xeps/xep-0440.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY rfc5056 "<span class='ref'><link url='http://tools.ietf.org/html/rfc5056'>RFC 5056</link></span> <note>RFC 5056:  On the Use of Channel Bindings to Secure Channels &lt;<link url='http://tools.ietf.org/html/rfc5056'>http://tools.ietf.org/html/rfc5056</link>&gt;.</note>" >
  <!ENTITY iana-cb-types "<span class='ref'><link url='https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml'>IANA Channel-Binding Types Registry</link></span> <note>IANA Channel-Binding Types Registry &lt;<link url='https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml'>https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml</link>&gt;.</note>" >
  <!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>SASL Channel-Binding Type Capability</title>
  <abstract>This specification allows servers to annouce their supported SASL channel-binding types to clients.</abstract>
  &LEGALNOTICE;
  <number>0440</number>
  <status>Experimental</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>sasl-cb-types</shortname>
  &flow;
  <revision>
    <version>0.1.0</version>
    <date>2020-06-14</date>
    <initials>XEP Editor (jsc)</initials>
    <remark>Accepted by vote of Council on 2020-05-27.</remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2020-05-20</date>
    <initials>fs</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>

  <p>SASL channel-binding is a technique to increase the security of
  connections (&rfc5056;). Unfortunately, the SASL profile specified
  in &rfc6120; lacks a method for the server to announce its supported
  channel-binding types. This hinders the adoption of channel-binding,
  especially since the error protocol to execute after a client
  requested a channel-binding type unsupported by the server is
  basically unspecified.</p>

  <p>The extension defined herein fills the gap left by &rfc6120;, by
  allowing the server the announce its supported channel-binding
  types.</p>

</section1>

<section1 topic='Announcing the SASL Channel-Binding Type Capability' anchor='sasl-cb-type'>

  <p>This protocol consists of a single optional extension element
  named 'sasl-channel-binding' qualified by the 'urn:xmpp:sasl-cb:0'
  namespace. The 'sasl-channel-binding' element MUST contain one or
  more 'channel-binding' elements, of which each MUST have an
  attribute with the name 'type'. The value of the 'type' attribute
  SHOULD be the "Channel-binding unique prefix" of a channel-binding
  type which was registered with the &iana-cb-types;.</p>

  <p>A server declares that it supports particular channel-binding
  types by listing the supported types via the 'sasl-channel-binding'
  element defined herein. The 'sasl-channel-binding' element could
  appear as child element to the SASL &lt;mechanisms/&gt;
  stream-feature element, qualified by the
  'urn:ietf:params:xml:ns:xmpp-sasl' namespace, as specified in
  &rfc6120;. Another potential appearance of
  &lt;sasl-channel-binding&gt; is as child element of the
  &lt;mechanisms/&gt; stream-feature element as specified in the
  &xep0388;.</p>

  <example caption='Example &lt;mechanisms/&gt; stream feature with SASL Channel-Binding Type Capability.'><![CDATA[
<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>EXTERNAL</mechanism>
    <mechanism>SCRAM-SHA-1-PLUS</mechanism>
    <mechanism>PLAIN</mechanism>
    <sasl-channel-binding xmlns='urn:xmpp:sasl-cb:0'>
      <channel-binding type='tls-server-end-point'/>
      <channel-binding type='tls-exporter'/>
    </sasl-channel-binding>
  </mechanisms>
</stream:features>]]></example>

</section1>

<section1 topic='Security Considerations' anchor='security'>

  <p>The author belives that this document itself does not yield any
  new security considerations.<note>Hopefully somebody will correct him, in
  case he is wrong.</note></p>

</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>This document requires no interaction with the XMPP registrar.</p>

</section1>

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

  <p>TODO: Add if the XEP is scheduled for the state after 'experimental'.</p>

</section1>

<section1 topic='Acknowledgements' anchor='acknowledgements'>

  <p>Thanks to Sam Whited for the discussion about the underlying
  issue and incentivizing me to come up with this extension.</p>

</section1>

</xep>