This document provides a technical specification for Digital Signatures in Extensible Messaging and Presence Protocol (XMPP ) based upon End-to-End Object Encryption (E2EEncrypt ) "work in progress".
The S/MIME approach defined in RFC 3923  has never been implemented in XMPP clients to the best of our knowledge, but has some attractive features, especially the ability to store-and-forward a signed message at a user's server if the user is not online when the message is received (in the XMPP community this is called "offline storage" and the message is referred to as an "offline message"). The authors surmise that RFC 3923 has not been implemented mainly because it adds several new dependencies to XMPP clients, especially MIME (along with the CPIM and MSGFMT media types). This document explores the possibility of an approach that is similar to but simpler than RFC 3923.
The process that a sending agent follows for securing stanzas is very similar regardless of the form of stanza (i.e., <iq/>, <message/>, or <presence/>).
The sender begins with the cleartext version of the <message/> stanza "S":
The sender then performs the steps 1 through 4 from above to generate:
And then performs steps 5 through 9 steps, causing the following to be sent:
To be added....
The following limitations and caveats apply:
Several scenarios are possible when an entity receives an encrypted stanza:
In Case #1, the receiving application MUST do one and only one of the following: (1) ignore the <signed/> extension, (2) ignore the entire stanza, or (3), except where precluded by the protocol (rfc3920bis ), return a <service-unavailable/> error to the sender.
In Case #2, the receiving application MUST NOT return a stanza error to the sender, since this is the success case.
In Case #3, the receiving application MAY, except where precluded by the protocol, return a <not-acceptable/> error to the sender, optionally supplemented by an application-specific error condition element of <bad-timestamp/> as shown below:
In Case #4, the receiving application SHOULD, except as precluded by the protocol, return a <bad-request/> error to the sender, optionally supplemented by an application-specific error condition element of <bad-signature/> as shown below:
Additionally in Case #4, the receiving application SHOULD NOT present the stanza to the intended recipient (human or application) and SHOULD provide some explicit alternate processing of the stanza (which may be to display a message informing the recipient that it has received a stanza that cannot be verified).
Timestamps are included to help prevent replay attacks. All timestamps MUST conform to DATETIME  and be presented as UTC with no offset, always including the seconds and fractions of a second to three digits (resulting in a datetime 24 characters in length). Absent a local adjustment to the sending agent's perceived time or the underlying clock time, the sending agent MUST ensure that the timestamps it sends to the receiver increase monotonically (if necessary by incrementing the seconds fraction in the timestamp if the clock returns the same time for multiple requests). The following rules apply to the receiving application:
The foregoing timestamp checks assume that the recipient is online when the message is received. However, if the recipient is offline then the server will probably store the message for delivery when the recipient is next online (offline storage does not apply to <iq/> or <presence/> stanzas, only <message/> stanzas). As described in Best Practices for Handling Offline Messages (XEP-0160) , when sending an offline message to the recipient, the server SHOULD include delayed delivery data as specified in Delayed Delivery (XEP-0203)  so that the recipient knows that this is an offline message and also knows the original time of receipt at the server. In this case, the recipient SHOULD verify that the timestamp received in the encrypted message is within five minutes of the time stamped by the recipient's server in the <delay/> element.
All implementations MUST support the following algorithms. Implementations MAY support other algorithms as well.
To participate in end-to-end signing using the methods defined in this document, a client needs to possess an X.509 certificate. It is expected that many clients will generate their own (self-signed) certificates rather than obtain a certificate issued by a certification authority (CA). In any case the certificate MUST include an XMPP address that is represented using the ASN.1 Object Identifier "id-on-xmppAddr" as specified in Section 5.1.1 of RFC 3920bis.
A URN sub-namespace of signed content for the Extensible Messaging and Presence Protocol (XMPP) is defined as follows.
This document borrows ideas and text from End-to-End Object Encryption "work in progress" by Matthew Miller and Peter Saint-Andre.
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This XMPP Extension Protocol is copyright © 1999 – 2020 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.
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".
2. End-to-End Object Encryption for the Extensible Messaging and Presence Protocol (XMPP), Miller, M. and P. Saint-Andre, work in progress <http://datatracker.ietf.org/doc/draft-miller-3923bis>.
6. RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/draft-ietf-saintandre-rfc3920bis>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/