X.509 version 3 certificates can be used to provide a strong cryptographic identity of an XMPP entity, i.e. an association of an XMPP address (RFC 7622 ) with its cryptographic key (formally defined under Section 18.104.22.168 of RFC 6120 ). They were initially intended for the use in SASL EXTERNAL for c2s and mutual s2s authentication (see Best Practices for Use of SASL EXTERNAL (XEP-0178) ). This document extends their usage for end-to-end (e2e) authentication of any entities attached to the XMPP network. A separate document also defines how Certificate Signing Request (CSR) of an XMPP account can be issued and signed using the XMPP protocol (XEP-EAX-SIGN).
Note that an XMPP client may use the same certificate for different kinds of e2e authentication.
Conceptually, the idea is to build PKIX trees where in each tree a node corresponds to a certificate of a CA signing certificates of its successors. A leaf in the tree represents a certificate assigned to an XMPP account, a "leaf certificate". The leaf certificates are supposed to be used for (but not limited to) e2e authentication.
In the example above, XMPP servers of domains "capulet.lit" and "montague.lit" do not have associated certificate authorities, so Root CA 1 and Intermediate CA 2.1 sign certificates of "firstname.lastname@example.org" and "email@example.com" directly. An XMPP server of domain "shakespeare.lit" has an associated CA (whose certificate is signed by Root CA N) and thus is able to sign certificates for users of "shakespeare.lit" (and only for them). As long as all root CAs are trusted by all parties, "firstname.lastname@example.org", "email@example.com" and "firstname.lastname@example.org" may mutually authenticate each other using their certificates (for sharing resources, exchanging messages, etc).
The following rules apply to any certificate:
The following rules apply to leaf certificates:
Note that the rules for leaf certificates comply with the rules defined for client certificates under Sections 22.214.171.124 and 126.96.36.199 of RFC 6120 . Thus they can be used for c2s SASL EXTERNAL authentication.
The requirement to possess a RELOAD URI and an rfc822Name address makes it possible to use the certificate for RELOAD authentication. Even if XOR extension (XEP-0415) is unused, the RELOAD URI uniquely identifies a user device: a user MAY have several certificates assigned to their XMPP address but with different RELOAD URIs.
The following rules apply to domain-associated certificates:
The following rules apply to intermediate certificates, excluding domain-associated certificates:
The following rules apply to root certificates:
CA Requirements are outlined in XEP-EAX-CAR.
The certificate is considered valid if it follows the rules specified in Certificate Requirements and, in the case when it is signed by a domain-associated certificate, it is a leaf certificate and the domain from the domain-associated certificate matches the domain part of the XmppAddr of the certificate. Otherwise, the certificate MUST be considered invalid.
In the case of a certificate chain, the rules for certification path validation are applied (RFC 5280 ).
An XMPP entity MAY maintain its own list of root certificates. However, in practice it's convenient to retrieve this list from a trusted source. For example, several organizations in the Internet maintain and provide such lists for certificates verification in the Web. This section specifies how the list of root certificates can be retrieved for the purpose of e2e authentication in XMPP.
Since the authentication is intended to be compliant with RELOAD and creating new document formats or DNS TXT records without exigency are in general discouraged, the Overlay Configuration document is reused to provide the list of root certificates (see Section 11.1 of RFC 6940 ). The root certificates are PEM-encoded (RFC 7468 ) with encapsulation boundaries removed and are included in <root-cert/> elements of the Overlay Configuration.
In order to retrieve the Overlay Configuration, an HTTP GET request is performed to "https://xmpp.org/.well-known/reload-config". The requesting UA MUST be prepared to process HTTP redirects. In the case of a failure, the UA MAY repeat the request. In this case exponential backoff MUST be applied. Since the list of root certifcates is not a subject for frequent updates, under normal conditions, the UA SHOULD NOT request the Overlay Configuration more often than once per day. Usage of 'If-Modified-Since' is RECOMMENDED (RFC 7232 ).
Further versions of this specification MAY extend the Overlay Configuration with new XML elements.
An XMPP entity MAY want to publish its certificate so other XMPP entities MAY retrieve it. The method to accomplish this depends on the usage:
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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 <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".
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/