ns>jingle</ns>
    <url>http://xmpp.org/schemas/jingle.xsd</url>
  </schemaloc>
  <schemaloc>
    <ns>jingle:errors</ns>
    <url>http://xmpp.org/schemas/jingle-errors.xsd</url>
  </schemaloc>
  <registry/>
  <discuss>jingle</discuss>
  &paulschaub;
  <revision>
    <version>0.1</version>
    <date>2017-11-29</date>
    <initials>XEP Editor (jwi)</initials>
    <remark><p>Accepted by Council as Expremental XEP</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2017-10-06</date>
    <initials>vv</initials>
    <remark><p>First draft</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>
  <p>&xep0391; can be used to utilize different end-to-end encryption methods to secure Jingle Transports, eg. in the context of &xep0234;. This document aims to extend &xep0391; to allow the use of OMEMO encryption with Jingle transports. To achieve this goal, this protocol extension makes use of OMEMOs <link url='https://xmpp.org/extensions/xep-0384.html#usecases-keysend'>KeyTransportElements</link>.</p>
</section1>
<section1 topic='Mappings' anchor='mappings'>
  <p>Conveniently the OMEMO protocol already provides a way to transport key material to another entity. So called KeyTransportElements are basically normal OMEMO MessageElements, but without a payload, so the contained key can be used for something else (see Section 4.6 of <cite>XEP-0384</cite>). This extension uses the key encrypted in the KeyTransportMessages &lt;key&gt; attribute and initialization vector from the &lt;iv&gt; attribute to secure Jingle Transports. The key corresponds to the <cite>Transport Key</cite> of <cite>XEP-0391</cite>, while the iv corresponds to the <cite>Initialization Vector</cite>. The KeyTransportMessage is the equivalent to the <cite>Envelope Element</cite>. Note that within the Envelope Element, the Transport Key is encrypted with the OMEMO ratchet.</p>
</section1>
<section1 topic='Limitations' anchor='limitations'>
  <p>Unfortunately &xep0384; determines the type of the transported key to be AES-128-GCM-NoPadding, so no other configuration can be used in the context of this extension.</p>
  <p>Since OMEMO deviceIds are not bound to XMPP resources, the initiator MUST encrypt the Transport Key for every device of the recipient.</p>
</section1>
<section1 topic='Key Transport' anchor='transport'>
  <p>In order to transport a key to the responder, the initiator creates a fresh AES-128-GCM-NoPadding Transport Key and Initialization Vector and generates an OMEMO KeyTransportElement from it as described in <cite>XEP-0384</cite>. This is then added as a child of the JET &lt;security&gt; element. The 'cipher' attribute MUST be set to 'aes-128-gcm-nopadding:0' (see the <link url='https://xmpp.org/extensions/xep-0391.html#ciphers'>ciphers</link> section of <cite>XEP-0391</cite>). The value of the 'type' attribute must be set to the namespace of the used version of <cite>XEP-0384</cite> &VNOTE;.</p>
  <p></p>
  <example caption="Romeo initiates an OMEMO encrypted file offer"><![CDATA[
<iq from='romeo@montague.example/dr4hcr0st3lup4c'
    id='nzu25s8'
    to='juliet@capulet.example/yn0cl4bnw0yr3vym'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-initiate'
          initiator='romeo@montague.example/dr4hcr0st3lup4c'
          sid='851ba2'>
    <content creator='initiator' name='a-file-offer' senders='initiator'>
      <description xmlns='urn:xmpp:jingle:apps:file-transfer:5'>
        <file>
          <date>1969-07-21T02:56:15Z</date>
          <desc>This is a test. If this were a real file...</desc>
          <media-type>text/plain</media-type>
          <name>test.txt</name>
          <range/>
          <size>6144</size>
          <hash xmlns='urn:xmpp:hashes:2'
                algo='sha-1'>w0mcJylzCn+AfvuGdqkty2+KP48=</hash>
        </file>
      </description>
      <transport xmlns='urn:xmpp:jingle:transports:s5b:1'
                 mode='tcp'
                 sid='vj3hs98y'>
        <candidate cid='hft54dqy'
                   host='192.168.4.1'
                   jid='romeo@montague.example/dr4hcr0st3lup4c'
                   port='5086'
                   priority='8257636'
                   type='direct'/>
      </transport>
      <security xmlns='urn:xmpp:jingle:jet:0'
                name='a-file-offer'
                cipher='urn:xmpp:ciphers:aes-128-gcm-nopadding'
                type='eu.siacs.conversations.axolotl'>
        <encrypted xmlns='eu.siacs.conversations.axolotl'>
          <header sid='27183'>
            <key rid='31415'>BASE64ENCODED...</key>
            <key prekey="true" rid='12321'>BASE64ENCODED...</key>
            <!-- ... -->
            <iv>BASE64ENCODED...</iv>
          </header>
        </encrypted>
      </security>
    </content>
  </jingle>
</iq>]]></example>
  <p>The recipient decrypts the OMEMO KeyTransportElement to retrieve the Transport Secret. Transport Key and Initialization Vector are later used to encrypt/decrypt data as described in &xep0391;.</p>
</section1>
<section1 topic='Determining Support' anchor='support'>
  <p>To advertise its support for JET-OMEMO, when replying to service discovery information ("disco#info") requests an entity MUST return URNs for any version of this extension, as well as of the JET extension that the entity supports -- e.g., "urn:xmpp:jingle:jet-omemo:0" for this version, or "urn:xmpp:jingle:jet:0" for &xep0391; &VNOTE;.</p>
  <example caption="Service discovery information request"><![CDATA[
<iq from='romeo@montague.example/dr4hcr0st3lup4c'
    id='uw72g176'
    to='juliet@capulet.example/yn0cl4bnw0yr3vym'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>]]></example>
  <example caption="Service discovery information response"><![CDATA[
<iq from='juliet@capulet.example/yn0cl4bnw0yr3vym'
    id='uw72g176'
    to='romeo@montague.example/dr4hcr0st3lup4c'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='urn:xmpp:jingle:jet:0'/>
    <feature var='urn:xmpp:jingle:jet-omemo:0'/>
  </query>
</iq>]]></example>
  <p>In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.</p>
</section1>
</xep>