Note: The protocols developed according to the cryptographic design described in this document are described in Encrypted Session Negotiation (XEP-0116) , Simplified Encrypted Session Negotiation (XEP-0217) , Offline Encrypted Sessions (XEP-0187)  and Stanza Encryption (XEP-0200) . The information in those documents should be sufficient for implementors. This purely informative document is primarily for people interested in the design and analysis of those protocols.
As specified in RFC 3920 , XMPP is an XML streaming protocol that enables the near-real-time exchange of XML fragments between any two (or more) network endpoints. To date, the main application built on top of the core XML streaming layer is instant messaging (IM) and presence, the base extensions for which are specified in RFC 3921 . There are three first-level elements of XML streams (<message/>, <presence/>, and <iq/>); each of these "XML stanza" types has different semantics, which can complicate the task of defining a generalized approach to end-to-end encryption for XMPP. In addition, XML stanzas can be extended (via properly-namespaced child elements) for a wide variety of functionality.
XMPP is a session-oriented communication technology: normally, a client authenticates with a server and maintains a long-lived connection that defines the client's XMPP session. Such stream-level sessions may be secured via channel encryption using Transport Level Security (RFC 2246 ), as specified in Section 5 of RFC 3920. However, there is no guarantee that all hops will implement or enforce channel encryption (or that intermediate servers are trustworthy), which makes end-to-end encryption desirable.
This document specifies a method for encrypted sessions ("ESessions") that takes advantage of the inherent possibilities and strengths of session encryption as opposed to object encryption. The detailed requirements for encrypted sessions are defined in Requirements for Encrypted Sessions (XEP-0210) .
The conceptual model for the approach specified in this document was inspired by "off-the-record" (OTR) communication, as implemented in the Gaim encryption plugin and described in Off-the-Record Communication . The basic concept is that of an encrypted session which acts as a secure tunnel between two endpoints. Once the tunnel is established, the content of all one-to-one XML stanzas exchanged between the endpoints will be encrypted and then transmitted within a "wrapper" protocol element.
Note: In order to gain a thorough understanding of this document, it is recommended that the Off-the-Record Communication paper and RFC 6189  are read first.
This document introduces two characters to help the reader follow the necessary exchanges:
While Alice and Bob are introduced as "end users", they are simply meant to be examples of XMPP entities. Any directly addressable XMPP entity may participate in an ESession.
Authenticated key-exchange is the most challenging part of the design of any secure communication protocol. The ESessions key exchange essentially translates the SIGMA   key-exchange protocol into the syntax of XMPP. The SIGMA approach to Diffie-Hellman Key Agreement (see RFC 2631 ) underpins several standard key-exchange protocols including the Internet Key Exchange (IKE) protocol versions 1 and 2 (see RFC 2409  and RFC 4306 ).
Note: Although this section provides an overview of SIGMA, it is strongly recommended that the SIGMA paper is read first in order to gain a thorough understanding of this document.
The 3-message SIGMA-I-based key exchange protects the identity of the initiator against active attacks. This SHOULD NOT be used to establish client to client sessions since the responder's identity is not protected against active attacks. However, it SHOULD be used to establish client to service (server) sessions, especially where the identity of the service is well known to third parties.
The two 4-message SIGMA-R-based key exchanges with hash commitment defend the responder's identity against active attacks and facilitate detection of a Man in the Middle attack. They SHOULD be used to establish client to client sessions.
Note: The block cipher function, cipher, uses CTR mode.
|x, y||Alice and Bob's private Diffie-Hellman keys|
|gx, gy||Alice and Bob's public Diffie-Hellman keys|
|Hgx||Hash of Alice's public Diffie-Hellman key|
|KSA, KSB||The MAC keys (derived from K) that Alice and Bob use to calculate macA and macB|
|pubKeyA, pubKeyB||The public keys that represent the identity of Alice and Bob, and are used to verify their signatures|
|macA, macB||The MAC values that associate the shared secret with the identity of Alice or Bob|
|signKeyA, signKeyB||The private keys that Alice and Bob use to sign|
|signA, signB||Alice's and Bob's signatures of the shared secret|
|KCA, KCB||The cipher keys (derived from K) that Alice and Bob use to encrypt|
|IDA, IDB||The encrypted parameters that identify Alice and Bob to each other|
|SAS||Short Authentication String|
|e, d||Alice and Bob's public Diffie-Hellman keys (the same as gx, gy)|
|He||Hash of Alice's public Diffie-Hellman key|
|K||Shared secret (derived by Alice from gy and x, or by Bob from gx and y)|
|HASH||Selected hash algorithm|
|NA, NB||Alice and Bob's session freshness nonces (ESession IDs)|
|CA, CB||Block cipher initial counter value for blocks sent by Alice and Bob|
|n||Block size of selected cipher algorithm in bits|
|KMA, KMB||The MAC keys (derived from K) that Alice and Bob use to protect the integrity of encrypted data|
|MA, MB||The MAC values that Alice and Bob use to confirm the integrity of encrypted data|
|SRS||Shared retained secret (derived from K in previous session between the clients)|
|RS1A...RSZA||Retained secrets Alice shares with Bob (one for each client he uses)|
|RS1B...RSZB||Retained secrets Bob shares with Alice (one for each client she uses)|
|RSH1A...RSHZA||HMACs of retained secrets Alice shares with Bob|
|SRSH||Bob's HMAC of SRS|
|OSS||Other shared secret of Alice and Bob (e.g. a shared password) defaults to "secret"|
|isPKA, isPKB||Whether or not Alice and Bob prefer to receive a public key (booleans)|
The diagram below demonstrates the barest cryptographic skeleton of the SIGMA-I key exchange protocol. Here Bob allows Alice to protect her identity from active attacks, by allowing her to authenticate him before she communicates her identity. Note: The cipher keys (KCA and KCB) are different in each direction, making this exchange slightly more conservative than SIGMA.
The diagram below demonstrates the skeleton of the Diffie-Hellman key exchange that employs out-of-band Short Authentication String (SAS) verification. If Alice and Bob's public keys are not yet trusted, or if their private keys have been compromised, then the hash commitment sent in the first step enables Alice and Bob to verify their copies of each other's Diffie-Hellman (and public) keys and detect a Man in the Middle more easily.
If a Man in the Middle changes the public Diffie-Hellman keys that Alice and Bob receive, then he could potentially use his knowledge of the SAS that Bob will eventually calculate when choosing the key he will send to Alice in the second step. However, the fact that the value he received in the first step is only a hash means the Man in the Middle must choose the key he sends to Alice before he can predict the SAS that she will calculate with it. Therefore, even if the SAS is very short, he is unable to use his resources to choose a key that will (have a better than random chance to) result in a SAS that matches Bob's. So only a truncated version of the HASH of Alice and Bob's keys needs to be verified out-of-band in the final step.
The logic of the four-step SIGMA-R protocol is similar to the three-step SIGMA-I protocol. The difference being that Bob protects his identity from active attacks by by delaying communicating his identity to Alice until he has authenticated her. The diagram below demonstrates the skeleton of the key exchange. Note that it also takes advantage of the extra step required for SIGMA-R to incorporate a hash commitment, thus enabling optional out-of-band SAS authentication.
The diagram below describes exactly the same SIGMA-I key exchange protocol as the SIGMA-I Overview above. It provides much more detail, without specifying any ESession-specific details. The differences between it and the SIGMA-R with SAS Key Exchange are highlighted.
The Short Authentication String technique enables protection against a Man in the Middle without the need to generate, distribute or authenticate any public keys. As long as a hash commitment is used at the start of the key exchange then only a short human-friendly string needs to be verified out-of-band (e.g. by recognizable voice communication).
Furthermore, if retained secrets associated with a client/user combination are employed consistently during key exchanges, then the Man in the Middle would need to be present for every session, including the first, and the out-of-band verification would only need to be performed once to verify the absence of a Man in the Middle for all sessions between the parties (past, present and future). 
Public keys are optional in the diagram below. It describes the same SIGMA-R with SAS key exchange protocol as the SIGMA-R Overview. It provides much more detail including the use of retained secrets and other secrets. The use of public keys is negotiated in the first two messages. Note: These optional security enhancements are especially important when the protocol is being used without public keys.
The diagram does not specify any ESession-specific details. The differences between it and the SIGMA-I Key Exchange are highlighted.
This section provides an overview of the full ESession key-exchange protocol from a cryptographic point of view. This protocol is based on the full fledge protocol, as described in Appendix B of the SIGMA paper. It also uses variant (ii), as described in Secion 5.4 of the same paper.
The table below describes the parameters that are not found in the Parameter Descriptions tables above.
|options||Includes a set of possible values for each and every ESession parameter (see the ESession Request sub-section in Encrypted Session Negotiation), including sets of possible values for p, g, HASH, CIPHER, SIGN|
|chosen||Includes a chosen value for each ESession parameter|
|CIPHER||Selected CTR-mode block cipher algorithm|
|DECIPHER||Selected CTR-mode block decipher algorithm (corresponds to CIPHER)|
|SIGN||Selected signature algorithm|
|VERIFY||The selected signature verification algorithm (corresponds to SIGN)|
|SASGEN||The selected SAS generation algorithm|
|x1...xZ||Alice's private Diffie-Hellman keys - each value corresponds to one of Z different DH groups|
|e1...eZ||The choice of public Diffie-Hellman keys that Alice offers Bob - each value corresponds to one of Z different DH groups (and a different value of x)|
|He1...HeZ||The list of hash commitments that Alice sends to Bob (hashes of e1...eZ)|
|*signKeysA||All the private keys that Alice is able to use to create signatures|
|*signsB||The set of signatures of formB (one for each of Bob's private keys)|
|*pubKeysA||All of Alice's public keys that Bob has access to|
* Offline negotiation only
Alice uses this protocol when Bob is Online. In addition to the key exchange described in the SIGMA-I Key Exchange protocol above, she offers Bob a choice of Diffie-Hellman groups with her corresponding values of e, various algorithms and other parameters. The differences between this protocol and Online ESession-R Negotiation are highlighted.
This protocol is similar to the Online ESession-I Negotiation above, except that Bob's identity is protected from active attacks (by by delaying communicating his identity to Alice until he has authenticated her). The optional use of SAS, retained secrets and other secrets means the protocol may be used without any public keys. The differences between this protocol and Online ESession-I Negotiation are highlighted.
Bob uses this protocol to send stanzas to Alice when she is Offline. Note: Since the full SIGMA protocol cannot be used if Alice is offline, her identity is not protected at all.
The diagram is split into three phases. First Alice publishes her ESession options before going offline. Later Bob completes the key exchange (and sends her encrypted stanzas that are not shown below) these are all stored by Alice's server. Finally when Alice comes online again she verifies and calculates the decryption key.
The differences between this offline protocol and the Online ESession-I Negotiation protocol above are highlighted in the diagram below.
Note: KMB is necessary only to allow Bob to terminate the ESession if he comes online before Alice terminates it. The calculation of KCB and KSB is not strictly necessary.
The security considerations are described in Encrypted Session Negotiation and Offline Encrypted Sessions.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This document requires no interaction with the XMPP Registrar .
The author would like to thank: Ian Goldberg for the time he spent reviewing an early version of this protocol and for his invaluable suggestions and comments; and Hugo Krawczyk for his general advice and encouragment. The author of this document is entirely responsable for any errors it contains.
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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 <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.
Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <email@example.com> list might also be appropriate.
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".
12. Like RFC 2409, this protocol uses variant (ii), as described in Secion 5.4 of the SIGMA paper.
16. This combination of techniques underpins the ZRTP key agreement protocol.
17. 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/>.
18. 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/
SIGMA-R negotiates both isPKA and isPKB fields
Moved requirements, approach and scope to new XEP-0210
Incorporated whole forms into SAS calculation
Added PKI Independence and Robustness requirements; added optional public key independence, hash commitment, SAS authentication, retained secrets and other secrets to SIGMA-R key exchange
Removed public key IDs from Offline options
Initial version (extracted from XEP-0116 version 0.9).