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Add Channel Binding Pseudomechanisms protoxep
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Sam Whited
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inbox/cb-pseudomechanisms.xml
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inbox/cb-pseudomechanisms.xml
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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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<!ENTITY ianacb "<span class='ref'><link url='https://www.iana.org/assignments/channel-binding-types'>IANA Channel-Binding Types Registry</link></span> <note>IANA registry of channel binding types <<link url='https://www.iana.org/assignments/channel-binding-types'>https://www.iana.org/assignments/channel-binding-types</link>>.</note>" >
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<!ENTITY rfc7677 "<span class='ref'><link url='http://tools.ietf.org/html/rfc7677'>RFC 7677</link></span> <note>RFC 7677: SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms <<link url='http://tools.ietf.org/html/rfc7677'>http://tools.ietf.org/html/rfc7677</link>>.</note>" >
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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>Channel Binding Pseudomechanisms</title>
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<abstract>
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A method for advertising and negotiating types of channel binding supported
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by SCRAM based SASL mechanisms.
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</abstract>
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&LEGALNOTICE;
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<number>xxxx</number>
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<status>ProtoXEP</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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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>pseudomechanisms</shortname>
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&sam;
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<revision>
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<version>0.0.1</version>
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<date>2020-05-01</date>
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<initials>ssw</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>
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The SCRAM (&rfc5802;) family of SASL (&rfc4422;) mechanisms is widely used
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in XMPP.
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SCRAM supports channel binding to the underlying security layer to prevent
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replay attacks, but does not provide a means of negotiating the type of
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channel binding performed.
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Normally this means that XMPP defaults to the tls-unique channel binding
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method defined in &rfc5929;.
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</p>
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<p>
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This specification provides servers with a way to advertise what forms of
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channel binding they support, and for clients to choose a channel binding
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mechanism to use.
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</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<ul>
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<li>
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Servers must be able to advertise multiple channel binding mechanisms.
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</li>
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<li>
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Clients must be able to pick a single supported SCRAM and channel binding
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mechanism.
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</li>
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<li>
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No extra round trips should be added to facilitate channel binding
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negotiation.
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</li>
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</ul>
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</section1>
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<section1 topic='Use Cases' anchor='usecases'>
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<p>
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As a server operator I want to support different channel bindings depending
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on what version of TLS was negotiated.
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</p>
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<p>
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As a client I want to know if the server supports tls-server-end-point
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channel binding to verify that the server's certificate has not changed
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between negotiation attempts.
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</p>
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</section1>
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<section1 topic='Pseudomechanisms' anchor='mechanism'>
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<p>
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The SCRAM based mechanisms advertise their channel binding support by
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listing themselves with an added suffix, "-PLUS".
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This advertises that channel binding is possible, but not the actual channel
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binding types supported by the server.
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The "SCRAM-SHA-1-PLUS" mechanism is the same as the "SCRAM-SHA-1" mechanism,
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the "-PLUS" suffix does not fundamentally change the mechanisms behavior.
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This concept of a "pseudomechanism", a mechanism that is implemented in
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terms of another mechanism with minor changes to the name and behavior, can
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be extended to allow for the advertising of specific channel binding types
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in addition to whether channel binding is supported or not.
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</p>
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<p>
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To create such a pseudomechanism the server MUST concatenate the name of any
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"-PLUS" suffixed mechanism that it supports from the IANA SASL SCRAM Family
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Mechanisms Subregistry of the &ianasasl; with a colon (":") followed by the
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name of any channel binding data that it supports with the currently
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negotiated security layer from the &ianacb;.
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For example, if a TLS connection has been established and the server wants
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to advertise that it supports the SCRAM-SHA-1-PLUS and SCRAM-SHA-256-PLUS
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mechanisms with the tls-unique and tls-server-end-point channel binding
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types, it would list the following mechanisms:
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</p>
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<p>
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While the channel binding variants of SCRAM are registered in the
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&ianasasl;, the pseudomechanisms described in this document depend on values
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in the registry themselves and so are explicitly not meant to be listed.
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A mechanism with a conflicting name cannot be registered because these
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pseudomechanisms are deliberately in violation of the naming convention of
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the SCRAM family of mechanisms as defined in &rfc7677; to prevent such
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conflicts.
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Because they are not meant to represent full mechanisms for use outside of
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XMPP, they do not require registration or the issuance of a GSS-API
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mechanism OID.
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</p>
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<example caption='Host Advertises Mechanisms'><![CDATA[
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<stream:features>
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<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
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<mechanism>SCRAM-SHA-256-PLUS</mechanism>
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<mechanism>SCRAM-SHA-256-PLUS:tls-unique</mechanism>
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<mechanism>SCRAM-SHA-256-PLUS:tls-server-end-point</mechanism>
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<mechanism>SCRAM-SHA-1-PLUS</mechanism>
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<mechanism>SCRAM-SHA-1-PLUS:tls-unique</mechanism>
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<mechanism>SCRAM-SHA-1-PLUS:tls-server-end-point</mechanism>
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<mechanism>SCRAM-SHA-256</mechanism>
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<mechanism>SCRAM-SHA-1</mechanism>
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</mechanisms>
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</stream:features>]]></example>
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<p>
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The original "-PLUS" suffixed mechanisms SHOULD continue to be listed and
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function as they did before (likely by defaulting to tls-unique) for the
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purpose of backwards compatibility.
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</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>
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The Security Considerations sections from &rfc4422;, &rfc5802;, and
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&rfc7677; apply to this document.
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</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document has no actions for IANA.</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<p>This document has no actions for the ®ISTRAR;.</p>
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</section1>
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</xep>
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