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.
This extension is designed to meet the following criteria:
This document defines a new XML element that can be used in any XMPP protocol extension. An example follows.
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:
In certain scenarios it makes sense to communicate the hash algorithm that is used prior to the calculation of the hash value.
The value of the 'algo' attribute MUST be one of the values from the IANA Hash Function Textual Names Registry  maintained by the Internet Assigned Numbers Authority (IANA) , or one of the values defined in the following table.
|Hash Function Name||Reference|
|"sha3-256"||FIPS PUB 202: SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions |
|"sha3-512"||FIPS PUB 202: SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions |
|"blake2b-256"||RFC 7693 |
|"blake2b-512"||RFC 7693 |
The digest produced by the used hash algorithm is included as the XML character data of the <hash/> element after being encoded using Base64 as specified in Section 4 of RFC 4648 . Thus the character data MUST conform to the base64Binary datatype  as defined in XML Schema Part 2 . The Base64 output MUST NOT include whitespace and MUST set padding bits to zero.
The MD5 algorithm was commonly used in earlier generations of Internet technologies. As explained in RFC 6151 , 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 currently known best attack against the pre-image resistance property of the MD5 algorithm is slightly better than the generic attack and was released 2009 .
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.
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 RFC 4270  in 2005, attacks have been found against the collision resistance property of SHA-1. RFC 6194  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.
In fall 2015 the SHA-1 collision cost has been estimated between 75K$ to 120K$ .
The SHA-1 algorithm is used in a number of XMPP protocols. See Analysis of Existing XMPP Extensions for details.
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).
The SHA-3 family of algorithms (SHA3-224, SHA3-256, SHA3-384, and SHA3-512) is based on the Keccak algortihm developed by Guido Bertoni, Joan Daemen, Michaël Peeters, and Gilles Van Assche, and was pubished by NIST on August 5, 2015 in FIPS PUB 202: SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions  after a public hash function competition.
The BLAKE2 family of algorithms was designed by Jean-Philippe Aumasson, Samuel Neves, Zooko Wilcox-O'Hearn, and Christian Winnerlein. It is described in RFC 7693  and is designed to be highly secure and run well on both software and hardware platforms.
Support for version 1 of the 'urn:xmpp:hashes' namespace implies the following:
|MD2||128 bits||MUST NOT|
|MD4||128 bits||MUST NOT|
|MD5||128 bit||MUST NOT|
|SHA-1||160 bits||SHOULD NOT|
These recommendations ought to be reviewed yearly by the XMPP Council .
If an entity supports the protocol defined herein, it MUST report that by including a Service Discovery (XEP-0030)  feature of "urn:xmpp:hashes:2" in response to disco#info requests, along with one service discovery feature for each algorithm it supports:
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 Entity Capabilities (XEP-0115) . However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
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).
As mentioned, several existing XMPP extensions make use of the SHA-1 algorithm. This section analyzes those extensions. The final subsection provides recommendations.
Both SOCKS5 Bytestreams (XEP-0065)  and Jingle SOCKS5 Bytestreams Transport Method (XEP-0260)  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.
In User Avatar (XEP-0084) , the Publish-Subscribe (XEP-0060)  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.
Entity Capabilities (XEP-0115)  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.
BOSH (XEP-0124)  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.
Link-Local Messaging (XEP-0174)  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.
Of the foregoing, the use in XEP-0115 has the most significant security implications. However, there are other security issues with XEP-0115 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.
This entire document discusses security.
This document requires no interaction with the IANA. However, it reuses entries from the relevant IANA registry.
This specification defines the following XML namespace:
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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.
Thanks to Dave Cridland, Waqas Hussain, Glenn Maynard, Remko Tronçon, Paul Schaub, Christian Schudt, and Florian Schmaus for their input.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <firstname.lastname@example.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
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The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
1. IANA registry of Hash Function Textual Names <http://www.iana.org/assignments/hash-function-text-names>.
2. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
17. The XMPP Council is a technical steering committee, authorized by the XSF Board of Directors and elected by XSF members, that approves of new XMPP Extensions Protocols and oversees the XSF's standards process. For further information, see <https://xmpp.org/about/xmpp-standards-foundation#council>.
28. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Clarify textual content of the <hash/> element.
Add hash-used element
Use xs:base64Binary instead of xs:string in the schema
Explicitly specify encoding format. Namespace version bump to urn:xmpp:hashes:2.
Updating to current knowledge on security of algorithms. Adding SHA-3 and BLAKE families of hashes.
Modified XML structure to remove wrapper element; added recommendations for new XMPP extensions; softened recommendations for existing extensions.
Updated to reflect initial analysis of existing XMPP protocol extensions.
Initial published version.
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.
Rough draft based on list discussion.