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protoXEP OpenPGP for XMPP Pubsub: version 0.0.2
Remove useless reference to XEP-0374 + acknowledgements
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Jérôme Poisson
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b06ddd6b8c
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<spec>XEP-0004</spec>
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<spec>XEP-0060</spec>
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<spec>XEP-0373</spec>
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<spec>XEP-0374</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<email>goffi@goffi.org</email>
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<jid>goffi@jabber.fr</jid>
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</author>
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<revision>
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<version>0.0.2</version>
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<date>2022-10-10</date>
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<initials>jp</initials>
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<remark><p>Remove useless references to XEP-0374 + acknowledgements</p></remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2022-10-09</date>
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<section1 topic='Introduction' anchor='intro'>
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<p>&xep0060; and &xep0163; are widely used XMPP features. However, items are published in plain text, making them readable by server administrators or anybody having administrator level access on the pubsub/PEP services.</p>
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<p>Although this is not a problem for data intended to be published publicly, there are many cases where it may be desirable to make the data inaccessible to anybody but the selected recipients, or in other words to end-to-end encrypt the data.</p>
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<p>This specification aims to provide an easy way to e2e encrypt items of a pubsub node, and to share access with other entities. It is compatible with all existing pubsub/PEP features, which makes it possible to use securily features such a geolocation, private blogs, private calendar events, data forms, etc. This specification is based on &xep0373;.</p>
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<p>This specification aims to provide an easy way to e2e encrypt items of a pubsub node, and to share access with other entities. It is compatible with all existing pubsub/PEP features, which makes it possible to use securily features such a geolocation, private blogs, private calendar events, data forms, etc.</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<p>The design goals of this XEP are:</p>
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@ -61,7 +66,7 @@
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</ul>
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</section1>
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<section1 topic='Overview' anchor='overview'>
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<p>To encrypt a pubsub node, each item is symmetrically encrypted with a shared secret using OpenPGP symmetric encryption. The secret is shared with other entities using e2e encrypted messages with &xep0374;. If an entity's access is revoked or a shared secret may have been compromised, a new shared secret is generated and new items are encrypted with it.</p>
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<p>To encrypt a pubsub node, each item is symmetrically encrypted with a shared secret using OpenPGP symmetric encryption. The secret is shared with other entities using e2e encrypted messages with &xep0373;. If an entity's access is revoked or a shared secret may have been compromised, a new shared secret is generated and new items are encrypted with it.</p>
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</section1>
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<section1 topic='Use Cases' anchor='usecases'>
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@ -91,7 +96,7 @@
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</section2>
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<section2 topic='Transmit Shared Secret' anchor='transmit_secret'>
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<p>Now that Juliet has started to publish encrypted items on a blog, she wants to share the secret key with her confidante and her lover. To do so she uses a e2ee &MESSAGE; stanza as specified in &xep0374;, in which she encrypt a <shared-secret/> element qualified by the 'urn:xmpp:openpgp:pubsub:0' namespace. This element MUST have the following attributes:</p>
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<p>Now that Juliet has started to publish encrypted items on a blog, she wants to share the secret key with her confidante and her lover. To do so she uses a e2ee &MESSAGE; stanza by using a <signcrypt/> as specified in <link url='https://xmpp.org/extensions/xep-0373.html#exchange'>XEP-0373 "Exchanging OpenPGP Encrypted and Signed Data"</link>. The encrypted payload of the <signcrypt/> MUST contain a <shared-secret/> element qualified by the 'urn:xmpp:openpgp:pubsub:0' namespace. This element MUST have the following attributes:</p>
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<ul>
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<li>a 'timestamp' attribute whose value is the datetime of the shared key generation. Datetime format is specified in &xep0082;.</li>
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<li>a 'jid' attribute with bare JID of the pubsub/PEP service;</li>
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<p>Entities MUST always use the most recently generated shared secret to encrypt new items (the 'timestamp' attribute is used to check generation date and time).</p>
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<p>
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Following example only show the <shared-secret> payload, as it is normally put as an encrypted payload of <openpgp/> element of a &xep0374; message.
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Following example only show the <shared-secret/> element, as it is normally put as an encrypted payload of &xep0373; <signcrypt/> payload.
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</p>
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<example caption="shared-secret payload (normally encrypted as part of XEP-0374 message)"><![CDATA[
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<example caption="shared-secret element (normally encrypted as part of XEP-0373's signcrypt payload)"><![CDATA[
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<shared-secret
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xmlns='urn:xmpp:openpgp:pubsub:0'
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jid='pubsub.capulet.lit'
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</shared-secret>
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]]></example>
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<p><strong>note:</strong> we use a &xep0374; &MESSAGE; instead of PEP or Pubsub to transmit the shared secret for several reasons:</p>
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<p><strong>note:</strong> we use &MESSAGE; instead of PEP or Pubsub to transmit the shared secret for several reasons:</p>
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<ul>
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<li>Shared secret would have to be encrypted for all entities allowed to decrypt the items, and re-encrypted each time there is a new entity. While acceptable for a small number of entities, this doesn't scale as well as separated &MESSAGE;;</li>
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<li>The item containing the shared secret would need to keep all previous shared secrets in case of secret rotation (see <link url='#revocation_rotation'>below</link>). At some point, there would be a risk to reach stanza maximum size limit of one server;</li>
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<section2 topic='Revocation and Shared Secret Rotation' anchor='revocation_rotation'>
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<p>If there is any doubt about the compromise of a shared secret or if access to the encrypted node is revoked for an entity, the shared secret SHOULD BE rotated.</p>
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<p>To rotate a key, a &xep0374; message must be sent to all people which got the shared secret, with an encrypted <revoke/> element qualified by the 'urn:xmpp:openpgp:pubsub:0' namespace, which MUST have an 'id' attribute whose value is the shared secret ID. Optionally, the <revoke/> element MAY have one or more <reason/> child element(s) which contain a human readable message explaining the reason of the revocation. The <reason/> element MAY contain an 'xml:lang' attribute with the language code of the text, and there MAY be several <reason/> elements if and only if they have distinct xml:lang. A revoked key MUST NOT be used to encrypt new items.</p>
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<p>To rotate a key, a &MESSAGE; must be sent to all people which got the shared secret. The &MESSAGE; MUST contain a XEP-0373 <signcrypt/> element whose payload is an encrypted <revoke/> element qualified by the 'urn:xmpp:openpgp:pubsub:0' namespace, which MUST have an 'id' attribute whose value is the shared secret ID. Optionally, the <revoke/> element MAY have one or more <reason/> child element(s) which contain a human readable message explaining the reason of the revocation. The <reason/> element MAY contain an 'xml:lang' attribute with the language code of the text, and there MAY be several <reason/> elements if and only if they have distinct xml:lang. A revoked key MUST NOT be used to encrypt new items.</p>
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<p>Then a new shared secret MUST be generated and transmitted to all participants (excluding those who have seen their access revoked) as explained in <link url='#transmit_secret'>Transmit Shared Secret.</link></p>
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<p>Note that if an entity's access is revoked, it SHOULD also be removed from the node's whitelist (if "whitelist" has been used as access model as recommended).</p>
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<p>
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Following example only show the <revoke/> payload, as it is normally put as an encrypted payload of <openpgp/> element of a &xep0374; message.
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Following example only show the <revoke/> element, as it is normally put as an encrypted payload of &xep0373; <signcrypt/> payload.
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</p>
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<example caption="revoke Payload (normally encrypted as part of XEP-0374 message)"><![CDATA[
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<example caption="revoke element (normally encrypted as part of XEP-0373's signcrypt payload)"><![CDATA[
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<revoke xmlns='urn:xmpp:openpgp:pubsub:0' id='1234-abcd-5678-efgh'>
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<reason>Revoked access from an entity</reason>
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</revoke>
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]]></example>
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<p>Items published before the key rotation SHOULD NOT be re-encrypted as it would be resource intensive, and revoked entities may have made a copy.</p>
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<p>When access to the shared secret is granted to a new entity, all previously used keys SHOULD be transmitted (with their 'revoked' attribute set to "true" as explained in <link url='#transmit_secret'>Transmit Shared Secret.</link>) along with the currently used shared secret. This allows the new entity to decrypt the items encrypted with the old keys. The <shared-secret/> elements can either be sent in the same encrypted payload of a single &xep0374; &MESSAGE;, or split into multiple encrypted &MESSAGE;, it's up to the implementation to decide which is better (keep in mind that too many shared secrets in a single message may reach the maximum stanza size limit at some point, even if this limit is not likely to be reached for most usual cases).</p>
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<p>When access to the shared secret is granted to a new entity, all previously used keys SHOULD be transmitted (with their 'revoked' attribute set to "true" as explained in <link url='#transmit_secret'>Transmit Shared Secret.</link>) along with the currently used shared secret. This allows the new entity to decrypt the items encrypted with the old keys. The <shared-secret/> elements can either be sent in the same encrypted payload of a single &MESSAGE;, or split into multiple encrypted &MESSAGE;, it's up to the implementation to decide which is better (keep in mind that too many shared secrets in a single message may reach the maximum stanza size limit at some point, even if this limit is not likely to be reached for most usual cases).</p>
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</section2>
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</section1>
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<p>When &xep0470; are used, the attachment node SHOULD have the same access model (i.e. whitelist as recommended in <link url="#security">Security Considerations</link>) as the encrypted node, with authorized entities synchronized (this should be done automatically for <link url='https://xmpp.org/extensions/xep-0470.html#full-compliance'>fully compliant services</link>). The items published to attachment node itself MUST be encrypted using this protocol. Due to <link url='https://xmpp.org/extensions/xep-0470.html#validity-check'>validity check of attachment items</link>, the encrypted element MUST be a child of a XEP-0470's <attachments/> element, instead of being the <item/> payload as usual. Note that this will prevent the <link url="https://xmpp.org/extensions/xep-0470.html#summary-node">summary feature</link> to work with encrypted elements, and the end-user client will have to do the summary itself, this is an inevitable trade-off when using e2ee.</p>
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<p>The "pubsub#type" of an encrypted item is always "urn:xmpp:openpgp:pubsub:0", thus no information is leaked on the content of the node, and all encrypted node can easily be retrieved by using &xep0462;.</p>
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<p>Note that encrypted items MAY be mixed with plain text items: for instance if a blog is public, but some of its items are private. However the proper handling of this use case is out of scope of this specification.</p>
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<p>To transmit the shared secret, an other e2e encryption algorithm than &xep0374; MAY be used as long as the elements containing the secret are end-to-end encrypted. For instance, &xep0384; (version 2 or more) with &xep0420; could be used. As OpenPGP is necessary to decrypt items, &xep0374; seems the most appropriate choice.</p>
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</section1>
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<section1 topic='Discovering Support' anchor='disco'>
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<li>Some protocols define prefixes for nodes, this is for instance the case for <link url='https://xmpp.org/extensions/xep-0277.html#comments'>XEP-0277 Comments</link>. If the prefix is not essential to make the feature work (which is the case for XEP-0277 comments), the client SHOULD NOT use the usual prefix to avoid leaking informations on the content of the node. The prefix defined in &xep0470; MUST be used as usually, as it is essential to retrieve the attachment node, and it's generic and thus doesn't leak information on the node content.</li>
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<li>To decrypt archives and future items, clients most probably cache plain text version of items, and shared secret. If one device with access to an encrypted node is compromised, all items from the node should be considered compromised, and shared secret should be rotated.</li>
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<li>For the same reason, client SHOULD encrypt data at rest (even if the device is stolen, data should not be accessible without some cryptographic key).</li>
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<li>Note that only the shared key sender is authenticated (by &xep0374;), not the items publishers, meaning that encrypted items could be published by anybody in possession of the shared secret. Pubsub items authentication will be treated in a separated XEP.</li>
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<li>Note that only the shared key sender is authenticated (by &xep0373;'s <signcrypt/>), not the items publishers, meaning that encrypted items could be published by anybody in possession of the shared secret. Pubsub items authentication will be treated in a separated XEP.</li>
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</ul>
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</section1>
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<p>TODO</p>
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</section1>
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<section1 topic='Acknowledgements' anchor='acks'>
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<p></p>
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<p>Thanks to Tim Henkes for his advices and NLNet foundation/NGI0 Discovery for funding.</p>
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</section1>
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</xep>
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