Dialback Key Generation and Validation This document provides a recommended method for generating and validating the keys used in the XMPP server dialback protocol. This XMPP Extension Protocol is copyright © 1999 – 2024 by the XMPP Standards Foundation (XSF). Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation. ## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ## In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages. This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA). 0185 Active Informational Standards Council XMPP Core N/A Philipp Hancke fippo@andyet.com fippo@goodadvice.pages.de Peter Saint-Andre stpeter@stpeter.im stpeter@jabber.org https://stpeter.im/ 1.0 2007-02-15 psa

Per a vote of the XMPP Council, advanced specification to Active.

 0.4 2007-02-09 psa/ph

Modified order of explanation to ease understanding; removed discussion of alternate algorithms, which is better left to a more in-depth security analysis.

 0.3 2006-11-01 ph

Recommended hashing the secret to satisfy length requirement; hostnames and Stream ID should be separated by spaces to avoid ambiguity; updated example to match revisions to RFC 3920.

 0.2 2006-05-10 ph

Clarified and corrected roles of originating and receiving servers; updated recommendation and main example to use HMAC-SHA256 for key generation.

 0.1 2006-04-11 psa

Initial version.

 0.0.1 2006-03-30 ph

First draft.

RFC 3920 RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>. does not specify in detail a recommended algorithm for generating the keys used in the server dialback protocol. This document provides such a recommendation as an aid to implementors of XMPP servers. This document is not meant to supersede any text in RFC 3920 RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>.; however, the recommendations in this document have been incorporated into Server Dialback (XEP-0220) XEP-0220: Server Dialback <https://xmpp.org/extensions/xep-0220.html>..

The process for generating and validating a dialback key SHOULD take into account the following four inputs:

In particular, the following algorithm is RECOMMENDED:

key = HMAC-SHA256 ( SHA256(Secret), { Receiving Server, ' ', Originating Server, ' ', Stream ID } )

Note the following:

  1. The resulting dialback key is a Keyed-Hash Message Authentication Code (see HMAC The Keyed-Hash Message Authentication Code (HMAC): Federal Information Processing Standards Publication 198 <http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf>.) generated using the SHA256 hashing algorithm (see SHA Secure Hash Standard: Federal Information Processing Standards Publication 180-2 <http://csrc.nist.gov/publications/fips/fips180-2/fips186-2withchangenotice.pdf>.).
  2. The Secret is used as a "key" within the HMAC generation process; because HMAC recommends that the length of the HMAC key should be at least half the size of the hash function output, the Secret SHOULD be hashed via SHA256 prior to use in the in HMAC generation process.
  3. The Secret MUST either be set up in a configuration option for each host or process, or generated as a random string when starting the XMPP server. Creation of the Secret MUST NOT require communication between the Originating Server, the Authoritative Server, and optionally a third party (such as a database).
  4. The output of the SHA256 hashing algorithm MUST be provided in the hexadecimal representation; this helps to avoid encoding problems.
  5. The hostname of the Receiving Server, the hostname of the Originating Server, and the Stream ID SHOULD be concatenated with a Unicode space character (U+0020) as the delimiter; this helps to avoid ambiguity in concatenation. For example, the string "example.inform.example.org" could be construed as a concatenation of "example.info" and "rm.example.org" or of "example.in" and "form.example.org".

This document closely follows the description of the dialback protocol in RFC 3920 RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>. and Server Dialback (XEP-0220) XEP-0220: Server Dialback <https://xmpp.org/extensions/xep-0220.html>., but omits steps that are not important for the generation and validation of the dialback keys. For ease of comparison the numbering of the steps is the same as in section 8.3 of RFC 3920 and Appendix C.3 of XEP-0220. Any line breaks in the examples are included for the purpose of readability only.

The following data values are used in the examples:

Originating Server example.org
Authoritative Server example.org
Receiving Server xmpp.example.com
Secret s3cr3tf0rd14lb4ck
Stream ID D60000229F

3. Receiving Server sends a stream header back to the Originating Server, including a unique ID for this interaction:

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The Originating Server now generates a dialback key to be sent to the Receiving Server:

key = HMAC-SHA256( SHA256(secret), { Receiving server, ' ', Originating server, ' ', Stream ID} ) = HMAC-SHA256( SHA256('s3cr3tf0rd14lb4ck'), { 'xmpp.example.com', ' ', 'example.org', ' ', 'D60000229F' } ) = HMAC-SHA256( 'a7136eb1f46c9ef18c5e78c36ca257067c69b3d518285f0b18a96c33beae9acc', 'xmpp.example.com example.org D60000229F' ) = '37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643'

4. The Originating Server sends the generated dialback key to the Receiving Server:

37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643 ]]>

8. The Receiving Server sends the Authoritative Server a request for verification of the key:

37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643 ]]>

The Authoritative Server calculates the valid key for this verify request, using data supplied in the packet and the secret shared with the Originating Server.

key = HMAC-SHA256( SHA256(secret), { Receiving Server, ' ', Authoritative Server, ' ', Stream ID } ) = HMAC-SHA256( SHA256('s3cr3tf0rd14lb4ck'), { 'xmpp.example.com', ' ', 'example.org', ' ', 'D60000229F' } ) = HMAC-SHA256( 'a7136eb1f46c9ef18c5e78c36ca257067c69b3d518285f0b18a96c33beae9acc', 'xmpp.example.com example.org D60000229F' ) = '37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643'

9. The Authoritative Server compares this value to the key contained in the verification requests and informs the Originating Server of the result, in our example a success:

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This document introduces no security considerations or concerns above and beyond those discussed in RFC 3920 RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>. and Server Dialback (XEP-0220) XEP-0220: Server Dialback <https://xmpp.org/extensions/xep-0220.html>..

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) 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/>..

This document requires no interaction with the XMPP Registrar 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/>..

Thanks to Ian Paterson and Matthias Wimmer for their feedback.