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 Server Dialback (XEP-0220) [2].
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:
This document closely follows the description of the dialback protocol in RFC 3920 and XEP-0220, 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:
<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'/>
This document introduces no security considerations or concerns above and beyond those discussed in RFC 3920 and XEP-0220.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].
This document requires no interaction with the XMPP Registrar [7].
Thanks to Ian Paterson and Matthias Wimmer for their feedback.
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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 <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>.
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. RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>.
2. XEP-0220: Server Dialback <https://xmpp.org/extensions/xep-0220.html>.
3. The Keyed-Hash Message Authentication Code (HMAC): Federal Information Processing Standards Publication 198 <http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf>.
4. Secure Hash Standard: Federal Information Processing Standards Publication 180-2 <http://csrc.nist.gov/publications/fips/fips180-2/fips186-2withchangenotice.pdf>.
5. 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".
6. 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/>.
7. 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/
Per a vote of the XMPP Council, advanced specification to Active.
Modified order of explanation to ease understanding; removed discussion of alternate algorithms, which is better left to a more in-depth security analysis.
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.
Clarified and corrected roles of originating and receiving servers; updated recommendation and main example to use HMAC-SHA256 for key generation.
Initial version.
First draft.
END