XEP-0185: Dialback Key Generation and Validation

NOTICE: This Informational specification defines a best practice or protocol profile that has been approved by the XMPP Council and/or the XSF Board of Directors. Implementations are encouraged and the best practice or protocol profile is appropriate for deployment in production systems.

Table of Contents

1. Introduction

RFC 3920 [1] 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; however, the recommendations in this document have been incorporated into rfc3920bis [2].

2. Recommended Algorithm

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

• the hostname of the Originating Server
• the hostname of the Receiving Server
• the Stream ID generated by the Receiving Server
• a secret known by the Authoritative Server's network

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 [3]) generated using the SHA256 hashing algorithm (see SHA [4]).
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. [5]

3. Example

This document closely follows the description of the dialback protocol in RFC 3920 and rfc3920bis, 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 rfc3920bis. Any line breaks in the examples are included for the purpose of readability only.

The following data values are used in the examples:

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

<stream:stream
  xmlns:stream='http://etherx.jabber.org/streams'
  xmlns='jabber:server'
  xmlns:db='jabber:server:dialback'
  to='xmpp.example.com'
  from='example.org'
  id='D60000229F'>
    

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:

<db:result
  to='xmpp.example.com'
  from='example.org'>
    37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643
</db:result>
    

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

<db:verify
  to='example.org'
  from='xmpp.example.com'
  id='D60000229F'>
    37c69b1cf07a3f67c04a5ef5902fa5114f2c76fe4a2686482ba5b89323075643
</db:verify>
    

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:

<db:verify
  to='xmpp.example.com'
  from='example.org'
  id='D60000229F'
  type='valid'/>
    

4. Security Considerations

This document introduces no security considerations or concerns above and beyond those discussed in RFC 3920 and rfc3920bis.

5. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].

6. XMPP Registrar Considerations

This document requires no interaction with the XMPP Registrar [7].

7. Acknowledgements

Thanks to Ian Paterson and Matthias Wimmer for their feedback.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0185
Publisher: XMPP Standards Foundation
Status: Active
Type: Informational
Version: 1.0
Last Updated: 2007-02-15
Approving Body: XMPP Council
Dependencies: XMPP Core
Supersedes: None
Superseded By: None
Short Name: N/A
Source Control: HTML  RSS
This document in other formats: XML  PDF

Appendix B: Author Information

Appendix C: Legal Notices

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) 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.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.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.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

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".

Appendix G: Notes

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

END