<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY signcrypt "&lt;signcrypt/&gt;">
  <!ENTITY sign "&lt;sign/&gt;">
  <!ENTITY crypt "&lt;crypt/&gt;">
  <!ENTITY openpgp "&lt;openpgp/&gt;">
  <!ENTITY payload "&lt;payload/&gt;">
  <!ENTITY rfc3629 "<span class='ref'><link url='http://tools.ietf.org/html/rfc3629'>RFC 3629</link></span> <note>RFC 3629: UTF-8, a transformation format of ISO 10646 &lt;<link url='http://tools.ietf.org/html/rfc3629'>http://tools.ietf.org/html/rfc3629</link>&gt;.</note>" >
  <!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>OpenPGP for XMPP Instant Messaging</title>
  <abstract>Specifies a OpenPGP for XMPP (XEP-0373) profile for the Instant
    Messaging (IM) use case.</abstract>
  &LEGALNOTICE;
  <number>0374</number>
  <status>Experimental</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0030</spec>
    <spec>XEP-0373</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>oxim</shortname>
  &flow;
  <author>
    <firstname>Dominik</firstname>
    <surname>Schürmann</surname>
    <email>dominik@dominikschuermann.de</email>
    <jid>dominik@dominikschuermann.de</jid>
  </author>
  <author>
    <firstname>Vincent</firstname>
    <surname>Breitmoser</surname>
    <email>look@my.amazin.horse</email>
    <jid>valodim@stratum0.org</jid>
  </author>
  <revision>
	<version>0.1.2</version>
	<date>2017-01-20</date>
	<initials>fs</initials>
	<remark><p>Make use of XEP-0380: Explicit Message Encryption.</p></remark>
  </revision>
  <revision>
    <version>0.1.1</version>
    <date>2016-06-04</date>
    <initials>fs</initials>
    <remark><p>Minior editorial fixes.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2016-05-10</date>
    <initials>XEP Editor (ssw)</initials>
    <remark><p>Initial published version approved by the XMPP Council.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2016-03-25</date>
    <initials>fs</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>

  <p>This XMPP extension protocol specifies a profile of &xep0373; for OpenPGP
    secured Instant Messaging (IM).</p>

  <p>Unlike similar XEPs, e.g., OMEMO, this XEP <em>does not</em>
  provide Forward Secrecy (FS), but as an advantage in return, allows
  users to read their archived conversations (respectively their
  encrypted data) later on. Of course, only as long as they still
  possess the according secret key. FS and being able to decrypt
  archived messages are mutually exclusive, i.e., one can not have
  both. The authors therefore consider this XEP complementary to
  similar ones which also provide end-to-end encryption but with a
  different feature set.</p>

</section1>

<section1 topic='OX Instant Messaging Profile' anchor='ox-im-profile'>

    <section2 topic='Discovering Support' anchor='disco'>

    <p>If an entity supports exchanging OpenPGP encrypted and signed
    instant messages over XMPP, i.e., what is specified herein, it MUST
    advertise that fact by announcing a &xep0030; feature of
    'urn:xmpp:openpgp:im:0'. It thus includes this feature in response
    to a service discovery request.</p>

    <example caption="Service Discovery information request"><![CDATA[
<iq type='get'
    from='juliet@example.org/balcony'
    to='romeo@example.org/orchard'
    id='disco1'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>]]></example>
  <example caption="Service Discovery information response"><![CDATA[
<iq type='result'
    from='romeo@example.org/orchard'
    to='juliet@example.org/balcony'
    id='disco1'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    ...
    <feature var='urn:xmpp:openpgp:im:0'/>
    ...
  </query>
</iq>]]></example>

    <p>Because of possible downgrade attacks, users should be given an
    option to force the usage of the protocol defined herein no matter
    if the remote announces support or not.</p>

  </section2>

  <section2 topic='OpenPGP Secured Instant Messaging' anchor='openpgp-secured-im'>

    <p>In order to establish a OpenPGP secured IM communication, IM
    clients first need to determine the public key of their
    interlocutor(s). OpenPGP historically provides public keyservers
    which can be used for key retrieval. Additional there are methods
    to store OpenPGP key information in the Domain Name
    System (DNS). This specification does not restrict the mechanism
    of key discovery and retrieval, but compliant clients MUST support
    the public key announcement as described in <cite>XEP-0373</cite>
    <link url='./xep-0373.html#announcing-discover-pubkey'>§ 4</link>.</p>

    <p>After the required public keys have been discovered, XMPP
    clients engage in an OpenPGP secured IM
    conversation by exchanging &openpgp; extension elements. They MUST
    use the &signcrypt; OpenPGP content element specified in
    <cite>XEP-0373</cite><link url='./xep-0373.html#exchange'>§ 3.1</link>.</p>

    <p>The child elements of the OpenPGP content element's &payload;
    can be seen as stanza extension elements which are encrypted and
    signed. After the &openpgp; element and the including &signcrypt;,
    element was verified, they SHOULD be processed similar as if they
    had been direct extension elements of the stanza. For example,
    direct child elements found in &payload; in the context of IM
    could be:</p>

    <ul>
      <li>Message bodies (&rfc6121; § 5.2.3): &lt;body xmlns='jabber:client'/&gt; </li>
      <li>&xep0085;: &lt;active xmlns='http://jabber.org/protocol/chatstates'/&gt;</li>
      <li>&xep0071;: &lt;html xmlns='http://jabber.org/protocol/xhtml-im'/&gt;</li>
    </ul>

    <p>But just as with stanza extension elements, child elements of
    &payload; can be any extension element. The example above uses
    the &lt;body/&gt; element as defined in RFC 6121. Note that it
    uses 'jabber:client' as namespace, but since the same
    &lt;body/&gt; element is also defined in the 'jabber:server'
    namespace, recipients MUST accept both.</p>

  </section2>

  <section2 topic='OpenPGP Key Handling' anchor='openpgp-key-handling'>

    <section3 topic='Choosing Public Keys' anchor='choose-pubkey'>

      <p>Clients MUST expect multiple public keys to be announced for a single remote entity. In this case all keys MUST be used for encryption.</p>

    </section3>

      <section3 topic='OpenPGP Secret Key Synchronization' anchor='openpgp-secret-key-sync'>

      <p>Clients MAY want to use the mechanism in <cite>XEP-0373 § 5</cite> to
      synchronize their secret key(s) over multiple devices. Thus, they
      should query the user's PEP service for an eventually stored
      encrypted secret key.</p>

    </section3>

  </section2>

</section1>

<section1 topic='Business Rules' anchor='rules'>

  <section2 topic='Always Use &signcrypt;' anchor='signcrypt-im-use-case'>

    <p>Only &signcrypt; MUST be used for the IM
    use case. Encrypted but unsigned messages (&crypt;) do not provide
    an advantage over unencrypted ones since the sender can not be
    verified. As result of this rule, the user interface of
    IM clients implementing the protocol defined herein MUST NOT
    provide an option for the user to select between sign+crypt, sign
    or crypt. This also increases the usability.</p>

  </section2>

  <section2 topic='Provide Hints' anchor='hint-im-use-case'>

    <p>In the IM use case every &MESSAGE; equipped with &openpgp;
    SHOULD include an unencrypted &lt;body/&gt; explaining that the actual
    message is encrypted. Furthermore the message SHOULD contain a
    'store' hint as defined in &xep0334; <link
    url='https://xmpp.org/extensions/xep-0334.html#sect-idp1502160'>§
    4.4</link> and a "this message contains an encrypted body text" hint
	in form of an &lt;encryption/&gt; extension element as specified by
	&xep0380;.</p>

    <example caption='An encrypted and signed message with hints.'><![CDATA[
<message to='juliet@example.org'>
  <body>This message is encrypted using OpenPGP.</body>
  <store xmlns='urn:xmpp:hints'/>
  <encryption xmlns='urn:xmpp:eme:0' namespace='urn:xmpp:openpgp:0'/>
  <openpgp xmlns='urn:xmpp:openpgp:0'>
    BASE64_OPENPGP_MESSAGE_CONTAINING_CONTENT_ELEMENT
  </openpgp>
</message>]]></example>

  </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'>

  <section2 topic='Protocol Namespaces' anchor='registrar-ns'>

    <p>The &REGISTRAR; includes 'urn:xmpp:openpgp:0' in its registry of protocol namespaces (see &NAMESPACES;).</p>

  </section2>

</section1>

<section1 topic='XML Schema' anchor='schema'>

  <p>This XEP does not define a Schema, since it exclusively uses elements from
  <cite>XEP-0373</cite> and other XEPs.</p>

</section1>

<section1 topic='Acknowledgements' anchor='acknowledgements'>
  <p>
    Please refer to the
    <link url='./xep-0373.html#acknowledgements'>Acknowledgements section</link>
    of <cite>XEP-0373</cite> since the two XEPs where designed together.
  </p>
</section1>
</xep>
