Personal Eventing Protocol (XEP-0163) [1] introduced the idea of a virtual Publish-Subscribe (XEP-0060) [2] service associated with an IM user's bare JID <localpart@domain.tld>. However, the default configuration of PEP nodes is not optimized for the persistent storage of data objects that are meant to be accessed only by the account owner, à la Private XML Storage (XEP-0049) [3]. Therefore this document defines a set of best practices that enable IM users to persistently store private information at their virtual pubsub service; in effect, we "sub-class" PEP by showing how a particular pubsub node can be configured for storing private data.
Imagine that you are a Shakespearean character named Juliet and that you want to persistently store some private information such as bookmarks (Bookmark Storage (XEP-0048) [4]).
We assume that your server (capulet.lit) supports PEP along with the "publish-options" feature, and that your client discovered that support when you logged in.
You want to start bookmarking Multi-User Chat (XEP-0045) [5] rooms, so your client stores that data privately.
<iq from='juliet@capulet.lit/balcony' type='set' id='pdp1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<publish node='storage:bookmarks'>
<item>
<conference xmlns='storage:bookmarks'
autojoin='true'
jid='theplay@conference.shakespeare.lit'
name='The Play's the Thing'>
<nick>JC</nick>
<password>Gl0b3</password>
</conference>
</item>
</publish>
<publish-options>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/pubsub#publish-options</value>
</field>
<field var='pubsub#persist_items'>
<value>true</value>
</field>
<field var='pubsub#access_model'>
<value>whitelist</value>
</field>
</x>
</publish-options>
</pubsub>
</iq>
Your publish request is a standard pubsub request except that:
If all goes well (see Storing Data), your bookmarks will be stored and the information will be pushed out to all of your resources (here "balcony" and "chamber").
<message from='juliet@capulet.lit'
to='juliet@capulet.lit/balcony'
type='headline'
id='bmfoo1'>
<event xmlns='http://jabber.org/protocol/pubsub#event'>
<items node='storage:bookmarks'>
<item id='SomeID'>
<conference xmlns='storage:bookmarks'
autojoin='true'
jid='theplay@conference.shakespeare.lit'
name='The Play's the Thing'>
<nick>JC</nick>
<password>Gl0b3</password>
</conference>
</item>
</items>
</event>
</message>
<message from='juliet@capulet.lit'
to='juliet@capulet.lit/chamber'
type='headline'
id='bmfoo2'>
<event xmlns='http://jabber.org/protocol/pubsub#event'>
<items node='storage:bookmarks'>
<item id='SomeID'>
<conference xmlns='storage:bookmarks'
autojoin='true'
jid='theplay@conference.shakespeare.lit'
name='The Play's the Thing'>
<nick>JC</nick>
<password>Gl0b3</password>
</conference>
</item>
</items>
</event>
</message>
So that's the general idea.
The best practices defined herein re-use the concepts already defined in Publish-Subscribe (XEP-0060) [2] and XEP-0163. In order to optimize for object persistence of private information instead of transient event notifications related to semi-public data, a node MUST be configured as follows:
An account owner publishes an item to a node by following the protocol specified in Publish-Subscribe (XEP-0060) [2]:
<iq from='juliet@capulet.lit/balcony' type='set' id='pdp1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<publish node='storage:bookmarks'>
<item>
<conference xmlns='storage:bookmarks'
autojoin='true'
jid='theplay@conference.shakespeare.lit'
name='The Play's the Thing'>
<nick>JC</nick>
<password>Gl0b3</password>
</conference>
</item>
</publish>
</pubsub>
</iq>
If the node does not already exist, the virtual pubsub service MUST create the node. As described in Personal Eventing Protocol (XEP-0163) [1], this "auto-create" feature (defined in XEP-0060) MUST be supported by a PEP service. (Naturally, the account owner's client MAY follow the node creation use case specified in XEP-0060 before attempting to publish an item.)
In order for the client to reliably persist private information, the virtual pubsub service must also support the "publish-options" feature defined in Publish-Subscribe (XEP-0060) [2]. Typically, a client will publish with options so that the object is privately stored.
<iq from='juliet@capulet.lit/balcony' type='set' id='pdp1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<publish node='storage:bookmarks'>
<item>
<conference xmlns='storage:bookmarks'
autojoin='true'
jid='theplay@conference.shakespeare.lit'
name='The Play's the Thing'>
<nick>JC</nick>
<password>Gl0b3</password>
</conference>
</item>
</publish>
<publish-options>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/pubsub#publish-options</value>
</field>
<field var='pubsub#persist_items'>
<value>true</value>
</field>
<field var='pubsub#access_model'>
<value>whitelist</value>
</field>
</x>
</publish-options>
</pubsub>
</iq>
If the publication logic dictates that event notifications shall be sent, the account owner's server generates notifications and sends them to all appropriate entities as described in the Receiving Event Notifications section of Personal Eventing Protocol (XEP-0163) [1].
Each item published to the node is a logically separate instance of the data to be stored. It is the responsibility of the publishing and receiving entities to construct a complete view of all such items, if desired. For example, each bookmark published to a private data node is a separate piece of data, whereas the history of all items published to the node provides a complete list of the user's bookmarks. This history may include items that are republished with an existing ItemID (thus overwriting the previous version of that item).
Before an account owner attempts to complete any of the use cases defined herein, its client MUST verify that the account owner's server supports both PEP and the "publish-options" feature; to do so, it MUST send a Service Discovery (XEP-0030) [6] information request to the server (or cache Entity Capabilities information received from the server).
<iq from='juliet@capulet.lit/balcony'
to='capulet.lit'
id='disco1'
type='get'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
The server MUST return an identity of "pubsub/pep" and include the "publish-options" feauture in the list of the namespaces and other features it supports:
<iq from='capulet.lit'
to='juliet@capulet.lit/balcony'
id='disco1'
type='result'>
<query xmlns='http://jabber.org/protocol/disco#info'>
<identity category='server' type='im'/>
<identity category='pubsub' type='pep'/>
...
<feature var='http://jabber.org/protocol/pubsub#publish-options'/>
...
</query>
</iq>
Since private data is to be stored in a mechanism originally intended to publish data, it is REQUIRED for entities to ensure that the restrictive <publish-options/> will actually be honored by the server by performing the feature discovery procedure as specified in Determining Support. If an entity using that procedure finds that the server does not support <publish-options/>, it MUST NOT store private data in PubSub, unless it can ensure privacy of the data with other means.
The configuration of a local pubsub node does not prevent an attacker or a contact with a misconfigured node from sending pubsub events with the same payload. Therefore clients MUST verify that the ‘from’ attribute on incoming event messages is either absent or equal to their own account JID.
The Security Considerations specified in Publish-Subscribe (XEP-0060) [2] and XEP-0163 need to be taken into account.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [7].
This document requires no interaction with the XMPP Registrar [8].
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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.
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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. XEP-0163: Personal Eventing Protocol <https://xmpp.org/extensions/xep-0163.html>.
2. XEP-0060: Publish-Subscribe <https://xmpp.org/extensions/xep-0060.html>.
3. XEP-0049: Private XML Storage <https://xmpp.org/extensions/xep-0049.html>.
4. XEP-0048: Bookmark Storage <https://xmpp.org/extensions/xep-0048.html>.
5. XEP-0045: Multi-User Chat <https://xmpp.org/extensions/xep-0045.html>.
6. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
7. 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/>.
8. 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 https://xmpp.org/extensions/attic/
Add notes about checking event origin (in reaction to CVE-2023-28686).
Per a vote of the XMPP Council, advanced specification to Active.
Added text about composition model, and modified examples accordingly.
Removed hokey acronym; changed type to Informational; clarified text.
Initial published version.
First draft.
@report{saint-andre2007n/a,
title = {Persistent Storage of Private Data via PubSub},
author = {Saint-Andre, Peter},
type = {XEP},
number = {0223},
version = {1.1.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-0223.html},
date = {2007-07-18/2023-03-23},
}END