XEP-0189: Public Key Publishing

WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.

Table of Contents

1. Introduction

End-to-end encryption between XMPP clients and mutual authentication between a client and a server are desirable goals for the XMPP network. To enable these features, typically an XMPP user will need to possess a public key (or multiple keys), along with an associated private key that is not made public. The goal of this document is to provide simple methods for the exchange of public keys among XMPP users -- specifically, data formats that enable an XMPP entity to complete the following use cases:

• Publish its current public keys.
• Revoke keys that are no longer in use.
• Publish signed copies of its public keys (called attestations).
• Discover and retrieve public keys published by other entities.

2. Approach

This document assumes that each user will have a privately-generated key. A user might have more than one key (e.g., different keys for different clients) and the user's key might be signed by other entities, such as a certification authority (CA) or another user. However, the simplest case is a single key per user.

The data formats defined here are deliberately kept very simple, with the key itself represented as ASCII output (of the kind supported by most cryptographic libraries) and any metadata represented using a few straightforward XML elements and attributes. This document does not use the 'http://www.w3.org/2000/09/xmldsig#' namespace as specified in XML Signature [1]; although that format is quite powerful, it introduces unnecessary levels of complexity for this use case.

We define three payload elements:

1. The <pubkey/> element (qualified by the 'urn:xmpp:pubkey:1' namespace) specifies a single public key for a user.
2. The <revoke/> element (qualified by the 'urn:xmpp:revoke:1' namespace) specifies old keys that a user once used but has now revoked.
3. The <attest/> element (qualified by the 'urn:xmpp:attest:1' namespace) specifies one or more signed copies of a user's public key.

An entity MAY support only public key publishing (the 'urn:xmpp:pubkey:1' namespace) and not support revocations and attestations.

3. Public Keys

A single public key is contained in a <pubkey/> element qualified by the 'urn:xmpp:pubkey:1' namespace (see Namespace Versioning regarding the possibility of incrementing the version number), for which the defined child elements are as follows:

An example follows.

Example 1. A public key

<pubkey xmlns='urn:xmpp:pubkey:1'>
  <begin>2009-12-11T20:12:37</begin>
  <end>2010-12-11T23:59:59</end>
  <jid>peter@jabber.org</jid>
  <key>
    MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA25/Y7oonF3+pRjZCk0cQ
    lQFiUFvyMeUOnn01NLfqfBeT4xjahsxaYFPd+V8jIHtpO3nbkpfrBh5aLPCkqDzS
    UFvmLaVSIMUt085kcT/Yv72BUWUGaISt7swSNfictRbVZ0k8TbXUDLIAYWN8lsie
    NVxL13PS9pu3WNoORuKAV5RuzDS3djnjb6fti7fTwEUpQVwRrJXY6jNhv4c4zE3x
    Pjm48gIlHbZrxuWNrXnKT6uwXnr6PJ6O3YspferJG1oA3kmOwm41yWDkalbKgkNZ
    WeS+ahB/i9LIG+41CdekPpI0Kr0Ll9X7+zaPV6xzjlG8dYy7ZGm4eTL9STuWlPdL
    +wIDAQAB
  </key>
  <print>13475c8e27399908b4447d7c52ab30822872832eba3a654f0d80e07fb4157673</print>
</pubkey>
  

4. Revocations

A user can revoke his own key. The revocation is contained in a <revoke/> element qualified by the 'urn:xmpp:revoke:1' namespace (see Namespace Versioning regarding the possibility of incrementing the version number), for which the defined child elements are as follows:

The data that is signed MUST be the revoked public key (i.e., the XML character data of the <key/> element), the SHA-256 fingerprint of the revoked public key (i.e., the XML character data of the <keyprint/> element), the SHA-256 fingerprint of the key used to sign the revocation (i.e., the XML character data of the <revocationprint/> element), and the time of the revocation (i.e., the XML character data of the <revocationtime/> element), concatenated together with no spaces or other additional characters.

As an example, consider a revocation of the key from Example 1, where the SHA-256 fingerprint of the revoked key is 13475c8e27399908b4447d7c52ab30822872832eba3a654f0d80e07fb4157673, the SHA-256 fingerprint of the key used to sign the revocation is becb78566783166f4a1a7c64e28dae288fe1a0f2825f6b593b336ce186c6b056, and the time of the revocation is 2009-12-14T20:49:16Z. The string to be signed would be as follows (where line breaks are not significant).

    MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA25/Y7oonF3+pRjZCk0cQ
    lQFiUFvyMeUOnn01NLfqfBeT4xjahsxaYFPd+V8jIHtpO3nbkpfrBh5aLPCkqDzS
    UFvmLaVSIMUt085kcT/Yv72BUWUGaISt7swSNfictRbVZ0k8TbXUDLIAYWN8lsie
    NVxL13PS9pu3WNoORuKAV5RuzDS3djnjb6fti7fTwEUpQVwRrJXY6jNhv4c4zE3x
    Pjm48gIlHbZrxuWNrXnKT6uwXnr6PJ6O3YspferJG1oA3kmOwm41yWDkalbKgkNZ
    WeS+ahB/i9LIG+41CdekPpI0Kr0Ll9X7+zaPV6xzjlG8dYy7ZGm4eTL9STuWlPdL
    +wIDAQAB13475c8e27399908b4447d7c52ab30822872832eba3a654f0d80e07f
    b4157673becb78566783166f4a1a7c64e28dae288fe1a0f2825f6b593b336ce1
    86c6b0562009-12-14T20:49:16Z
  

An example follows.

Example 2. A revocation

<revocation xmlns='urn:xmpp:revoke:1'>
  <key>
    MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA25/Y7oonF3+pRjZCk0cQ
    lQFiUFvyMeUOnn01NLfqfBeT4xjahsxaYFPd+V8jIHtpO3nbkpfrBh5aLPCkqDzS
    UFvmLaVSIMUt085kcT/Yv72BUWUGaISt7swSNfictRbVZ0k8TbXUDLIAYWN8lsie
    NVxL13PS9pu3WNoORuKAV5RuzDS3djnjb6fti7fTwEUpQVwRrJXY6jNhv4c4zE3x
    Pjm48gIlHbZrxuWNrXnKT6uwXnr6PJ6O3YspferJG1oA3kmOwm41yWDkalbKgkNZ
    WeS+ahB/i9LIG+41CdekPpI0Kr0Ll9X7+zaPV6xzjlG8dYy7ZGm4eTL9STuWlPdL
    +wIDAQAB
  </key>
  <keyprint>60561AEE-4187-4122-8A71-ACF9195DE572</keyprint>
  <revocationprint>becb78566783166f4a1a7c64e28dae288fe1a0f2825f6b593b336ce186c6b056</revocationprint>
  <revocationtime>2009-12-14T20:49:16Z</revocationtime>
  <signature>
    iEYEARECAAYFAksnvfsACgkQNL8k5A2w/vyVHgCglI5fPQfcu1e5PTA3bMPKiL0F
    J5cAnirpKzJ45OD+03b66UHoIosKQ31O
    =PGFT
  </signature>
</revocation>
  

5. Attestations

Whether in the context of the public key infrastructure (PKI) or a web of trust, a user might want to publish signed copies of his public key, where the signer might be another user or a trusted third party such as a certification authority (CA). The signing key could even be another key owned by the user (e.g., a primary key used to sign a secondary key associated with a particular device controlled by the user). Furthermore, the signing material might be an RSA key, a DSA key, an X.509 certificate, an OpenPGP key, or any other format.

Each signed copy is contained in an <attest/> element qualified by the 'urn:xmpp:attest:1' namespace (see Namespace Versioning regarding the possibility of incrementing the version number), for which the defined child elements are as follows:

The data that is signed MUST be the user's public key (i.e., the XML character data of the <key/> element), the SHA-256 fingerprint of the user's public key (i.e., the XML character data of the <keyprint/> element), the signer's JID (i.e., the XML character data of the <signerjid/> element), the SHA-256 fingerprint of the signer's key (i.e., the XML character data of the <signerprint/> element), and the time of the attestation (i.e., the XML character data of the <signtime/> element), concatenated together with no spaces or other additional characters.

As an example, consider an attestation of the key from Example 1, where the signer's JID is stpeter@jabber.org, the fingerprint of the signer's key is "becb78566783166f4a1a7c64e28dae288fe1a0f2825f6b593b336ce186c6b056", and the time of the attestation is 2009-12-14T18:43:00Z. The string to be signed would be as follows (where line breaks are not significant).

    MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA25/Y7oonF3+pRjZCk0cQ
    lQFiUFvyMeUOnn01NLfqfBeT4xjahsxaYFPd+V8jIHtpO3nbkpfrBh5aLPCkqDzS
    UFvmLaVSIMUt085kcT/Yv72BUWUGaISt7swSNfictRbVZ0k8TbXUDLIAYWN8lsie
    NVxL13PS9pu3WNoORuKAV5RuzDS3djnjb6fti7fTwEUpQVwRrJXY6jNhv4c4zE3x
    Pjm48gIlHbZrxuWNrXnKT6uwXnr6PJ6O3YspferJG1oA3kmOwm41yWDkalbKgkNZ
    WeS+ahB/i9LIG+41CdekPpI0Kr0Ll9X7+zaPV6xzjlG8dYy7ZGm4eTL9STuWlPdL
    +wIDAQAB13475c8e27399908b4447d7c52ab30822872832eba3a654f0d80e07f
    b4157673stpeter@jabber.orgbecb78566783166f4a1a7c64e28dae288fe1a0
    f2825f6b593b336ce186c6b0562009-12-14T18:43:00Z
  

Example 3. RSA public key signed by OpenPGP key

<attest xmlns='urn:xmpp:attest:1'>
  <keyprint>60561AEE-4187-4122-8A71-ACF9195DE572</keyprint>
  <signature>
    iEYEARECAAYFAksnyJ0ACgkQNL8k5A2w/vy9DgCfc1eOYZlp1P456NkjgvCtJaRX
    Wi4AoIGhfzr7XMzeOzOBi9A/bxopH8O4
    =MMuz
  </signature>
  <signerjid>stpeter@jabber.org</signerjid>
  <signerprint>becb78566783166f4a1a7c64e28dae288fe1a0f2825f6b593b336ce186c6b056</signerprint>
  <signtime>2009-12-14T18:43:00Z</signtime>
</attest>
  

6. Publication via PEP

A user SHOULD publish public keys, attestations, and revocations via Personal Eventing Protocol [4], in particular adhering to the best practices defined in Best Practices for Persistent Storage of Public Data via Publish-Subscribe [5].

Any other authorized entity can then receive notifications related to the user's public keys, and retrieve keys, attestations, and revocations.

If the user has only one public key (the simplest and most common case), then the pubkey PEP node SHOULD have only one item, with an ItemID of "current".

7. Requesting a Public Key Directly

If it is not possible for the user to publish his key via PEP, or for a contact to retrieve the user's key via PEP, the contact MAY request the user's key directly by sending an <iq/> of type 'get' to the user's particular full JID, containing an empty <pubkey/> element qualified by the 'urn:xmpp:pubkey:1' namespace (see Namespace Versioning regarding the possibility of incrementing the version number).

Example 4. Public key request

<iq from='maineboy@jabber.org/bar'
    to='peter@jabber.org/foo'
    id='hfgt654s'
    type='get'>
  <pubkey xmlns='urn:xmpp:pubkey:1'/>
</iq>
  

The receiving client then return its public key (which might be different from the overall key for the user, e.g. a key for only a particular device).

Example 5. Public key response

<iq from='peter@jabber.org/foo'
    to='maineboy@jabber.org/bar'
    id='hfgt654s'
    type='result'>
  <pubkey xmlns='urn:xmpp:pubkey:1'>
    <begin>2009-12-11T20:12:37</begin>
    <end>2010-12-11T23:59:59</end>
    <jid>peter@jabber.org</jid>
    <key>
      MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA25/Y7oonF3+pRjZCk0cQ
      lQFiUFvyMeUOnn01NLfqfBeT4xjahsxaYFPd+V8jIHtpO3nbkpfrBh5aLPCkqDzS
      UFvmLaVSIMUt085kcT/Yv72BUWUGaISt7swSNfictRbVZ0k8TbXUDLIAYWN8lsie
      NVxL13PS9pu3WNoORuKAV5RuzDS3djnjb6fti7fTwEUpQVwRrJXY6jNhv4c4zE3x
      Pjm48gIlHbZrxuWNrXnKT6uwXnr6PJ6O3YspferJG1oA3kmOwm41yWDkalbKgkNZ
      WeS+ahB/i9LIG+41CdekPpI0Kr0Ll9X7+zaPV6xzjlG8dYy7ZGm4eTL9STuWlPdL
      +wIDAQAB
    </key>
    <print>13475c8e27399908b4447d7c52ab30822872832eba3a654f0d80e07fb4157673</print>
  </pubkey>
</iq>
  

8. Sending a Public Key Directly

An entity might need to send its public key to another entity, for example if it has generated a new key but does not have a way to publish the new key (or does not wish to publish the key in a world-readable fashion). In this case the entity MAY include the key directly in a <message/> stanza.

Example 6. Sending a key in a message

<message from='peter@jabber.org/foo'
         to='maineboy@jabber.org/bar'>
  <pubkey xmlns='urn:xmpp:pubkey:1'>
    <begin>2009-12-11T20:12:37</begin>
    <end>2010-12-11T23:59:59</end>
    <jid>peter@jabber.org</jid>
    <key>
      MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA25/Y7oonF3+pRjZCk0cQ
      lQFiUFvyMeUOnn01NLfqfBeT4xjahsxaYFPd+V8jIHtpO3nbkpfrBh5aLPCkqDzS
      UFvmLaVSIMUt085kcT/Yv72BUWUGaISt7swSNfictRbVZ0k8TbXUDLIAYWN8lsie
      NVxL13PS9pu3WNoORuKAV5RuzDS3djnjb6fti7fTwEUpQVwRrJXY6jNhv4c4zE3x
      Pjm48gIlHbZrxuWNrXnKT6uwXnr6PJ6O3YspferJG1oA3kmOwm41yWDkalbKgkNZ
      WeS+ahB/i9LIG+41CdekPpI0Kr0Ll9X7+zaPV6xzjlG8dYy7ZGm4eTL9STuWlPdL
      +wIDAQAB
    </key>
    <print>13475c8e27399908b4447d7c52ab30822872832eba3a654f0d80e07fb4157673</print>
  </pubkey>
</message>
  

9. Signalling Key Generation

There are several situations in which it is helpful for an entity to signal that it is currently generating a key. For example, a client that does not have access to permanent storage might generate a key on startup, but key generation might not be complete when the client sends initial presence upon establishing an XMPP session. In this case the client might signal support for the public key format in the entity capabilities data that it includes in its initial presence broadcast, but also include an indication that it is currently generating a key.

Example 7. Key generation in progress

<presence from='peter@jabber.org/foo'>
  <generating xmlns='urn:xmpp:pubkey:1'/>
</presence>
  

After key generation is complete, the entity could publish the new key to the appropriate PEP node (if available) and signal that key generation is complete so that any interested parties can request the new key.

Example 8. Key generation completed

<presence from='peter@jabber.org/foo'>
  <generated xmlns='urn:xmpp:pubkey:1'/>
</presence>
  

10. Determining Support

To advertise its support for public keys, revocations, and attestations, when replying to Service Discovery [6] information requests an entity MUST return features for 'urn:xmpp:pubkey:1', 'urn:xmpp:revoke:1', and 'urn:xmpp:attest:1' respectively.

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 [7]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

11. Security Considerations

The reliable association between a user or entity and its public keys is beyond the scope of this document. However, each client SHOULD maintain its own secure library of the public keys it associates with other users.

12. IANA Considerations

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

13. XMPP Registrar Considerations

14. XML Schemas

To follow.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0189
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Standards Track
Version: 0.11
Last Updated: 2009-12-17
Approving Body: XMPP Council
Dependencies: XMPP Core, XEP-0060, XEP-0163
Supersedes: None
Superseded By: None
Short Name: NOT_YET_ASSIGNED
Source Control: HTML  RSS

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