This XMPP extension protocol specifies a profile of OpenPGP for XMPP (XEP-0373)  for OpenPGP secured Instant Messaging (IM).
Unlike similar XEPs, e.g., OMEMO, this XEP does not provide Forward Secrecy (FS), but as an advantage in return, allows users to read their archived conversations (respectively their encrypted data) later on. Of course, only as long as they still possess the according secret key. FS and being able to decrypt archived messages are mutually exclusive, i.e., one can not have both. The authors therefore consider this XEP complementary to similar ones which also provide end-to-end encryption but with a different feature set.
If an entity supports exchanging OpenPGP encrypted and signed instant messages over XMPP, i.e., what is specified herein, it MUST advertise that fact by announcing a Service Discovery (XEP-0030)  feature of 'urn:xmpp:openpgp:im:0'. It thus includes this feature in response to a service discovery request.
Because of possible downgrade attacks, users should be given an option to force the usage of the protocol defined herein no matter if the remote announces support or not.
In order to establish a OpenPGP secured IM communication, IM clients first need to determine the public key of their interlocutor(s). OpenPGP historically provides public keyservers which can be used for key retrieval. Additional there are methods to store OpenPGP key information in the Domain Name System (DNS). This specification does not restrict the mechanism of key discovery and retrieval, but compliant clients MUST support the public key announcement as described in XEP-0373 § 4.
After the required public keys have been discovered, XMPP clients engage in an OpenPGP secured IM conversation by exchanging <openpgp/> extension elements. They MUST use the <signcrypt/> OpenPGP content element specified in XEP-0373 § 3.1.
The child elements of the OpenPGP content element's <payload/> can be seen as stanza extension elements which are encrypted and signed. After the <openpgp/> element and the including <signcrypt/>, element was verified, they SHOULD be processed similar as if they had been direct extension elements of the stanza. For example, direct child elements found in <payload/> in the context of IM could be:
But just as with stanza extension elements, child elements of <payload/> can be any extension element. The example above uses the <body/> element as defined in RFC 6121. Note that it uses 'jabber:client' as namespace, but since the same <body/> element is also defined in the 'jabber:server' namespace, recipients MUST accept both.
Clients MUST expect multiple public keys to be announced for a single remote entity. In this case all keys MUST be used for encryption.
Clients MAY want to use the mechanism in XEP-0373 § 5 to synchronize their secret key(s) over multiple devices. Thus, they should query the user's PEP service for an eventually stored encrypted secret key.
Only <signcrypt/> MUST be used for the IM use case. Encrypted but unsigned messages (<crypt/>) do not provide an advantage over unencrypted ones since the sender can not be verified. As result of this rule, the user interface of IM clients implementing the protocol defined herein MUST NOT provide an option for the user to select between sign+crypt, sign or crypt. This also increases the usability.
In the IM use case every <message/> equipped with <openpgp/> SHOULD include an unencrypted <body/> explaining that the actual message is encrypted. Furthermore the message SHOULD contain a 'store' hint as defined in Message Processing Hints (XEP-0334)  § 4.4 and a "this message contains an encrypted body text" hint in form of an <encryption/> extension element as specified by Explicit Message Encryption (XEP-0380) .
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This XEP does not define a Schema, since it exclusively uses elements from XEP-0373 and other XEPs.
Please refer to the Acknowledgements section of XEP-0373 since the two XEPs where designed together.
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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.
The primary venue for discussion of XMPP Extension Protocols is the <firstname.lastname@example.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 <email@example.com>.
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".
8. 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/>.
9. 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 http://xmpp.org/extensions/attic/
Make use of XEP-0380: Explicit Message Encryption.
Minior editorial fixes.
Initial published version approved by the XMPP Council.