SASL channel-binding is a technique to increase the security of connections (RFC 5056 [1]). Unfortunately, the SASL profile specified in RFC 6120 [2] lacks a method for the server to announce its supported channel-binding types. This hinders the adoption of channel-binding, especially since the error protocol to execute after a client requested a channel-binding type unsupported by the server is basically unspecified.
The extension defined herein fills the gap left by RFC 6120 [2], by allowing the server the announce its supported channel-binding types.
This protocol consists of a single optional extension element named 'sasl-channel-binding' qualified by the 'urn:xmpp:sasl-cb:0' namespace. The 'sasl-channel-binding' element MUST contain one or more 'channel-binding' elements, of which each MUST have an attribute with the name 'type'. The value of the 'type' attribute SHOULD be the "Channel-binding unique prefix" of a channel-binding type which was registered with the IANA Channel-Binding Types Registry [3].
A server declares that it supports particular channel-binding types by listing the supported types via the 'sasl-channel-binding' element defined herein. The 'sasl-channel-binding' element could appear as child element to the SASL <mechanisms/> stream-feature element, qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace, as specified in RFC 6120 [2]. Another potential appearance of <sasl-channel-binding> is as child element of the <mechanisms/> stream-feature element as specified in the Extensible SASL Profile (XEP-0388) [4].
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>SCRAM-SHA-1-PLUS</mechanism>
<mechanism>PLAIN</mechanism>
<sasl-channel-binding xmlns='urn:xmpp:sasl-cb:0'>
<channel-binding type='tls-server-end-point'/>
<channel-binding type='tls-exporter'/>
</sasl-channel-binding>
</mechanisms>
</stream:features>The author belives that this document itself does not yield any new security considerations. [5]
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].
This document requires no interaction with the XMPP registrar.
TODO: Add if the XEP is scheduled for the state after 'experimental'.
Thanks to Sam Whited for the discussion about the underlying issue and incentivizing me to come up with this extension.
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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.
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Errata can be sent to <editor@xmpp.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. RFC 5056: On the Use of Channel Bindings to Secure Channels <http://tools.ietf.org/html/rfc5056>.
2. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.
3. IANA Channel-Binding Types Registry <https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml>.
4. XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>.
5. Hopefully somebody will correct him, in case he is wrong.
6. 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/>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
First draft.
@report{schmaus2020sasl-cb-types,
title = {SASL Channel-Binding Type Capability},
author = {Schmaus, Florian},
type = {XEP},
number = {xxxx},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2020-05-20/2020-05-20},
}END