mirror of
https://github.com/moparisthebest/xeps
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646 lines
40 KiB
XML
646 lines
40 KiB
XML
<?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 SIGNATURE "<Signature/>">
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<!ENTITY REFERENCE "<Reference/>">
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<!ENTITY SIGNEDINFO "<SignedInfo/>">
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<!ENTITY KEYINFO "<KeyInfo/>">
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<!ENTITY SIGNATUREPROPERTIES "<SignatureProperties/>">
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<!ENTITY MANIFEST "<Manifest/>">
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<!ENTITY XMPPprop "<XMPPproperties/>">
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<!ENTITY SMIME "<span class='ref'><link url='http://tools.ietf.org/html/rfc3851'>S/MIME</link></span> <note>RFC 3851: Secure/Multipurpose Internet Mail Extensions (S/MIME) <<link url='http://tools.ietf.org/html/rfc3851'>http://tools.ietf.org/html/rfc3851</link>>.</note>" >
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<!ENTITY CMS "<span class='ref'><link url='http://tools.ietf.org/html/rfc3852'>CMS</link></span> <note>RFC 3852: Cryptographic Message Syntax (CMS) <<link url='http://tools.ietf.org/html/rfc3852'>http://tools.ietf.org/html/rfc3852</link>>.</note>" >
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<!ENTITY CDCIE "<span class='ref'><link url='http://www.jfcom.mil/about/facts_prt/MNIS.pdf'>CDCIE</link></span> <note>USFCOM fact sheet: Multinational Information Sharing and the Cross Domain Collaborative Information Environment <<link url='http://www.jfcom.mil/about/facts_prt/MNIS.pdf'>http://www.jfcom.mil/about/facts_prt/MNIS.pdf</link>>.</note>" >
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<!ENTITY CDCIE-CCP "<span class='ref'>CDCIE-CCP</span> <note>Cross Domain Collaborative Information Environment (CDCIE) Chat Client Protocol Specification, Version 2.0, Trident Systems, Inc., 12 March 2008</note>" >
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<!ENTITY XMLDSIG "<span class='ref'><link url='http://www.w3.org/TR/xmldsig-core/'>XMLDSIG</link></span> <note>XML Signature Syntax and Processing, W3C Recommendation, 10 June 2008 <<link url='http://www.w3.org/TR/xmldsig-core/'>http://www.w3.org/TR/xmldsig-core/</link>>.</note>" >
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<!ENTITY XPointer "<span class='ref'><link url='http://www.w3.org/TR/xptr'>XPointer</link></span> <note>XML Pointer Language (XPointer), W3C Recommendation, 8 June 2001 <<link url='http://www.w3.org/TR/xptr'>http://www.w3.org/TR/xptr</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>Design Considerations for Digital Signatures in XMPP</title>
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<abstract>This document discusses considerations for the design of Digital Signatures in XMPP,
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including use cases and requirements. The document also discusses various ways XML Digital
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Signatures could be used in XMPP.</abstract> &LEGALNOTICE; <number>0274</number>
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<status>Experimental</status>
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<type>Informational</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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<spec>XEP-0001</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>N/A</shortname>
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&kdz;
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<revision>
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<version>0.4</version>
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<date>2011-01-28</date>
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<initials>kdz</initials>
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<remark>
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<p>Update/fix references.</p>
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</remark>
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</revision>
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<revision>
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<version>0.3</version>
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<date>2011-01-12</date>
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<initials>kdz</initials>
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<remark>
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<p>Update discussions based upon introduction of Encapsulated Digital Signatures in XMPP, an
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alternative to XEP-0285.</p>
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</remark>
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</revision>
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<revision>
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<version>0.2</version>
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<date>2010-09-29</date>
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<initials>kdz</initials>
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<remark>
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<p>Update discussions based upon introduction of Digital Signatures in XMPP.</p>
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</remark>
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</revision>
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<revision>
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<version>0.1</version>
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<date>2009-09-15</date>
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<initials>psa</initials>
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<remark>
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<p>Initial published version as accepted for publication by the XMPP Council.</p>
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</remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2009-08-20</date>
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<initials>kdz</initials>
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<remark>
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<p>Proto-XEP draft.</p>
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</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>Digital signatures may be used to provide a number of security services, including message
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authentication, message integrity and non-repudiation. There are many use cases for Digital
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Signatures in the Extensible Messaging and Presence Protocol (&xmpp;).</p>
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<p>XMPP can be described as a mean for exchanging structured information or stanzas between two
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or more entities. To accomplish this exchange, a number of other entities may be involved. For
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instance, communication of a stanza between two client entities will typically involve one or
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more server entities. Entities may exchange stanzas through service entities, such as a chat
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room service, to effect one-to-many communications.</p>
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<p>Any entity involved in the exchange of a stanza may have wish to include one or more digital
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signatures for the benefit of any entity involved in the exchange:</p>
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<ul>
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<li>A client might wish to sign information it exchanges with another client for the benefit
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of this client (e.g, to provide message origin authentication service and content integrity
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service)</li>
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<li>A client might wish to sign a message in order to bind a Security Label to that
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message.</li>
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<li>A client may wish to sign information it sends to a chat room for the benefit of the chat
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room service and/or for the benefit of room occupants.</li>
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<li>The chat room service may wish to sign information it forwards to room occupants for the
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benefit of room occupants, such as to bind the client's JID to the client's room JID.</li>
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<li>A server involved in the exchange of a stanza between two clients may wish to sign
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information for the benefit of another server involved in the exchange (e.g., to provide
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delivery path validation).</li>
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<li>A server may wish to add additional data to a message, for example a Security Label, and
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bind that data to the message with a digital signature.</li>
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</ul>
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<p>Digital signatures are provided to serve specific purposes. These purposes might include
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authentication of a particular entity involved in the exchange and integrity of information
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that entity provided.</p>
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<p>This document discusses <link url="#design">considerations for the design</link> of
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general-purpose digital signature extension for XMPP. The document discusses <link url="#uses"
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>use cases</link> and <link url="#requires">requirements</link>, as well as explores the
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solution space. The document also discusses <link url="#existing">existing solutions</link> in
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this area.</p>
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<p>This document contains a numerous examples intended to aide in the discussion of design
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issues. The examples are examples generally abbreviated and often use informal syntaxes.</p>
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</section1>
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<section1 topic="Use Cases" anchor="uses">
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<section2 topic="Use in directed one-to-one stanzas" anchor="direct-1to1-use">
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<p>Directed one-to-one stanzas are stanzas which are exchanged between two entities, the
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originator of the stanza and intended recipient of that stanza, without exchanging through
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services which provide re-direction of stanzas (such as a groupchat service). The stanza may
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be handled by one or more other entities.</p>
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<p>Examples of directed one-to-one stanzas include chat &MESSAGE; used in one-to-one chat
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sessions and &IQ; stanzas (excepting those exchanged through services providing <link
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url="redirect-1to1-use">re-direction</link>).</p>
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<p>The originator may wish to provide a signature for the benefit of the intended recipient.
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The intended recipient could use this signature to authenticate the originator and to ensure
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integrity of originator provided information.</p>
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<p>Entities handling the stanza may wish to provide a signature for the benefit of the
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intended recipient. For instance, where a originator is a client and does not provide a
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signature, the client's server may wish to provide a signature for the benefit of the
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intended recipient. The intended recipient could use this signature to authenticate this
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server and to ensure integrity of the information as forwarded by this server. </p>
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</section2>
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<section2 topic="Use in redirected one-to-one stanzas" anchor="redirect-1to1-use">
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<p>Redirected one-to-one stanzas which are exchanged between two entities, the originator of
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the stanza and intended recipient of that stanza, through a service which provides
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re-direction of stanzas. The stanza may be handled by one or more other entities. </p>
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<p>A multi-user chat (MUC) 'private message' is an example of redirected one-to-one
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stanza.</p>
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<p>The originator's server may wish to provide a signature for the benefit of the re-direction
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service. The service could use this signature to authenticate the originator and to ensure
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integrity of originator provided information.</p>
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<p>The originator may wish to provide a signature for the benefit of the intended recipient.
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The intended recipient could use this signature to authenticate the originator and to ensure
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integrity of originator provided information. However, this signature would by itself not
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establish any relationship between the signer and 'from' address in the stanza as received,
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nor does it establish this signature establish that the stanza was processed by the
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re-direction service. As in the <link url="#direct-1to1-use">directed one-to-one
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stanza</link>, a originating client's server may wish to provide a signature for the
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benefit of the intended recipient.</p>
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<p>The re-direction service may wish to provide a signature for the benefit of the intended
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recipient. The intended recipient could use this signature to authenticate the service and
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hence establish the service processed the stanza. The intended recipient could also use the
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signature to ensure the integrity of the information as redirected by the service.</p>
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</section2>
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<section2 topic="Use in redirected one-to-group stanzas" anchor="redirect-1toG-use">
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<p>Redirected one-to-many stanzas which are exchanged between two or more entities, the
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originator of the stanza and a group of recipients, through a service which provides
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re-direction of stanzas of a single stanza to a set of recipients. The stanza may be handled
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by one or more other entities. </p>
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<p>A multi-user chat (MUC) message to all occupants is an example of redirected one-to-group
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stanza.</p>
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<p>The originator's server may wish to provide a signature for the benefit of the re-direction
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service. The service could use this signature to authenticate the originator and to ensure
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integrity of originator provided information.</p>
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<p>The originator may wish to provide a signature for the benefit of each recipient in the
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group. Each recipient could use this signature to authenticate the originator and to ensure
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integrity of originator provided information. However, this signature would by itself not
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establish any relationship between the signer and the 'from' address in the stanza as
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received, nor does it establish this signature establish that the stanza was processed by
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the re-direction service. As in the <link url="#direct-1to1-use">directed one-to-one
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stanza</link>, a originating client's server may wish to provide a signature for the
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benefit of the each recipient.</p>
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<p>The re-direction service may wish to provide a signature for the benefit of each recipient
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in the group. Each recipient could use this signature to authenticate the service and hence
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establish the service processed the stanza. Each could also use the signature to ensure the
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integrity of the information as redirected by the service.</p>
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</section2>
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<section2 topic="Use in presence stanzas" anchor="presence-use">
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<p>The presence can be viewed as a specialized "publish-subscribe" mechanism. Commonly the
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publishing entity sends a &PRESENCE; stanza to a presence service and the presence service
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than forwards the stanza to each subscriber. In basic user presence, the publishing entity
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is the user's client and the presence service is presence service is the provided by this
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client's server. In this case, the 'to' address is empty.</p>
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<p>The publisher may wish to sign the signature for the benefit of each subscriber. Each
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subscriber could use this signature to authenticate the publisher and to ensure integrity of
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publisher provided information.</p>
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<p>The presence service may wish to provide a signature for the benefit of each subscriber.
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Each subscriber could use this signature to authenticate the service and hence establish the
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service processed the stanza. Each could also use the signature to ensure the integrity of
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the information as redirected by the service.</p>
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<p>A presence stanza may also directed to another entity, possibly through a re-direction
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service. This use is similar to the <link url="#direct-1to1-use">directed one-to-one</link>
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and <link url="#redirect-1to1-use">redirected one-to-one</link> cases detailed above.</p>
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</section2>
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</section1>
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<section1 topic="General Requirements" anchor="requires">
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<p>For the purposes of this memo, the following requirements are stipulated for a general
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solution: </p>
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<ol start="1">
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<li>The extension shall support client signing of stanzas.</li>
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<li>The extension shall support service (e.g., multi-user chat service) signing of
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stanzas.</li>
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<li>The extension shall support server signing stanzas.</li>
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<li>The extension shall support multiple signatures in a stanza. That is, multiple entities
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can sign a stanza.</li>
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<li>The extension shall support signing of &IQ; stanzas.</li>
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<li>The extension shall support signing of &MESSAGE; stanzas, including chat and
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groupchat.</li>
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<li>The extension shall support signing of &PRESENCE; stanzas.</li>
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<li>The extension shall support selective signing of stanzas. That is, a signer can sign
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select portions of a stanza.</li>
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<li>The extension shall support signing of externally referenced object. That is, the
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signature may include a message digest of an external object, such as an HTTP accessible
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content.</li>
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<li>The extension shall allow selective verification of signed elements. </li>
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<li>The extension shall allow independent handling of verification errors in signed
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content.</li>
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<li>The extension shall allow signers to provide signed copies of data likely to be modified
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by intermediate entities, such as stanza 'to' and 'from' attributes.</li>
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<li>The extension should avoid duplication of content.</li>
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<li>The extension must provide a means for relating signed content with unsigned content.</li>
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<li>The extension should support querying for key information in XMPP (e.g., &IQ;).</li>
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<li>The extension should support communicating key information through their XMPP-published
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vCard.</li>
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<li>The extension should be designed such that the successful verification of a signature is
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independent of the extension support in entities involved in the exchange.</li>
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<li>The extension should be compatible with object encryption, supporting encryption of signed
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content, signing of encrypted content, and signing of encrypted signed content (e.g., triple
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wrap content).</li>
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</ol>
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<p>Some of above requirements may well be, if not outright mutually exclusive, in opposition to
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each other. It is suspected that set of reasonable solutions meeting all of the above
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requirements may be empty. To produce a reasonable solution, it is expected that some of the
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above requirements be eliminated and hence limiting the solution to some subset of the
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applications of digital signatures in XMPP.</p>
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</section1>
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<section1 topic="Existing Solutions" anchor="existing">
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<section2 topic="XMPP E2E" anchor="xmpp-e2e">
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<p>The &IETF; standardized a signing and encryption facility for XMPP known as &xmppe2e;. XMPP
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E2E is based upon Secure/Multipurpose Internet Message Extensions (&SMIME;) and the
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Cryptographic Message Syntax (&CMS;). As it's name implies, XMPP E2E is intended to be an
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end-to-end solution. That is, it enables a sender to sign content sent to a specific
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recipient.</p>
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<p>An advantage of the XMPP E2E approach is that it uses an encapsulating signature which
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protects the signed content from alteration as it is exchanged over an XMPP network. A
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disadvantage is that implementations which do not support XMPP E2E cannot make use of the
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signed content.</p>
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<p>At the time of this writing, XMPP E2E has not been widely implemented. XMPP E2E appears to
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have limited applicability. </p>
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</section2>
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<section2 topic="PGP signatures in XMPP" anchor="xmpp-e2e">
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<p>The &xep0027; (XMPP PGP), like the XMPP E2E, uses an encapsulating signature to protects
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the signed content from alteration as it is exchanged over an XMPP network. Like XMPP E2E,
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it is intended to be an end-to-end solution.</p>
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<p>At the time of this writing, XMPP PGP has not been widely implemented (though some
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implementations do exist). XMPP PGP appears to have limited applicability.</p>
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</section2>
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<section2 topic="Encapsulating Digital Signatures in XMPP" anchor="xmpp-e2e">
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<p>The &xep0285; (XMPP EgDSIG), like the XMPP E2E and XMPP PGP, uses an encapsulating
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signature to protects the signed content from alteration as it is exchanged over an XMPP
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network.</p>
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<p>Unlike XMPP E2E and XMPP PGP, the solution is not intended to be strictly end-to-end in
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that multiple signers and verifiers where contemplated, now of which is necessarily an end
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entity. However, like XMPP E2E, it does not supported <em>optimistic signing</em>.</p>
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</section2>
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<section2 topic="CDCIE-CCP" anchor="cdcie-ccp">
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<p>Alternative approaches have been developed. For instance, the Cross Domain Collaborative
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Information Environment (&CDCIE;) Client Chat Protocol (&CDCIE-CCP;), an XMPP-based
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protocol, supports signing of XMPP stanzas utilizes XML digital signatures (&XMLDSIG;)
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"enveloped" signatures over the whole stanza.</p>
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<p>An advantage of the CDCIE-CCP approach is that, because it uses an encapsulated signature,
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implementations need not support CDCIE-CPP to make use of the stanza. The disadvantage is
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that the signature always over the entire stanza. Alteration of the stanza, as is common
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(often required) when exchanging stanzas over an XMPP network, will invalidate the
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signature.</p>
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<p>While this approach has been implemented and deployed to some extent, the approach appears
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to have applicability limited to the CDCIE.</p>
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</section2>
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<section2 topic="Encapsulated Digitial Signatures in XMPP" anchor="xmpp-ed-dsig">
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<p>The &xep0290; (XMPP DSIG) is an encapsulated signature proposal similar to that
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signed manifest approach suggested below. This approach is intended to support a wide range of
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uses and applications including <em>optimistic signing</em> in general communciations.</p>
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<p>Unlike CDCIE-CCP approach, XMPP DSIG signatures are not "enveloped" signatures over the
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whole stanza but signatures over an object which details the stanza contents.</p>
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</section2>
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</section1>
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<section1 topic="Protocol Design Discussion" anchor="design">
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<section2 topic="Encapsulated v. Encapsulating Signatures" anchor="encap">
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<p>An encapsulating signature is a signature approach that encapsulates the signed content
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within the signature syntax. An encapsulated signature is a signature approach where the
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signature syntax in encapsulated within the structure of the signed content. XMPP E2E and
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XMPP PGP are examples of the former. CDCIE-CCP and XMPP DSIG are examples of the latter.</p>
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<p>The following example illustrates, using pseudo language, an encapsulating signature over a
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&MESSAGE; stanza.</p>
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<example caption="Encapsulating Signature"><![CDATA[
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<encapslating-signature>
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<signedInfo>
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<message to='romeo@example.net' type='chat'>
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<body>Art thou not Romeo, and a Montague?</body>
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</message>
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</signedInfo>
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<signature-over-signedInfo/>
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</encapslating-signature>
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]]></example>
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<p>To transfer a signed &MESSAGE; using an encapsulating signature, one needs to send it
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within &MESSAGE; stanza. </p>
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<example caption="Transfer of an Encapsulating Signature"><![CDATA[
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<message to='romeo@example.net' type='chat'>
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<encapslating-signature>
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<signedInfo>
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<message to='romeo@example.net' type='chat'>
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<body>Art thou not Romeo, and a Montague?</body>
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</message>
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</signedInfo>
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<signature-over-signedInfo/>
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</encapslating-signature>
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</message>
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]]></example>
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<p>The following example illustrates, using pseudo language, an encapsulated signature over a
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&MESSAGE; stanza. </p>
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<example caption="Encapsulated Signature"><![CDATA[
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<message to='romeo@example.net' type='chat'>
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<body>Art thou not Romeo, and a Montague?</body>
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<encapsulated-signature>
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<signature-over-message/>
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</encapsulated-signature>
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</message>
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]]></example>
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<p>Applicability of a simple (non-nesting) encapsulating signatures, such as in XMPP E2E and
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XMPP PGP, are generally limited to end-to-end use cases. That is, cases where the originator
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of a stanza signs the stanza and send it through the XMPP network to its intended recipient,
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and only the intended recipient is expected to make use of the signed content. Entities
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between the signer and the intended recipient are expected to forward of the stanza without
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regard to the encapsulating signature, and without themselves signing the stanza. The
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approach does not require forwarding entities to support the signing extension.</p>
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<p>Simple encapsulating signatures have limited applicability in MUC and PubSub use cases. For
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instance, an occupant can sign its submissions to the service for the benefit of the service
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and the service can sign reflected stanzas to occupants. In providing non-anonymous chat
|
|
rooms, in addition to signing the reflected content, the service should include and sign the
|
|
stanza it received when it was signed. This allows the occupants verify the content the
|
|
service purports to have received, and to determine whether the reflected content is
|
|
consistent given this. The following example illustrates an encapsulating signature over a
|
|
groupchat &MESSAGE; stanza.</p>
|
|
<example caption="MUC submission with Encapsulating Signature"><![CDATA[
|
|
<message from='hag66@shakespeare.lit/pda' to='darkcave@chat.shakespeare.lit'
|
|
type='groupchat' xml:lang='en'>
|
|
<encapslating-signature>
|
|
<signed-info>
|
|
<message
|
|
to='darkcave@chat.shakespeare.lit'
|
|
type='groupchat'
|
|
xml:lang='en'>
|
|
<body>Harpier cries: 'tis time, 'tis time.</body>
|
|
</message>
|
|
</signed-info>
|
|
<signature-value>...</signature-value>
|
|
</encapslating-signature>
|
|
</message>
|
|
]]></example>
|
|
<p>The following examples illustrates the signed reflection of the above stanza.</p>
|
|
<example caption="MUC reflection with Encapsulating Signature"><![CDATA[
|
|
<message from='darkcave@chat.shakespeare.lit/thirdwitch'
|
|
to='crone1@shakespeare.lit/desktop' type='groupchat' xml:lang='en'>
|
|
<encapslating-signature>
|
|
<signed-info>
|
|
<message
|
|
from='darkcave@chat.shakespeare.lit/thirdwitch'
|
|
to='crone1@shakespeare.lit/desktop' type='groupchat' xml:lang='en'>
|
|
<body>Harpier cries: 'tis time, 'tis time.</body>
|
|
</message>
|
|
<derived-from>
|
|
<message from='hag66@shakespeare.lit/pda'
|
|
to='darkcave@chat.shakespeare.lit' type='groupchat' xml:lang='en'>
|
|
<encapslating-signature>
|
|
<signedInfo>
|
|
<message to='darkcave@chat.shakespeare.lit'
|
|
type='groupchat' xml:lang='en'>
|
|
<body>Harpier cries: 'tis time, 'tis time.</body>
|
|
</message>
|
|
</signedInfo>
|
|
<signature/>
|
|
</encapslating-signature>
|
|
</message>
|
|
</derived-from>
|
|
</signed-info>
|
|
<signature-value>...</signature-value>
|
|
</encapslating-signature>
|
|
</message>
|
|
]]></example>
|
|
<p>In encapsulated signature solutions, as in CDCIE-CCP, any entities can make use of the
|
|
signed content even if they do not support the signing extension. If the signature is over
|
|
the entire stanza, as in CDCIE-CCP, the signature is likely not to be valid when the stanza
|
|
is passed through multiple entities prior to verification. Hence, when the signature is over
|
|
the entire stanza, the encapsulating signature approach applicability is generally limited
|
|
to cases where there no entities between the signer and verifier. However, as discussed
|
|
<link url="#stanza-siging">below</link>, encapsulated selective signatures are generally
|
|
more applicable.</p>
|
|
</section2>
|
|
<section2 topic="Selective Signing" anchor="select">
|
|
<p>While an entity could provide a signature to be over the entire stanza, such signatures are
|
|
likely be invalidated as the stanza exchanged over the XMPP. This is because XMPP allows
|
|
and, in many cases, requires stanza to be modified as they are forwarded.</p>
|
|
<p>For instance, a client with the JID "juliet@example.com/Balcony" might send the following
|
|
signed stanza:</p>
|
|
<example caption="Signature over entire stanza"><![CDATA[
|
|
<message to='romeo@example.net' type='chat' xml:lang='en'>
|
|
<subject>Love</subject>
|
|
<body>Art thou not Romeo, and a Montague?</body>
|
|
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
|
|
<SignedInfo>
|
|
...
|
|
<Reference URI="">
|
|
<Transforms>
|
|
<Transform
|
|
Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
|
|
</Transforms>
|
|
...
|
|
</Reference>
|
|
</SignedInfo>
|
|
<SignatureValue>...</SignatureValue>
|
|
...
|
|
</Signature>
|
|
</message>
|
|
]]></example>
|
|
<p>The example.com server is required, per &xmppcore;, to add a 'from' attribute to the
|
|
&MESSAGE; element before forwarding it to the example.net server. The example.net server is
|
|
required to replace the 'to' attribute with the full JID of the romeo@example.net client it
|
|
intends to forward the message to. These alternatations will "break" the signature.</p>
|
|
|
|
<p>XMLDSIG provides for a facility to selective sign XML content. For instance, the client
|
|
could sign the &SUBJECT; and &BODY; element and their content. However, this by itself would
|
|
not cover key aspects of the stanza, such that it was a chat &MESSAGE; addressed to a
|
|
particular JID and sent from a particular JID. XMLDSIG allows for enveloping signatures,
|
|
that is a signature that signs a data object contained within the &SIGNATURE; element. The
|
|
solution could define an element, such as &XMPPprop; used below, for including properties of
|
|
the stanza in the signature. </p>
|
|
</section2>
|
|
|
|
<section2 topic="Replay attack protection" anchor="replay">
|
|
<p>The signature in Example 1 does not provide any protection against replay attack. To
|
|
address replay attack, as well as other concerns, XMLDSIG defines the &SIGNATUREPROPERTIES;
|
|
element for including information items about the generation of the Signature, such as the
|
|
date/time the signature was generated. </p>
|
|
</section2>
|
|
|
|
<section2 topic="Manifest Signing" anchor="manifest">
|
|
<p>While one could have &SIGNATURE; which included a &REFERENCE; element for each of four
|
|
elements discussed above within its &SIGNEDINFO; element, this would require reference
|
|
validation for each &REFERENCE; (See 2.3 of XMLDSIG). To provide greater flexibility over
|
|
handling of absent references and broken digest values, a &MANIFEST; can be constructed and
|
|
only it signed.</p>
|
|
|
|
|
|
<p>Putting all of the above together, the client might send the following signed stanza:</p>
|
|
<example caption="Client signed message"><![CDATA[
|
|
<message to='romeo@example.net' type='chat' xml:lang='en'>
|
|
<subject id='X-subj'>Love</subject>
|
|
<body id='X-body'>Art thou not Romeo, and a Montague?</body>
|
|
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#" Id="sig">
|
|
<SignedInfo>
|
|
...
|
|
<Reference URI="#X-manifest">...</Reference>
|
|
</SignedInfo>
|
|
<SignatureValue>...</SignatureValue>
|
|
<Object>
|
|
<Manifest id='X-manifest'>
|
|
<Reference URI="#X-subj">...</Reference>
|
|
<Reference URI="#X-body">...</Reference>
|
|
<Reference URI="#X-xmppprop">...</Reference>
|
|
<Reference URI="#X-sigprop">...</Reference>
|
|
</Manifest>
|
|
</Object>
|
|
<Object>
|
|
<XMPPproperties id='X-xmppprop'>
|
|
<stanza>message</stanza>
|
|
<type>chat</type>
|
|
<from>juliet@example.com</from>
|
|
<to>romeo@example.net</to>
|
|
</XMPPproperties>
|
|
</Object>
|
|
<Object>
|
|
<SignatureProperties id="X-sigprop" Target="#X-sig">
|
|
<SignatureProperty Target="#timestamp">
|
|
<timestamp>2009-08-03T13:33:00Z</timestamp>
|
|
</SignatureProperty>
|
|
</SignatureProperties>
|
|
</Object>
|
|
</Signature>
|
|
</message>
|
|
]]></example>
|
|
</section2>
|
|
<section2 topic="Unambigious identification of content" anchor="id">
|
|
<p>The signature references needs to unambiguously identify content in stanza even in face of
|
|
subsequent modification of that stanza. Failure to unambiguously identify signed content
|
|
would also be problematic.</p>
|
|
|
|
<p>In the above example, signed child elements of the stanza were identified by 'id'
|
|
attribute. As stanzas may be forwarded into any XMPP stream, such identifiers needs to
|
|
remain unique.</p>
|
|
|
|
<p>Use of an extension attribute to identify elements may be problematic. In particular, the
|
|
XMPP specifications provide no assurance that this attribute would be forwarded with
|
|
element. While one could identify signed content by other means, such as &XPointer;, these
|
|
means would not unambiguously identify the signed content in the face of subsequent stanza
|
|
modification. </p>
|
|
|
|
<p>The an 'id' attribute is could be used (or possibly 'xml:id'), it may be appropriate for
|
|
the XMPP entity inserting a child element into a stanza to provide an 'xml:id' attribute
|
|
regardless of what stanza content it might sign.</p>
|
|
</section2>
|
|
|
|
<section2 topic="Multiple Signatures" anchor="multisig">
|
|
<p>Multiple entities can sign a stanza. A single entity may sign a stanza multiple times,
|
|
typically on different occasions.</p>
|
|
|
|
<p>Each signer simply adds their &SIGNATURE; element to the stanza, typically as the last
|
|
element. A &SIGNATURE; may sign other signatures, or portions thereof.</p>
|
|
|
|
<p>While a simple chat &MESSAGE; typically transits through only one or two XMPP servers and a
|
|
groupchat &MESSAGE; may typically transits one to three XMPP servers, a stanza might include
|
|
far more than four &SIGNATURE; elements.</p>
|
|
</section2>
|
|
<section2 topic="Optimistic Signing" anchor="optimistic">
|
|
<p>Some users design the ability to <em>optimistic signing</em> of stanzas. That is, to sign
|
|
all stanzas adhere to a configured criteria, such as all &MESSAGE; stanzas, they send. A key
|
|
aspect of optimistic signing is that receiving entities not supporting the signing extension
|
|
should be able to make use the message content (excluding the signature information) while
|
|
those receiving entities supporting the extension can make use of the message content and
|
|
the signature information.</p>
|
|
<p>Optimistic signing is available in E-mail through the use of S/MIME detached signatures.
|
|
Use of S/MIME detached signatures can be problematic. Mail systems, especially restribution
|
|
services such as mailing lists, are notorious for changing the signed content and hence
|
|
breaking the signature.</p>
|
|
<p>In XMPP, as stanzas are generally altered in transit and hence optimistic signing will be
|
|
fragile at best. Through use of selective signing and manifesting, issues may be mitigated
|
|
to some degree. It is doubtful that a solution exists that provides optimistic signing and
|
|
reliability verification.</p>
|
|
<section3 topic="Dual content" anchor="dual">
|
|
<p>One possible optimistic signing solution is for stanzas to carry <em>alternative</em>
|
|
sets of content, an unsigned content alternative and a signed content alternative. The
|
|
premise of this approach is that an entity supporting the signing extension could make use
|
|
of the signed content alternative while an entity not supporting the signing extension
|
|
could make use of the unsigned content alternative. The approach has been suggested to as
|
|
a mechanism for support extension-unaware entities downstream of extension-unware
|
|
groupchat (or like) services use of the stanza content.</p>
|
|
<p>The following example not only illustrate this approach, but highlights some of the
|
|
issues with this approach:</p>
|
|
<example caption="Dual content message"><![CDATA[
|
|
<message from='hag66@shakespeare.lit/pda' to='darkcave@chat.shakespeare.lit/laptop'
|
|
type='groupchat' xml:lang='en'>
|
|
<body>No.</body>
|
|
<encapslating-signature>
|
|
<signed-info>
|
|
<message to='darkcave@chat.shakespeare.lit' type='groupchat' xml:lang='en'>
|
|
<body>Yes.</body>
|
|
</message>
|
|
</signed-info>
|
|
<signature-value>...</signature-value>
|
|
</encapslating-signature>
|
|
<delay xmlns='urn:xmpp:delay'
|
|
from='shakespeare.lit'
|
|
stamp='2002-09-10T23:08:25Z'/>
|
|
</message>
|
|
]]></example>
|
|
<p>But it should be obvious that the signed and unsigned contents are not proper
|
|
alternatives. The signed content presumedly is what the signer sent. The unsigned content
|
|
is presumedly a modified version of what the signer sent. The modifications are generally
|
|
important to the entity making use of the stanza. In the above example, note that the
|
|
to/from addresses of the signed content differ from the unsigned content. Note as well
|
|
that the unsigned content contains a >delay/< element indicating that the stanza was
|
|
delayed in transit. Such modifications are generally important to the proper processing of
|
|
the content by not only this entity, but entities to which the content might be forwarded
|
|
to. Dual content, even in absence of attacks, simply complicates such processing. </p>
|
|
<p>Note that the &BODY; element values differ between the signed and unsigned content. While
|
|
it reasonable straight forward (though significant work) to determine that the signed and
|
|
unsigned content differ, it is extermely difficult to to determine whether the changes are
|
|
due to normal processing or an attack.</p>
|
|
<p>Dual content adds significant blot. In simple cases, the approach effective doubles the
|
|
content. However, in some use cases, the appraoch may lead to multiple doublings of the
|
|
content.</p>
|
|
<p>It must be noted that verifying entities downstream of a redistribution will need some
|
|
mechanism to determine who signed the stanza, determine what signer is an appropriate
|
|
signer, and to obtain the public key of that signer. While certain information can be
|
|
placed in key data, the question of whether the signer is an appropriate signer for
|
|
purported sender (e.g., a room subscriber) generally would require information from the
|
|
redistribution service, and this would generally require the redistribution service to
|
|
support an extension to make that information available to entities desiring to verify the
|
|
signature(s). If one accepts the premise that downstream verification of redistributed
|
|
stanzas, such as via a MUC service, cannot be performed without extension and cooperation
|
|
of the redistribution service, then it follows that dual content can be avoided by having
|
|
the MUC service also support the signing extension.</p>
|
|
<p>Dual content approaches should be avoided.</p>
|
|
</section3>
|
|
</section2>
|
|
<section2 topic="Key Info" anchor="key-info">
|
|
<p>While a signer may provide a &KEYINFO; element within the &SIGNATURE;, doing so will
|
|
significantly increase the size of the &SIGNATURE; element. As implementations may enforce a
|
|
maximum stanza size as small as 10,000 bytes, use of &KEYINFO; in stanza signatures should
|
|
be limited.</p>
|
|
<p>It is also noted there are cases where the signer may not want to expose the key
|
|
information to all entities involved in the exchange of stanza.</p>
|
|
<p>There are a number of ways key information may be published, such as in user's vCard. Key
|
|
information can also be provided at request, such as by &IQ;.</p>
|
|
</section2>
|
|
</section1>
|
|
|
|
<section1 topic="Security Considerations" anchor="seccon">
|
|
<p>Care must be taken in the design of not only ensure it provides an effective digital
|
|
signature solution for XMPP, but is designed itself with security in mind. This section
|
|
discussions some security issues in providing a digital signature solution. The design should
|
|
consider a general digital signature issues as well issues specific to the technologies
|
|
used/involved, and particulars of the solution.</p>
|
|
<p>Due to the nature of XML and XMPP, an effective general digital signing solution for XMPP is
|
|
likely to be quite complex. This document suggests nothing less. With complexity comes
|
|
significant security risk. To minimize this risk, the solutions should avoid reinvention of
|
|
needed technology, such as signature and key information syntaxes, by reusing well established
|
|
and understood technologies such as XMLDSIG. Solutions should also favor simple and widely
|
|
used features of such technologies over esoteric or rarely used features</p>
|
|
<p>Designers of the solution should be mind full of security considerations discussed in XMLDSIG
|
|
(regardless of whether XMLDSIG is used in the solution)</p>
|
|
<p>If XMLDSIG is used, a number of security considerations would be introduced into the
|
|
solution. Implementations need to take special care in processing XMLDSIG &SIGNATURE; elements
|
|
to avoid a wide range of attacks. For instance, an attacker could attempt to mount a Denial of
|
|
Service attack by sending a &SIGNATURE; purporting to sign arbitrary large and complex
|
|
content. Or an attacker could attempt to mount a Distributed Denial of Service sending a
|
|
message to a chatroom that containing &SIGNATURE; with multiple references to large content
|
|
hosted at the attack target in hopes that each room participant will repeated fetch it. A
|
|
&SIGNATURE; element might also contain circler references.</p>
|
|
</section1>
|
|
</xep>
|