The XMPP Standards Foundation (XSF)  has defined a technology for end-to-end encryption of XMPP communications, called "Encrypted Sessions" (ESessions). This document describes ways for client and library developers to approach the task of implementing ESessions.
In essence, implementation of ESessions proceeds in two directions:
If client developers implement the "frontend" specifications, they should be able to integrate the "backend" code developed by library developers, enabling the two sets of developers to "meet in the middle" and offer complete implementations.
When working from the client/interface level down to the library/API level, it makes sense to implement the relevant specifications in the following order:
Best Practices for Message Threads (XEP-0201) 
This document describes what a "stanza session" is, i.e., it is defined by the life of a message thread. Clients that implement this specification are well on their way to implementing encrypted sessions, since it is necessary to have a clear session "object" in a client before implementing an encrypted version of such a session.
Stanza Session Negotiation (XEP-0155) 
Because this document describes how to negotiate a stanza session, it is a building block for developing how to negotiate an encrypted stanza session.
Stanza Encryption (XEP-0200) 
By hardcoding the initial parameters during an early phase of development, implementors can use this specification as a starting point for testing of encrypted sessions. A later phase would address rekeying (Section 9) and negotiation of the initial parameters (see below).
Simplified Encrypted Session Negotiation (XEP-0217) 
This specification (to be published concurrently) defines a simple subset of the process for negotiating the initial parameters used in an encrypted session.
Encrypted Session Negotiation (XEP-0116) 
This specification defines the "heavy lifting" involved in going from a cleartext state to an encrypted state, i.e., it specifies how to the initial parameters for an encrypted session.
Cryptographic Design of Encrypted Sessions (XEP-0188) 
Strictly speaking, a developer does not implement this specification, since it describes the theory behind the process of upgrading from cleartext to encryption that is defined in XEP-0116. However it is useful background knowledge for developers who implement XEP-0116.
Naturally, when developing from the library/API level up to the client/interface level, the order should be (roughly) reversed.
Once a library or client has implemented the specifications listed above, it may choose to implement the following additional specifications, which supplemented the core encrypted sessions specifications.
Public Key Publishing (XEP-0189) 
This specification defines a precondition for implementation of the specifications that follow.
Offline Encrypted Sessions (XEP-0187) 
We should be able to encrypt so-called "offline messages" (see Best Practices for Handling Offline Messages (XEP-0160) ) using the same basic principles used to encrypted messages sent while online.
Message Archiving (XEP-0136) 
This specification enables secure archiving of the messages sent and received in an encrypted session.
Incomplete implementations of the Encrypted Sessions technology will not have the same security properties as complete implementations.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This document requires no interaction with the XMPP Registrar .
This document in other formats: XML PDF
This XMPP Extension Protocol is copyright © 1999 – 2018 by the XMPP Standards Foundation (XSF).
Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.
## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##
In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.
This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
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 <email@example.com> 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 <firstname.lastname@example.org>.
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. The XMPP Standards Foundation (XSF) is an independent, non-profit membership organization that develops open extensions to the IETF's Extensible Messaging and Presence Protocol (XMPP). For further information, see <https://xmpp.org/about/xmpp-standards-foundation>.
12. 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/>.
13. 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/