<remark><p>Modified XML structure to remove wrapper element; added recommendations for new XMPP extensions; softened recommendations for existing extensions.</p></remark>
<remark><p>Adjusted format to include multiple hashes in one element; modified namespace versioning rules to align with common practice; added service discovery features for various algorithms.</p></remark>
<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 algorithms. 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>
<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>
<p>An XMPP protocol can include more than one instance of the <hash/> element, as long as each one has a different value for the 'algo' attribute:</p>
<p>The MD5 algorithm was commonly used in earlier generations of Internet technologies. 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.</p>
<p>The primary use of MD5 in XMPP protocols is &xep0096;, which will be obsoleted by &xep0234;.</p>
<p>The SHA-0 algorithm was developed by the U.S. National Security Agency and first published in 1993. It was never widely deployed and is not used in any XMPP protocols.</p>
<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 as of 2011 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". Furthermore, there is no indication that attacks on SHA-1 can be extended to HMAC-SHA-1. Nevertheless, 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.</p>
<p>The SHA-2 family of algorithms (SHA-224, SHA-256, SHA-384, and SHA-512) was 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>
<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>
<p>If an entity supports the protocol defined herein, it MUST report that by including a &xep0030; feature of "urn:xmpp:hashes:1" in response to disco#info requests, along with one service discovery feature for each algorithm it supports:</p>
<p>In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.</p>
<p>The XSF is strongly encouraged to incorporate hash agility into new XMPP extensions that it develops by mandating re-use of the protocol defined in this specification (instead of hash elements or attributes specific to each extension).</p>
<section1topic='Analysis of Existing XMPP Extensions'anchor='existing'>
<p>As mentioned, several existing XMPP extensions make use of the SHA-1 algorithm. This section analyzes those extensions. The final subsection provides recommendations.</p>
<p>Both &xep0065; and &xep0260; use SHA-1 to hash the Stream ID, Requester's JID, and Target's JID, and this hash can be communicated via the 'dstaddr' attribute. Although this usage is not security-critical, currently it has no agility to specify newer algorithms. Because the hash is communicated by means of an attribute, it cannot directly use the extension defined in this specification.</p>
<p>In &xep0084;, the &xep0060; ItemId for the metadata node is the SHA-1 hash of the image data for the "image/png" media type. There is no hash agility for this usage. Although attacks against the collision resistance property could potentially result in confusion over the avatar for a user, the fact that avatars cannot be uploaded without authentication as the node owner or authorization as a node publisher reduces the practicality of attacks. In addition, XEP-0084 ought to be updated to specify that avatars must not be compared across JIDs.</p>
<p>&xep0115; typically uses SHA-1 to compute the verification string, however hash agility is supported by use of the 'hash' attribute. Because the hash is communicated by means of an attribute, it cannot directly use the extension defined in this specification.</p>
<p>&xep0124; uses SHA-1 to generate the key sequence used to secure sessions that are not protected via SSL/TLS. Because these keys are ephemeral, it is unlikely that an attacker could reproduce or poison the key sequence quickly enough to successfully attack the session. However, attackers can be discouraged more significantly by protecting sessions with SSL/TLS (indeed, it is unclear how widely the key sequence feature is implemented). That said, this use of SHA-1 in BOSH does not support hash agility.</p>
<p>&xep0174; uses SHA-1 to hash the avatar image (i.e., the "phsh" field) advertised in the DNS TXT record for a user, mirroring the usage from XEP-0115. The "hash" field can be used to specify alternative hash algorithms, and thus supports hash agility. However, in practice it is likely that only SHA-1 is implemented. Because the hash is represented in a DNS TXT record, it cannot directly use the extension defined in this specification.</p>
<p>&xep0231; supports hash agility through the structure of values for the 'cid' attribute, but does not mandate support for any particular algorithm.</p>
<p>Of the foregoing, the use in <cite>XEP-0115</cite> has the most significant security implications. However, there are other security issues with <cite>XEP-0115</cite> that make it likely to be replaced in a more wholesale fashion. Although it would be desirable for all XMPP extensions that use cryptographic hashes to incorporate hash agility, realistically this is difficult to achieve after the fact. For now, the XSF is encouraged to focus on new protocols (e.g., XEP-0234 and a replacement for XEP-0115 if there is consensus to work on the latter) rather than spending effort on migrating its existing uses of SHA-1 to the SHA-2 family of algorithms, and to the SHA-3 family when available. Naturally, these priorities might change if XMPP technologies experience significant attacks on existing extensions that use SHA-1.</p>
<p>An entity SHOULD provide one service discovery feature for each algorithm it supports. Ideally these features would be of the form "urn:iana:hash-function-text-names:foo" (where "foo" is the name of an algorithm registered with the IANA); however there is no urn:iana namespace at present. Until there is, we use features of the form "urn:xmpp:hash-function-text-names:foo" instead. Therefore the registry submission is as follows.</p>
