Advance to Draft as per 2019-09-19 Council vote
Remove hash function recommendations to be able to advance this without tying down the recommendations. Recommendations are now in XEP-0414.
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.
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.
2XarmwTlNxDAMkvymloX3S5+VbylNrJt/l5QyPa+YoU=]]>
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:
2AfMGH8O7UNPTvUVAM9aK13mpCY=
2XarmwTlNxDAMkvymloX3S5+VbylNrJt/l5QyPa+YoU= ]]>
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
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
Previously, this document made recommendations for specific hash functions.
Those documentations have been removed in version 0.6.0 and are now found
in Cryptographic Hash Function Recommendations for XMPP (XEP-0414)
If an entity supports the protocol defined herein, it MUST report that by including a Service Discovery (XEP-0030)
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)
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).
Specifications should take the considerations in Cryptographic Hash Function Recommendations for XMPP (XEP-0414)
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:
The XMPP Registrar
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.
urn:xmpp:hash-function-text-names:md5
Support for the MD5 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha-1
Support for the SHA-1 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha-224
Support for the SHA-224 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha-256
Support for the SHA-256 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha-384
Support for the SHA-384 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha-512
Support for the SHA-512 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha3-224
Support for the SHA3-224 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha3-256
Support for the SHA3-256 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha3-384
Support for the SHA3-384 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:sha3-512
Support for the SHA3-512 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:id-blake2b160
Support for the BLAKE2b-160 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:id-blake2b256
Support for the BLAKE2b-256 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:id-blake2b384
Support for the BLAKE2b-384 hashing algorithm
XEP-0300
urn:xmpp:hash-function-text-names:id-blake2b512
Support for the BLAKE2b-512 hashing algorithm
XEP-0300
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Thanks to Dave Cridland, Waqas Hussain, Glenn Maynard, Remko Tronçon, Paul Schaub, Christian Schudt, and Florian Schmaus for their input.