While it is nowadays possible to encrypt pubsub items with OpenPGP for XMPP Pubsub, this specification is designed for pubsub nodes were all items are end-to-end encrypted, and it is using symmetric encryption with a system of key sharing, meaning that if a key is available, it can decrypt all items encrypted with it.
This is fine for most use cases, however it may be desirable to only encrypt a few items with properties such as Perfect Forward Secrecy. This specification describes a way on how to do that by adapting existing end-to-end encryption algorithms used in instant messaging to pubsub items. This may be used to implement restricted items (a feature known is some other software such as "aspects" or "circles") or for transient nodes.
The design goal of this specification is do adapt simply existing e2e encryption algorithms used for messages to pubsub items.
Juliet holds a public blog using Microblogging Over XMPP (XEP-0277) [1]. However, she wants to publish a new items that should be visible only to some well targeted users. To do so she encrypt the payload in the same way as she encrypt messages with algorithm such as OMEMO Encryption (XEP-0384) [2]. She wrap the encrypted payload in an <encrypted/> element qualified by the 'urn:xmpp:pte:0' namespace which MUST have a 'by' attribute with its own bare jid as value, and which MUST have a 'type' attribute whose value is the namespace of the algorithm used.
She decides to use OMEMO Encryption (XEP-0384) [2] to encrypt her items, her client publish an item like this:
<iq xmlns="jabber:client" id="pte_1" type="set" from="juliet@capulet.lit/chamber" to="juliet@capulet.lit">
<pubsub xmlns="http://jabber.org/protocol/pubsub">
<items node="urn:xmpp:microblog:0">
<item id="secret_blog_post" publisher="juliet@capulet.lit/chamber">
<encrypted xmlns="urn:xmpp:pte:0" by="juliet@capulet.lit" type="urn:xmpp:omemo:2">
<encrypted xmlns="urn:xmpp:omemo:2">
<header sid="878841001">
<keys jid="juliet@capulet.lit">
<key rid="673880319">ChDRqSBLTR+RtRIH8io7kf22EmgIARACGiCasIYfB6Zfe5SNyT8twIa+mEYA8h7uEQIjQ64dJx4vXiJAZSpXPRW+sVVSC7gc4lDEiTA4DT7AIh/woa82PFjgFdL0A8HTyBe7yh3UWThZGuTp5A3zmjXH7pAwKX85oxQ8XA==</key>
</keys>
</header>
<payload>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</payload>
</encrypted>
</encrypted>
</item>
</items>
</pubsub>
</iq>
The properties of the encryption algorithm applies. For instance in the case of OMEMO Encryption (XEP-0384) [2], there Perfect Forward Secrecy, meaning that once an item has been decrypted once by a targeted entity, it can't be decrypted anymore. Client shoud then handle pubsub caching of the decrypted item when necessary.
If a client supports the protocol specified in this XEP, it MUST advertise it by including the "urn:xmpp:pte:0" discovery feature in response to a Service Discovery (XEP-0030) [3] information request, Then the supported encryption algorithms are announced as explained in their respective XEPs.
<iq type='get'
from='juliet@example.org/balcony'
to='romeo@example.org/orchard'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<iq type='result'
from='romeo@example.org/orchard'
to='juliet@example.org/balcony'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'>
...
<feature var='urn:xmpp:pte:0'/>
<feature var='urn:xmpp:omemo:2'/>
...
</query>
</iq>Security Considerations of used encryption specifications apply.
TODO
TODO
TODO
Thanks to NLNet foundation/NGI0 Discovery for funding.
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1. XEP-0277: Microblogging over XMPP <https://xmpp.org/extensions/xep-0277.html>.
2. XEP-0384: OMEMO Encryption <https://xmpp.org/extensions/xep-0384.html>.
3. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
First draft.
@report{poisson2022pte,
title = {Pubsub Targeted Encryption},
author = {Poisson, Jérôme},
type = {XEP},
number = {xxxx},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2022-10-31/2022-10-31},
}END