xeps/xep-0364.xml

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  <header>
    <title>Current Off-the-Record Messaging Usage</title>
    <abstract>
      This document outlines the current usage of Off-the-Record messaging in
      XMPP, its drawbacks, its strengths, and recommendations for improving the
      end user experience.
    </abstract>
    &LEGALNOTICE;
    <number>0364</number>
    <status>Experimental</status>
    <type>Informational</type>
    <sig>Standards</sig>
    <approver>Council</approver>
    <dependencies>
      <spec>XMPP Core</spec>
    </dependencies>
    <supersedes/>
    <supersededby/>
    <shortname>NOT_YET_ASSIGNED</shortname>
    <author>
      <firstname>Sam</firstname>
      <surname>Whited</surname>
      <email>sam@samwhited.com</email>
      <jid>sam@samwhited.com</jid>
    </author>
    <revision>
      <version>0.2</version>
      <date>2016-04-24</date>
      <initials>ssw</initials>
      <remark>
        <p>
          Remove RFC 2119 language other than [NOT] RECOMMENDED, add session
          ending recommendations, add delivery receipt recommendation.
        </p>
      </remark>
    </revision>
    <revision>
      <version>0.1</version>
      <date>2015-08-27</date>
      <initials>XEP Editor (mam)</initials>
      <remark><p>Initial published version approved by the XMPP Council.</p></remark>
    </revision>
    <revision>
      <version>0.0.1</version>
      <date>2015-07-28</date>
      <initials>ssw</initials>
      <remark><p>Initial draft.</p></remark>
    </revision>
  </header>
  <section1 topic='Introduction' anchor='intro'>
    <p>
      The Off-the-Record messaging protocol (OTR) was originally introduced in
      the 2004 paper
      <i><link url='https://otr.cypherpunks.ca/otr-wpes.pdf'>
          Off-the-Record Communication, or, Why Not To Use PGP
      </link></i>
      <note>
        Nikita Borisov, Ian Goldberg, Eric Brewer (2004-10-28). "Off-the-Record
        Communication, or, Why Not To Use PGP"
        &lt;<link url='https://otr.cypherpunks.ca/otr-wpes.pdf'>
          https://otr.cypherpunks.ca/otr-wpes.pdf
        </link>&gt;
      </note>
      and has since become the de facto standard for performing end-to-end
      encryption in XMPP. OTR provides encryption, deniable authentication,
      forward secrecy, and malleable encryption.
    </p>
    <p>
      The OTR protocol itself is currently described by the document:
      <i><link url='https://otr.cypherpunks.ca/Protocol-v3-4.0.0.html'>
          Off-the-Record Messaging Protocol version 3
      </link></i>
      <note>
        "Off-the-Record Messaging Protocol version 3"
        &lt;<link url='https://otr.cypherpunks.ca/Protocol-v3-4.0.0.html'>
          https://otr.cypherpunks.ca/Protocol-v3-4.0.0.html
        </link>&gt;
      </note>
      and will not be redescribed here. Instead, this document aims to describe
      OTR's usage and best practices within XMPP. It is not intended to be a
      current standard, or technical specification, as better (albeit, newer and
      less well tested) methods of end-to-end encryption exist for XMPP.
    </p>
  </section1>
  <section1 topic='Overview' anchor='overview'>
    <p>
      Though this document will not focus on the OTR protocol itself, a brief
      overview is warranted to better understand the protocols strengths and
      weaknesses.
    </p>
    <p>
      OTR uses 128 bit AES symmetric-key encryption and the SHA-1 hash function.
      An OTR session can be held only between two parties, meaning that OTR is
      incompatible with &xep0045; and &xep0369;. It provides deniability in the
      form of malleable encryption (a third party may generate fake messages
      after the session has ended). This means that if you were not a part of
      the original conversation, you cannot prove, based on captured messages
      alone, that a message from the conversation was actually sent by a given
      party.  Unlike PGP, OTR also provides forward secrecy; even if a session
      is recorded and the primary key is compromised at a later date, the OTR
      messages will not be able to be decrypted as each was encrypted with an
      ephemeral key exchanged via Diffie-Hellman key exchange with a 1536 bit
      modulus.
    </p>
  </section1>
  <section1 topic='Discovery'>
    <p>
      Clients that support the OTR protocol do not advertise it in any of the
      normal XMPP ways. Instead, OTR provides its own discovery mechanism. If a
      client wishes to indicate support for OTR they include a special
      whitespace tag in their messages. This tag can appear anywhere in the body
      of the message stanza, but it is most often found at the end. The OTR tag
      comprises the following bytes:

      <example caption='OTR tag'>
\x20\x09\x20\x20\x09\x09\x09\x09 \x20\x09\x20\x09\x20\x09\x20\x20
      </example>

      and is followed by one or more of the following sequences to indicate the
      version of OTR which the client supports:

      <example caption='OTR tag version 1'>
\x20\x09\x20\x09\x20\x20\x09\x20
      </example>

      Note that this version 1 tag must come before other version tags for
      compatibility; it is, however, NOT RECOMMENDED to implement version 1 of
      the OTR protocol.

      <example caption='OTR tag version 2'>
\x20\x20\x09\x09\x20\x20\x09\x20
      </example>

      <example caption='OTR tag version 3'>
\x20\x20\x09\x09\x20\x20\x09\x09
      </example>
    </p>
    <p>
      When a client sees this special string in the body of a message stanza it
      may choose to start an OTR session immediately, or merely indicate support
      to the user and allow the user to manually start a session. This is done
      by sending a message stanza containing an OTR query message in the body
      which indicates the supported versions of OTR. In XMPP these are most
      commonly version 2 and version 3, which would be indicated by a message
      stanza which has a body that starts with the string:

      <example caption='OTR query'>
?OTR?v23?
      </example>
    </p>
    <p>
      Any message which begins with the afforementioned string (note that the
      version number[s] may be different), postfixed with a payload should be
      decrypted as an OTR message. The initialization message should not contain
      a payload, and should just be the initialization string by itself.
    </p>
  </section1>
  <section1 topic='OTR Messages'>
    <section2 topic='Construction and Decoding'>
      <p>
        Some clients in the wild have been known to insert XML in the
        &lt;body&gt; node of a message. Clients that support OTR should tolerate
        encrypted payloads which expand to unescaped XML, and treat it as plain
        text.
      </p>
    </section2>
    <section2 topic='Routing'>
      <p>
        XMPP is designed so that the client needs to know very little about
        where and how a message will be routed. Generally, clients are
        encouraged to send messages to the bare JID and allow the server to
        route the messages as it sees fit. However, OTR requires that messages
        be sent to a particular resource. Therefore clients should send OTR
        messages to a full JID, possibly allowing the user to determine which
        resource they wish to start an encrypted session with. Furthermore, if a
        client receives a request to start an OTR session in a carboned message
        (due to a server which does not support the aforementioned "private"
        directive, or a client which does not set it), it should be silently
        ignored.
      </p>
    </section2>
    <section2 topic='Processing Hints'>
      <p>
        &xep0334; defines a set of hints for how messages should be handled by
        XMPP servers. These hints are not hard and fast rules, but suggestions
        which the servers may or may not choose to follow. Best practice is to
        include the following hints on all OTR messages:

        <code><![CDATA[
<no-copy xmlns="urn:xmpp:hints"/>
<no-permanent-store xmlns="urn:xmpp:hints"/>
          ]]></code>
      </p>

      <p>
        Similarly the "private" directive from &xep0280; should also be included
        to indicate that carbons are not necessary (since no other resource will
        be able to read the message):

        <code><![CDATA[
<private xmlns="urn:xmpp:carbons:2"/>
          ]]></code>

        All together, an example OTR message might look like this (with the
        majority of the body stripped out for readability):

        <example caption='OTR message with processing hints'><![CDATA[
<message from='malvolio@stewardsguild.lit/countesshousehold'
         to='olivia@countess.lit/veiled'>
  <body>?OTR?v23?...</body>
  <no-copy xmlns="urn:xmpp:hints"/>
  <no-permanent-store xmlns="urn:xmpp:hints"/>
  <private xmlns="urn:xmpp:carbons:2"/>
</message>
          ]]></example>
      </p>
    </section2>
    <section2 topic='Delivery Receipts'>
      <p>
        If a client supports OTR and &xep0184; it is RECOMMENDED that the client
        send a delivery receipt only after successfully decrypting an encrypted
        message.
      </p>
    </section2>
  </section1>
  <section1 topic='OTR Sessions'>
    <section2 topic='Starting an OTR session'>
      <p>
        Most clients today provide options to automatically start an OTR
        session, to manually construct a session at the users request, or to
        always require the use of an OTR session even if the remote client does
        not support OTR.
      </p>
      <p>
        In the interest of user experience, it is NOT RECOMMENDED to start an
        OTR session with a previously unseen resource or one for which we do not
        have OTR keys cached without first discovering if the remote end
        supports OTR using one of the mechanisms described in the "Discovery"
        section of this document except in security critical contexts where user
        experience is not a concern.
      </p>
      <p>
        Instead, it is RECOMMENDED to always allow the user to manually start an
        OTR session and to indicate that OTR is known to be available when OTR
        support is discovered by any of the aforementioned mechanisms.
      </p>
    </section2>
    <section2 topic='Ending an OTR session'>
      <p>
        It is RECOMMENDED that the lifetime of OTR sessions be limited to the
        lifetime of the XMPP session in which the OTR session was established.
        If a resource associated with either end of the OTR session goes offline
        (a closing stream tag is received, or a fatal stream error occurs), it
        is RECOMMENDED that the other end terminate the OTR session.
      </p>
      <p>
        When an XMPP session that is hosting an OTR session ends, it is
        RECOMMENDED that XMPP session be completely torn down before the
        associated OTR session is ended. For instance, when receiving a closing
        stream tag, clients should send their own closing stream tag (as
        specified in &rfc6120;), close the underlying TCP connection (or
        connections), and then terminate the OTR session in that order. This
        prevents a race condition in some clients that attempt to automatically
        establish an OTR session where the OTR session is torn down and then
        re-established by an incomming message before the XMPP session can be
        closed.
      </p>
    </section2>
  </section1>
  <section1 topic='Use in XMPP URIs'>
    <p>
      &rfc5122; defines a Uniform Resource Identifier (URI) and
      Internationalized Resource Identifier (IRI) scheme for XMPP entities, and
      &xep0147; defines various query components for use with XMPP URI's. When
      an entity has an associated OTR fingerprint it's URI is often formed with
      "otr-fingerprint" in the query string. Eg.

      <example caption='OTR Fingerprint'>
xmpp:feste@allfools.lit?otr-fingerprint=AEA4D503298797D4A4FC823BC1D24524B4C54338
      </example>
    </p>
    <p>
      The &REGISTRAR; maintains a registry of queries and key-value pairs for
      use in XMPP URIs at &QUERYTYPES;. As of the date this document was
      authored, the 'otr-fingerprint' query string has not been formally defined
      and has therefore is not officially recognized by the registrar.
    </p>
  </section1>
  <section1 topic='Acknowledgements' anchor='acks'>
    <p>
      Thanks to Daniel Gultsch for his excellent
      <link url='https://github.com/siacs/Conversations/blob/development/docs/observations.md'>
        article
      </link>
      <note>
        Daniel Gultsch (Retreived on 2015-07-29). "Observations on Imlementing
        XMPP"
        &lt;<link url='https://github.com/siacs/Conversations/blob/development/docs/observations.md'>
          https://github.com/siacs/Conversations/blob/development/docs/observations.md
        </link>&gt;
      </note>
      on the pitfalls of implementing OTR, and to Georg Lukas and Chris
      Ballinger for their feedback and corrections.
    </p>
  </section1>
  <section1 topic='Security Considerations' anchor='security'>
    <p>
      While this document describes an existing protocol which is streamed over
      XMPP and therefore does not introduce any new security concerns itself, it
      is worth mentioning a few security issues with the underlying OTR
      protocol:
    </p>
    <p>
      Because Diffie-Hellman (D-H) key exchange is unauthenticated, the initial
      D-H exchange which sets up the encrypted channel is vulnerable to a
      man-in-the-middle attack. No sensitive information should be sent over the
      encrypted channel until mutual authentication has been performed inside
      the encrypted channel.
    </p>
    <p>
      OTR makes use of the SHA-1 hash algorithm. While no practical attacks have
      been observed in SHA-1 at the time of this writing, theoretical attacks
      have been constructed, and attacks have been performed on hash functions
      that are similar to SHA-1. One cryptographer estimated that the cost of
      generating SHA-1 collisions was $2.77 million dollars in 2012, and would
      drop to $700,000 by 2015.
      <note>
        Bruce Schneier (2012-10-05). "When Will We See Collisions for SHA-1?"
        &lt;<link url='https://www.schneier.com/blog/archives/2012/10/when_will_we_se.html'>
          https://www.schneier.com/blog/archives/2012/10/when_will_we_se.html
        </link>&gt;
      </note>.
      This puts generating SHA-1 collisions well within the reach of
      governments, malicious organizations, and even well-funded individuals.
    </p>
  </section1>
  <section1 topic='IANA Considerations' anchor='iana'>
    <p>
      This document requires no interaction with the Internet Assigned Numbers
      Authority (IANA).
    </p>
  </section1>
  <section1 topic='XMPP Registrar Considerations' anchor='registrar'>
    <p>
      No namespaces or parameters need to be registered with the XMPP Registrar
      as a result of this document.
    </p>
  </section1>
</xep>