XEP-xxxx: Pre-auth Registration Key Generation and Validation

Abstract
This specification updates XEP-0401 and XEP-0445 by specifying a shared format for the pre-authenticated registration token.
Author
Sam Whited
Copyright
© 1999 – 2021 XMPP Standards Foundation. SEE LEGAL NOTICES.
Status

ProtoXEP

WARNING: This document has not yet been accepted for consideration or approved in any official manner by the XMPP Standards Foundation, and this document is not yet an XMPP Extension Protocol (XEP). If this document is accepted as a XEP by the XMPP Council, it will be published at <https://xmpp.org/extensions/> and announced on the <standards@xmpp.org> mailing list.
Type
Standards Track
Version
0.0.1 (2021-06-06)
Document Lifecycle
  1. Experimental
  2. Proposed
  3. Stable
  4. Final

1. Introduction

Both Easy User Onboarding (XEP-0401) [1] and Pre-Authenticated In-Band Registration (XEP-0445) [2] specify a mechanism for requesting a token from a server that can be exchanged for registration at a later date. However, neither XEP defines the format of this token, or a recommended algorithm for generating it. This means that each server may choose a unique format and that any token issuing entity must either only support a specific servers token format, or must connect to the server with an admin account so that it can ask the server for tokens.

This specification rectifies this by specifying a server-agnostic format for pre-auth tokens and an algorithm for generating them. This enables third-party trusted services that share a private key to sign a token that can later be verified by the server to register a user.

2. Requirements

3. Glossary

Authentication server
The server authenticating the user using IBR or SASL, normally the XMPP server.
Authorization server
A server issuing a token authorizing the user to register. This may be the XMPP server, or another entity that shares its private key.
IBR
In-band Registration, as defined by either In-Band Registration (XEP-0077) [3] or Extensible In-Band Registration (XEP-0389) [4]
Key
A shared secret that is used to sign and validate tokens.

4. Use Cases

5. Business Rules

The following algorithm is used to generate tokens where "," is a separator and not part of any actual input and $key is the shared secret key. All uses of base64 are the Raw URL encoding (with no padding characters) defined in RFC 4648 [5] HMAC-SHA256 is a Keyed-Hash Message Authentication Code (see HMAC [6]) using the SHA256 hashing algorithm (see SHA [7]).

// Current time rounded up and converted to milliseconds.
expiration = ($currentTime + 1e6 -1) / 1e6
jids       = {
               JID1, ':',
               JID2, ':',
               JID3, ':',
               …
               JIDFinal
             }
signature  = HMAC-SHA256
             (
               $key,
               $jids, ':',
               $expiration
             )
token      = {
               base64-raw-url($signature), ':',
               base64-raw-url($jids), ':',
               $expiration,
             }
  

6. Implementation Notes

If the shared key is longer than the block size it will be hashed by some HMAC implementations, otherwise it is left unhashed. This is not represented in the algorithm above, but if it is not done the user must determine whether the input key needs hashing themselves. Input keys shorter than the block size are not hashed.

7. Security Considerations

The secret key SHOULD be at least half the length of the SHA256 output (ie. 16 bytes). No key stretching is performed by this algorithm, so the user should take care to pick a long key.

Creation of the secret key MUST NOT require communication between the authentication server, the authorization server, or a third party such as a database.

8. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [8]

9. XMPP Registrar Considerations

This document requires no action from the XMPP Registrar [9]

10. XML Schema

This document does not define an XML namespace requiring a schema.


Appendices

Appendix A: Document Information

Series
XEP
Number
xxxx
Publisher
XMPP Standards Foundation
Status
ProtoXEP
Type
Standards Track
Version
0.0.1
Last Updated
2021-06-06
Approving Body
XMPP Council
Dependencies
XMPP Core, XEP-0401, XEP-0445
Supersedes
None
Superseded By
None
Short Name
preauth-token

This document in other formats: XML  PDF

Appendix B: Author Information

Sam Whited
Email
sam@samwhited.com
JabberID
sam@samwhited.com
URI
https://blog.samwhited.com/

Copyright

This XMPP Extension Protocol is copyright © 1999 – 2020 by the XMPP Standards Foundation (XSF).

Permissions

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.

Disclaimer of Warranty

## 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. ##

Limitation of Liability

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.

IPR Conformance

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).

Visual Presentation

The HTML representation (you are looking at) is maintained by the XSF. It is based on the YAML CSS Framework, which is licensed under the terms of the CC-BY-SA 2.0 license.

Appendix D: Relation to XMPP

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.

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 <https://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

1. XEP-0401: Easy User Onboarding <https://xmpp.org/extensions/xep-0401.html>.

2. XEP-0445: Pre-Authenticated In-Band Registration <https://xmpp.org/extensions/xep-0445.html>.

3. XEP-0077: In-Band Registration <https://xmpp.org/extensions/xep-0077.html>.

4. XEP-0389: Extensible In-Band Registration <https://xmpp.org/extensions/xep-0389.html>.

5. RFC 4648: The Base16, Base32, and Base64 Data Encodings <http://tools.ietf.org/html/rfc4648>.

6. The Keyed-Hash Message Authentication Code (HMAC): Federal Information Processing Standards Publication 198 <http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf>.

7. Secure Hash Standard: Federal Information Processing Standards Publication 180-2 <http://csrc.nist.gov/publications/fips/fips180-2/fips186-2withchangenotice.pdf>.

8. 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/>.

9. 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/>.

Appendix H: Revision History

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

  1. Version 0.0.1 (2021-06-06)

    First draft.

    ssw

Appendix I: Bib(La)TeX Entry

@report{whited2021preauth-token,
  title = {Pre-auth Registration Key Generation and Validation},
  author = {Whited, Sam},
  type = {XEP},
  number = {xxxx},
  version = {0.0.1},
  institution = {XMPP Standards Foundation},
  url = {https://xmpp.org/extensions/xep-xxxx.html},
  date = {2021-06-06/2021-06-06},
}

END