mirror of
https://github.com/moparisthebest/xeps
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342 lines
17 KiB
XML
342 lines
17 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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<!ENTITY BASE64 "<span class='ref'><link url='http://tools.ietf.org/html/rfc4648'>BASE64</link></span> <note>RFC 4648: The Base16, Base32, and Base64 Data <<link url='http://tools.ietf.org/html/rfc4648'>http://tools.ietf.org/html/rfc4648</link>>.</note>" >
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<!ENTITY DATETIME "<span class='ref'><link url='http://tools.ietf.org/html/rfc3339'>DATETIME</link></span> <note>RFC 3339: Date and Time on the Internet Timestamps <<link url='http://tools.ietf.org/html/rfc3339'>http://tools.ietf.org/html/rfc3339</link>>.</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 E2EEncrypt "<span class='ref'><link url='http://datatracker.ietf.org/doc/draft-miller-3923bis'>E2EEncrypt</link></span> <note>End-to-End Object Encryption for the Extensible Messaging and Presence Protocol (XMPP), Miller, M. and P. Saint-Andre, work in progress <<link url='http://datatracker.ietf.org/doc/draft-miller-3923bis'>http://datatracker.ietf.org/doc/draft-miller-3923bis</link>>.</note>" >
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%ents;
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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>Encapsulating Digital Signatures in XMPP</title>
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<abstract>This document provides a technical specification for Encapsulating Digital Signatures
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in the Extensible Messaging and Presence Protocol (XMPP).</abstract> &LEGALNOTICE;
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<number>0285</number>
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<status>Deferred</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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<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>0.3</version>
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<date>2011-01-12</date>
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<initials>kdz</initials>
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<remark><p>Change title, and clarify in text, that this is an encapulating digital
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signature approach, an alternative to the encapulated digitial signatures proposal.</p></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><p>Minor changes (editorial, cleanup, etc.).</p></remark>
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</revision>
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<revision>
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<version>0.1</version>
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<date>2010-09-15</date>
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<initials>psa</initials>
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<remark><p>Initial published version.</p></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>2010-03-10</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 class='box'><em>This document is one of two proposals for digital signatures in XMPP. It is expected
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that only one of these proposals be progressed beyond Experimental on the Standards Track.</em></p>
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<p>This document provides a technical specification for Digital Signatures in Extensible
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Messaging and Presence Protocol (&xmpp;) based upon End-to-End Object Encryption
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(&E2EEncrypt;) "work in progress".</p>
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<p>The S/MIME approach defined in &rfc3923; has never been implemented in XMPP clients to the
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best of our knowledge, but has some attractive features, especially the ability to
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store-and-forward a signed message at a user's server if the user is not online when the
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message is received (in the XMPP community this is called "offline storage" and the message is
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referred to as an "offline message"). The authors surmise that RFC 3923 has not been
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implemented mainly because it adds several new dependencies to XMPP clients, especially MIME
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(along with the CPIM and MSGFMT media types).</p>
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<p>This document explores the possibility of an approach that is similar to but simpler than
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RFC 3923. Like the approach detailed in RFC 3923, the approach utilizes encapsulating
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digital signatures.</p>
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<p>Like other encapsulating signature approaches (e.g., &xep0027;), this approach does not
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support <em>optimistic signing</em>.</p>
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</section1>
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<section1 topic="Signing XMPP Stanzas" anchor="stanza">
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<p>The process that a sending agent follows for securing stanzas is very similar regardless of
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the form of stanza (i.e., <iq/>, <message/>, or <presence/>).</p>
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<ol>
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<li>Constructs a cleartext version of the stanza, S.</li>
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<li>Notes the current UTC date and time N when this stanza is constructed, formatted as
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described in Section 5.</li>
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<li>Converts the stanza to a UTF-8, as defined by &rfc3269;, encoded string, optionally
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removing line breaks and other insignificant whitespace between elements and attributes,
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i.e., UTF8-encode(S) = S'. We call S' a "stanza-string" because for purposes of signing and
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verification it is treated not as XML but as an opaque string (this avoids the need for
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complex canonicalization of the XML input).</li>
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<li>Constructs a plaintext envelope (E) <plain/> qualified by the "urn:xmpp:signed:0"
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namespace as follows: <ul>
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<li>The attribute 'timestamp' set to the UTC date and time value N</li>
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<li>The XML character data set to the base64-encoded form of S' (where the encoding
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adheres to the definition in Section 4 of &BASE64; and where the padding bits are set to
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zero). This encoding is necessary to preserve a canonicalized form of S'.</li>
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</ul>
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</li>
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<li>Converts the envelope (E) to a UTF-8 encoded string, optionally removing line breaks and
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other insignificant whitespace between elements and attributes, i.e., E' =
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UTF8-encode(E).</li>
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<li>Produce a signature of UTF8-encoded envelope (E') using the intended signature algorithm.
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T = signature(E'). (This step is underspecified and will be expanded upon in later revision
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of this document.)</li>
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<li>Base64-encodes T to produce the signature data T'.</li>
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<li>Constructs an <signed/> element qualified by the "urn:xmpp:signed:0" namespace as
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follows: <ul>
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<li>The child element <signature> (implicitly qualified by the "urn:xmpp:signed:0"
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namespace) as follows: <ul>
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<li>The attribute 'algorithm' set to a string identifying the signature algorithm
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used.</li>
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<li>The XML character data T'.</li>
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</ul></li>
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<li>The child element <data> (implicitly qualified by the "urn:xmpp:signed:0"
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namespace) as follows: <ul>
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<li>The XML character data E'.</li>
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</ul>
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</li>
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</ul>
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</li>
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<li>Sends the <signed> element as the payload of a stanza that SHOULD match the stanza
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from step 1 in kind (e.g., <message/>), type (e.g., "chat"), and addressing (e.g.
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to="romeo@montague.net" from="juliet@capulet.net/balcony"). If the original stanza (S) has a
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value for the "id" attribute, this stanza MUST NOT use the same value for its "id"
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attribute.</li>
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</ol>
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<section2 topic="Example of Signing Messages" anchor="ex-message">
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<p>The sender begins with the cleartext version of the <message/> stanza "S":</p>
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<example><![CDATA[
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<message xmlns='jabber:client'
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from='juliet@capulet.net/balcony'
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id='183ef129'
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to='romeo@montague.net'
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type='chat'>
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<thread>8996aef0-061d-012d-347a-549a200771aa</thread>
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<body>Wherefore art thou, Romeo?</body>
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</message>
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]]></example>
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<p>The sender then performs the steps 1 through 4 from above to generate:</p>
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<example><![CDATA[
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<plain xmlns="urn:xmpp:signed:0"
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timestamp="2010-06-29T02:15:21.012Z">
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PG1lc3NhZ2UgeG1sbnM9ImphYmJlcjpjbGllbnQiIGZyb209Imp1bGlldEBjYXB
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1bGV0Lm5ldC9iYWxjb255IiB0bz0icm9tZW9AbW9udGVndWUubmV0IiB0eXBlPS
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JjaGF0Ij48dGhyZWFkPmM2MzczODI0LWEzMDctNDBkZC04ZmUwLWJhZDZlNzI5O
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WFkMDwvdGhyZWFkPjxib2R5PldoZXJlZm9yZSBhcnQgdGhvdSwgUm9tZW8/PC9i
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b2R5PjwvbWVzc2FnZT4=
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</plain>
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]]></example>
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<p>And then performs steps 5 through 9 steps, causing the following to be sent:</p>
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<example><![CDATA[
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<message xmlns='jabber:client'
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from='juliet@capulet.net/balcony'
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id='6410ed123'
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to='romeo@montague.net'
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type='chat'>
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<signed xmlns="urn:xmpp:signed:0">
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<signature algorithm="RSA-SHA1">
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DxbxIziY1C1Ytcxkj0IFLsfmDLMv96JMlMAQZ7jh49IbsOIPsxI2LyLmqhKH/994UXDJKQLHvLJz
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gAmw8V2b+zmyZeItJzSmB+HHiLFVXkD2Dd4JfetsafsfIcB7uNWg0gAeiKrTHfFgiyEC/2WxwOj3
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JUMRyQ9ykEPIzS0GZ/k=
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</signature>
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<data>
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PHBsYWluIHhtbG5zPSJ1cm46eG1wcDpzaWduZWQ6MCIgdGltZXN0YW1wPSIyMDEwLTA2LTI5VDAy
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OjE1OjIxLjAxMloiPgogIFBHMWxjM05oWjJVZ2VHMXNibk05SW1waFltSmxjanBqYkdsbGJuUWlJ
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R1p5YjIwOUltcDFiR2xsZEVCallYQgogIDFiR1YwTG01bGRDOWlZV3hqYjI1NUlpQjBiejBpY205
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dFpXOUFiVzl1ZEdWbmRXVXVibVYwSWlCMGVYQmxQUwogIEpqYUdGMElqNDhkR2h5WldGa1BtTTJN
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emN6T0RJMExXRXpNRGN0TkRCa1pDMDRabVV3TFdKaFpEWmxOekk1TwogIFdGa01Ed3ZkR2h5WldG
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a1BqeGliMlI1UGxkb1pYSmxabTl5WlNCaGNuUWdkR2h2ZFN3Z1VtOXRaVzgvUEM5aQogIGIyUjVQ
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and2YldWemMyRm5aVDQ9CjwvcGxhaW4+Cg==
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</data>
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</signed>
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</message>
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]]></example>
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</section2>
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<section2 topic="Example of Securing IQs" anchor="ex-iq">
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<p>To be added....</p>
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</section2>
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</section1>
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<section1 topic="Interaction with Stanza Semantics" anchor="interact">
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<p>The following limitations and caveats apply:</p>
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<ul>
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<li>Undirected <presence/> stanzas SHOULD NOT be signed.</li>
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<li>Stanzas directed to multiplexing services (e.g. multi-user chat) SHOULD NOT be signed,
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unless the sender has established the service supports the handling of signed stanzas.</li>
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</ul>
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</section1>
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<section1 topic="Handling of Inbound Stanzas" anchor="inbound">
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<p>Several scenarios are possible when an entity receives an encrypted stanza:</p>
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<dl>
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<di>
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<dt>Case #1:</dt>
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<dd>The receiving application does not understand the protocol.</dd>
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</di>
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<di>
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<dt>Case #2:</dt>
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<dd>The receiving application understands the protocol and is able to verify the
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signature.</dd>
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</di>
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<di>
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<dt>Case #3:</dt>
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<dd>The receiving application understands the protocol and is able to verify the signature,
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but the timestamps fail the checks specified under Checking of Timestamps.</dd>
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</di>
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<di>
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<dt>Case #4:</dt>
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<dd>The receiving application understands the protocol and is unable to verify the
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signature.</dd>
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</di>
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</dl>
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<p>In Case #1, the receiving application MUST do one and only one of the following: (1) ignore
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the <signed/> extension, (2) ignore the entire stanza, or (3), except where precluded by
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the protocol (&rfc6120;), return a <service-unavailable/> error to the sender.</p>
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<p>In Case #2, the receiving application MUST NOT return a stanza error to the sender, since
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this is the success case.</p>
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<p>In Case #3, the receiving application MAY, except where precluded by the protocol, return a
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<not-acceptable/> error to the sender, optionally supplemented by an
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application-specific error condition element of <bad-timestamp/> as shown below:</p>
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<example><![CDATA[
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<message from='romeo@example.net/orchard'
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id='6410ed123'
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to='juliet@capulet.net/balcony'
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type='error'>
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<signed xmlns='urn:xmpp:signed:0'>
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<!-- original content -->
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</signed>
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<error type='modify'>
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<not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
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<bad-timestamp xmlns='urn:xmpp:signed:0'/>
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</error>
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</message>
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]]></example>
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<p>In Case #4, the receiving application SHOULD, except as precluded by the protocol, return a
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<bad-request/> error to the sender, optionally supplemented by an application-specific
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error condition element of <bad-signature/> as shown below:</p>
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<example><![CDATA[
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<message from='romeo@example.net/orchard'
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id='6410ed123'
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to='juliet@capulet.net/balcony'
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type='error'>
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<signed xmlns='urn:xmpp:signed:0'>
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<!-- original content -->
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</signed>
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<error type='modify'>
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<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
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<bad-signature xmlns='urn:ietf:params:xml:xmpp-signed:0'/>
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</error>
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</message>
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]]></example>
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<p>Additionally in Case #4, the receiving application SHOULD NOT present the stanza to the
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intended recipient (human or application) and SHOULD provide some explicit alternate
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processing of the stanza (which may be to display a message informing the recipient that it
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has received a stanza that cannot be verified).</p>
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</section1>
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<section1 topic="Inclusion and Checking of Timestamps" anchor="timestamps">
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<p>Timestamps are included to help prevent replay attacks. All timestamps MUST conform to
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&DATETIME; and be presented as UTC with no offset, always including the seconds and fractions
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of a second to three digits (resulting in a datetime 24 characters in length). Absent a local
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adjustment to the sending agent's perceived time or the underlying clock time, the sending
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agent MUST ensure that the timestamps it sends to the receiver increase monotonically (if
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necessary by incrementing the seconds fraction in the timestamp if the clock returns the same
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time for multiple requests). The following rules apply to the receiving application:</p>
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<ul style="symbols">
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<li>It MUST verify that the timestamp received is within five minutes of the current time,
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except as described below for offline messages.</li>
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<li>If the foregoing check fails, the timestamp SHOULD be presented to the receiving entity
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(human or application) marked with descriptive text indicating "old timestamp" or "future
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timestamp" and the receiving entity MAY return a stanza error to the sender (except as
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precluded in the protocol).</li>
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</ul>
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<p>The foregoing timestamp checks assume that the recipient is online when the message is
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received. However, if the recipient is offline then the server will probably store the message
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for delivery when the recipient is next online (offline storage does not apply to <iq/>
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or <presence/> stanzas, only <message/> stanzas). As described in &xep0160;, when
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sending an offline message to the recipient, the server SHOULD include delayed delivery data
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as specified in &xep0203; so that the recipient knows that this is an offline message and also
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knows the original time of receipt at the server. In this case, the recipient SHOULD verify
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that the timestamp received in the encrypted message is within five minutes of the time
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stamped by the recipient's server in the <delay/> element.</p>
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</section1>
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<section1 topic="Mandatory-to-Implement Cryptographic Algorithms" anchor="mti">
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<p>All implementations MUST support the following algorithms. Implementations MAY support other
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algorithms as well.</p>
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<ul>
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<li>TBD (RSA/SHA1? RSASSA-RKCS1-v1_5? RSASSA-PSS?)</li>
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</ul>
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</section1>
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<section1 topic="Certificates" anchor="certs">
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<p>To participate in end-to-end signing using the methods defined in this document, a client
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needs to possess an X.509 certificate. It is expected that many clients will generate their
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own (self-signed) certificates rather than obtain a certificate issued by a certification
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authority (CA). In any case the certificate MUST include an XMPP address that is represented
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using the ASN.1 Object Identifier "id-on-xmppAddr" as specified in Section 5.1.1 of
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RFC 3920bis.</p>
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</section1>
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<section1 topic="Security Considerations" anchor="security">
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<p>TBD.</p>
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</section1>
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<section1 topic="XMPP Registrar Considerations" anchor="reg">
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<section2 topic="XML Namespace Name for Signed Data in XMPP" anchor="ns">
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<p>A URN sub-namespace of signed content for the Extensible Messaging and Presence Protocol
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(XMPP) is defined as follows.</p>
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<dl>
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<di>
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<dt>URI:</dt>
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<dd>urn:xmpp:signed</dd>
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</di>
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<di>
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<dt>Specification:</dt>
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<dd>ProtoXEP</dd>
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</di>
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<di>
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<dt>Description:</dt>
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<dd>This is an XML namespace name of signed content for the Extensible Messaging and
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Presence Protocol as defined by ProtoXEP.</dd>
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</di>
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<di>
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<dt>Registrant Contact:</dt>
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<dd>XSF</dd>
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</di>
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</dl>
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</section2>
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</section1>
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<section1 topic="Acknowledgements" anchor="ack">
|
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<p>This document borrows ideas and text from End-to-End Object Encryption "work in progress" by
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Matthew Miller and Peter Saint-Andre.</p>
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</section1>
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</xep>
|