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 Jabber Software Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems should not deploy implementations of this protocol until it advances to a status of Draft.

XEP Information

Author Information

Ian Paterson

Email: ian.paterson@clientside.co.uk
JID: ian@zoofy.com

Legal Notice

Discussion Venue

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 Jabber Software 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.

Conformance Terms

The following 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. Introduction
2. Public Key Publication and Retrieval via PEP
3. Requesting Public Keys Directly From Another Entity
4. Security Considerations
5. IANA Considerations
6. XMPP Registrar Considerations
7. XML Schema
Notes
Revision History

1. Introduction

This document defines different methods an entity may use for publishing its long-term public keys:

An entity MAY have multiple public keys with different formats, signatures, algorithms, strengths and expiry dates. Each client used by a user may use different keys.

2. Public Key Publication and Retrieval via PEP

An entity SHOULD use Personal Eventing via Pubsub [1] to publish all its long-term public keys via its own server.

If the pubkeys PEP node does not exist already then the entity MUST create it first. In this case, the entity SHOULD specify that the keys will only be pushed to subscribers whenever new keys are published (i.e. not when subscribers become newly available or when a new subscription is created). If the user wants to control access to his/her identity (see Security Considerations) then the entity MUST also specify an appropriate access model other than "Open".

<iq from='juliet@capulet.com/balcony' type='set' id='create1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <create node='http://jabber.org/protocol/pubkeys'/>
    <configure>
      <x xmlns='jabber:x:data' type='form'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#node_config</value>
        </field>
        <field var='pubsub#send_last_published_item'>
          <option><value>never</value></option>
        </field>
        <field var='pubsub#access_model'>
          <option><value>roster</value></option>
        </field>
        <field var='pubsub#roster_groups_allowed'>
          <option><value>friends</value></option>
        </field>
      </x>
    </configure>
  </pubsub>
</iq>

Once the publishing node has been created, the entity can update the keys at any time. Each public key MUST be wrapped in a <KeyInfo/> element as specified in XML Signature [2]. Each <KeyInfo/> element MUST contain a <KeyName/> element with a unique (for the user) name to allow the key to be referenced by other protocols. All the <KeyInfo/> elements MUST be wrapped in a single <pubkeys/> element.

Note: Any change to the list of keys requires all keys to be republished. So clients SHOULD take care to request (see third example below) and then include all keys for the JID that were published previously by other clients (unless the keys have expired). [3]

<iq from='juliet@capulet.com/balcony' type='set' id='pub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='http://jabber.org/protocol/pubkeys'>
      <item>
        <pubkeys xmlns='http://jabber.org/protocol/pubkeys'>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietRSAkey1</KeyName>
            <KeyValue>
              <RSAKeyValue>
                <Modulus>...</Modulus>
                <Exponent>AQAB</Exponent>
              </RSAKeyValue>
            </KeyValue>
          </KeyInfo>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietDSAkey1</KeyName>
            <KeyValue>
              <DSAKeyValue>
                <P>...</P><Q>...</Q><G>...</G><Y>...</Y>
              </DSAKeyValue>
            </KeyValue>
          </KeyInfo>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietX509cert1</KeyName>
            <X509Data>
              <X509IssuerSerial>
                <X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM, L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName>
                <X509SerialNumber>12345678</X509SerialNumber>
              </X509IssuerSerial>
              <X509SKI>31d97bd7</X509SKI>
              <X509SubjectName>Subject of Certificate B</X509SubjectName>
              <X509Certificate>...</X509Certificate>
              <X509Certificate>...</X509Certificate>
              <X509Certificate>...</X509Certificate>
            </X509Data>
          </KeyInfo>
          <KeyInfo xmlnss="http://www.w3.org/2000/09/xmldsig#">
            <KeyName>julietPGPkey1</KeyName>
            <PGPData>
              <PGPKeyId>...</PGPKeyId>
              <PGPKeyPacket>...</PGPKeyPacket>
            </PGPData>
          </KeyInfo>
        </pubkeys>
      </item>
    </publish>
  </pubsub>
</iq>

<message to='romeo@montague.net/garden' from='juliet@capulet.com' type='headline'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='http://jabber.org/protocol/pubkeys'>
      <item>
        <pubkeys xmlns='http://jabber.org/protocol/pubkeys'>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietRSAkey1</KeyName>
            ...
          </KeyInfo>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietDSAkey1</KeyName>
            ...
          </KeyInfo>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietX509cert1</KeyName>
            ...
          </KeyInfo>
          <KeyInfo xmlnss="http://www.w3.org/2000/09/xmldsig#">
            <KeyName>julietPGPkey1</KeyName>
            ...
          </KeyInfo>
        </pubkeys>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.com/balcony'/>
  </addresses>
</message>

Note: The stanza containing the event notification (see example above) MAY also include 'replyto' data (as specified by the Extended Stanza Addressing [4] protocol) to provide an explicit association between the published data and the resource that published it.

<iq type='get'
    to='juliet@capulet.com'
    from='romeo@montague.net/garden'
    id='items1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <items node='http://jabber.org/protocol/pubkeys'/>
  </pubsub>
</iq>

<iq type='result'
    to='romeo@montague.net/garden'
    from='juliet@capulet.com'
    id='items1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <items node='http://jabber.org/protocol/pubkeys'>
      <item>
        <pubkeys xmlns='http://jabber.org/protocol/pubkeys'>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietRSAkey1</KeyName>
            ...
          </KeyInfo>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietDSAkey1</KeyName>
            ...
          </KeyInfo>
          <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
            <KeyName>julietX509cert1</KeyName>
            ...
          </KeyInfo>
          <KeyInfo xmlnss="http://www.w3.org/2000/09/xmldsig#">
            <KeyName>julietPGPkey1</KeyName>
            ...
          </KeyInfo>
        </pubkeys>
      </item>
    </items>
  </pubsub>
</iq>

3. Requesting Public Keys Directly From Another Entity

If an entity wishes to request the public keys of another entity and it cannot access the keys via Personal Eventing via Pubsub, then the entity MAY send an IQ to the other entity:

<iq type='get'
    id='keys1'
    to='juliet@capulet.com/balcony'
    from='romeo@montague.net/garden'>
  <pubkeys xmlns='http://jabber.org/protocol/pubkeys'/>
</iq>

The other entity MUST make a careful access control decision before returning only those public keys for which it holds the corresponding private key (not necessarily the full list of keys being published via Personal Eventing via Pubsub):

<iq type='result'
    id='keys1'
    to='romeo@montague.net/garden'
    from='juliet@capulet.com/balcony'>
    <pubkeys xmlns='http://jabber.org/protocol/pubkeys'>
      <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
        <KeyName>julietRSAkey1</KeyName>
        ...
      </KeyInfo>
      <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
        <KeyName>julietX509cert1</KeyName>
        ...
      </KeyInfo>
    </pubkeys>
</iq>

If the receiving entity decides not to return the public keys, it MUST return an IQ error, which SHOULD be <service-unavailable/> (to avoid divulging presence to unauthorized entities), but MAY be some other appropriate error, such as <forbidden/> or <not-allowed/>:

<iq type='error'
    id='keys1'
    to='romeo@montague.net/garden'
    from='juliet@capulet.com/balcony'>
  <pubkeys xmlns='http://jabber.org/protocol/pubkeys'/>
  <error code='503' type='cancel'>
    <service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

4. Security Considerations

The reliable association between a user or entity and its public keys is beyond the scope of this document. However, it is RECOMMENDED that each client maintains its own secure library of the public keys (or the "fingerprints" of the keys) it associates with other users (not necessarily JIDs).

Whenever public keys are published an identity is typically associated with a JID. Although the public keys are public information, it may be critically important for the user of the JID to keep his identity secret from all but a few specified people. Implementors MUST take great care to ensure the identity of the user of a JID is never divulged to anyone except the entities who have been permitted by the user to access the public key.

5. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [5].

6. XMPP Registrar Considerations

The XMPP Registrar [6] shall add 'http://jabber.org/protocol/pubkeys' to its registry of protocol namespaces.

7. XML Schema

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='http://jabber.org/protocol/pubkeys'
    xmlns='http://jabber.org/protocol/pubkeys'
    elementFormDefault='qualified'>

  <xs:element name='pubkeys'>
    <xs:complexType>
      <xs:choice maxOccurs='unbounded'>
        <any processContents="lax" namespace="##other"/>
      </xs:choice>
    </xs:complexType>
  </xs:element>

</xs:schema>

Notes

1. XEP-0163: Personal Eventing via Pubsub <http://www.xmpp.org/extensions/xep-0163.html>.

2. XML Signature Syntax and Processing <http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/>.

3. The keys are long-term so they will only be republished infrequently.

4. XEP-0033: Extended Stanza Addressing <http://www.xmpp.org/extensions/xep-0033.html>.

5. 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/>.

6. 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 Jabber Software Foundation. For further information, see <http://www.xmpp.org/registrar/>.