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stpeter 2011-06-16 13:10:01 -06:00
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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>Use of Cryptographic Hash Functions in XMPP</title>
<abstract>This document provides recommendations for the use of cryptographic hash functions in XMPP protocol extensions.</abstract>
&LEGALNOTICE;
<number>XXXX</number>
<status>ProtoXEP</status>
<type>Standards Track</type>
<sig>Standards</sig>
<approver>Council</approver>
<dependencies>
<spec>XMPP Core</spec>
</dependencies>
<supersedes/>
<supersededby/>
<shortname>N/A</shortname>
&stpeter;
<revision>
<version>0.0.1</version>
<date>2011-06-16</date>
<initials>psa</initials>
<remark><p>Rough draft based on list discussion.</p></remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>Various XMPP extensions make use of cryptographic hash functions, but they do so in different ways (e.g., some define XML elements and some define XML attributes) and often mandate support for different algorthms (e.g., &xep0096; uses MD5, &xep0115; uses SHA-1, and &xep0116; used SHA-256). The lack of a consistent approach to the use of cryptographic hash functions in XMPP extensions can lead to interoperability problems and security vulnerabilities. Therefore, this document recommends a common approach and XML element that can be re-used in any XMPP protocol extension.</p>
</section1>
<section1 topic='Requirements' anchor='reqs'>
<p>This extension is designed to meet the following criteria:</p>
<dl>
<di><dt>Agility</dt><dd>It is absolutely necessary to support more secure cryptographic hash functions as they become available, and to stop supporting less secure functions as they are deprecated.</dd></di>
<di><dt>Security</dt><dd>This document needs to be regularly maintained and revisited so that XMPP protocols are using the most up-to-date security technologies.</dd></di>
<di><dt>Reusability</dt><dd>The extension needs to be reusable in any XMPP protocol.</dd></di>
</dl>
</section1>
<section1 topic='XML Element' anchor='element'>
<p>This document defines a new XML element that can be used in any XMPP protocol extension. An example follows.</p>
<code><![CDATA[
<hash xmlns='urn:xmpp:hashes:0' algo='sha-256'>
2XarmwTlNxDAMkvymloX3S5+VbylNrJt/l5QyPa+YoU=
</hash>
]]></code>
<p>The value of the 'algo' attribute MUST be one of the values from the &ianahashes; maintained by &IANA;.</p>
</section1>
<section1 topic='Namespace Versioning' anchor='versioning'>
<p>This document defines the XML namespace 'urn:xmpp:hashes:0'. The document MUST be updated, and the namespace version MUST be incremented, whenever the XSF wishes to modify the list of mandatory, recommended, deprecated, and obsolete algorithms.</p>
</section1>
<section1 topic='Hash Functions' anchor='hashes'>
<section2 topic='MD2' anchor='hashes-md2'>
<p>The MD2 algorithm is not used in any XMPP protocols and has been deprecated by the IETF (see &rfc6149;).</p>
</section2>
<section2 topic='MD4' anchor='hashes-md4'>
<p>The MD4 algorithm is not used in any XMPP protocols and has been deprecated by the IETF (see &rfc6150;).</p>
</section2>
<section2 topic='MD5' anchor='hashes-md5'>
<p>The MD5 algorithm is used in several XMPP protocols. As explained in &rfc6151;, the MD5 algorithm "is no longer acceptable where collision resistance is required" (such as in digital signatures) and "new protocol designs should not employ HMAC-MD5" either. The XSF is working to deprecate the use of MD5 in XMPP protocols.</p>
</section2>
<section2 topic='SHA-0' anchor='hashes-sha0'>
<p>The SHA-0 algorithm was developed by the U.S. National Securitiy Agency and first published in 1993. It was never widely deployed and is not used in any XMPP protocols.</p>
</section2>
<section2 topic='SHA-1' anchor='hashes-sha1'>
<p>The SHA-1 algorithm was developed by the U.S. National Security Agency and first published in 1995 to fix problems with SHA-0. The SHA-1 algorithm is currently the most widely-deployed hash function. As described in &rfc4270; in 2005, attacks have been found against the collision resistance property of SHA-1. &rfc6194; notes that no published results indicate improvement upon those attacks. In addition, RFC 6194 notes that "[t]here are no known pre-image or second pre-image attacks that are specific to the full round SHA-1 algorithm". However, the U.S. National Institute of Standards and Technology (NIST) has recommended that SHA-1 not be used for generating digital signatures after December 31, 2010. However, there is no indication that attacks on SHA-1 can be extended to HMAC-SHA-1.</p>
<p>The XSF is strongly encouraged to consider migrating its existing uses of SHA-1 to the SHA-2 family of algorithms, and to the SHA-3 family when available.</p>
</section2>
<section2 topic='SHA-2' anchor='hashes-sha2'>
<p>The SHA-2 family of algorithms (SHA-224, SHA-256, SHA-384, and SHA-512) were developed by the U.S. National Security Agency and first published in 2001. Because SHA-2 is somewhat similar to SHA-1, it is thought that the security flaws with SHA-1 described above could be extended to SHA-2 (although no such attacks have yet been found on the full-round SHA-2 algorithms).</p>
</section2>
<section2 topic='SHA-3' anchor='hashes-sha3'>
<p>The U.S. National Institute of Standards and Technology (NIST) is currently holding a public competition to replace the SHA-1 and SHA-2 functions. The winner and resulting new standard will be announced in 2012. When this "SHA-3" technology is announced, the XSF will update this specification accordingly.</p>
</section2>
</section1>
<section1 topic='Algorithm Recommendations' anchor='recommendations'>
<p>Support for version 0 of the 'urn:xmpp:hashes' namespace implies the following:</p>
<table caption='Algorithm Recommendations'>
<tr>
<th>Algorithm</th>
<th>Support</th>
</tr>
<tr>
<td>MD2</td>
<td>MUST NOT</td>
</tr>
<tr>
<td>MD4</td>
<td>MUST NOT</td>
</tr>
<tr>
<td>MD5</td>
<td>MAY</td>
</tr>
<tr>
<td>SHA-1</td>
<td>MUST</td>
</tr>
<tr>
<td>SHA-256</td>
<td>MUST</td>
</tr>
<tr>
<td>SHA-512</td>
<td>SHOULD</td>
</tr>
</table>
<p>These recommendations ought to be reviewed yearly by the &COUNCIL;.</p>
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>This entire document discusses security.</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
<p>This document requires no interaction with the IANA. However, it reuses entries from the relevant IANA registry.</p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<p>This document requires no interaction with the &REGISTRAR;.</p>
</section1>
<section1 topic='Acknowledgements' anchor='ack'>
<p>Thanks to Dave Cridland, Waqas Hussain, Glenn Maynard, and Remko Tronçon for their input.</p>
</section1>
</xep>