| Internet-Draft | Channel Bindings for TLS 1.3 | May 2021 |
| Whited | Expires 27 November 2021 | [Page] |
This document defines a channel binding type, tls-exporter, that is compatible with TLS 1.3 in accordance with RFC 5056, On Channel Binding. Furthermore it updates the "default" channel binding to the new binding for versions of TLS greater than 1.2. This document updates RFC5801, RFC5802, RFC5929, and RFC8446.¶
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The "unique" channel binding types defined in [RFC5929] were found to be vulnerable to the "triple handshake vulnerability" [TRIPLE-HANDSHAKE] without the extended master secret extension defined in [RFC7627]. Because of this they were not defined for TLS 1.3 (see [RFC8446] section C.5). To facilitate channel binding with TLS 1.3, a new channel binding type is needed.¶
Throughout this document the acronym "EKM" is used to refer to Exported Keying Material as defined in [RFC5705].¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Channel binding mechanisms are not useful until TLS implementations expose the required data. To facilitate this, "tls-exporter" uses exported keying material (EKM) which is already widely exposed by TLS implementations. The EKM is obtained using the keying material exporters for TLS as defined in [RFC5705] and [RFC8446] section 7.5 by supplying the following inputs:¶
SCRAM [RFC5802] defines "tls-unique" as the default channel binding to use over TLS. As "tls-unique" is not defined for TLS 1.3 (and greater), this document updates [RFC5802] to use "tls-exporter" as the default channel binding to use over TLS 1.3 (and greater).¶
Channel bindings do not leak secret information about the channel and are considered public. Implementations MUST NOT use the channel binding to protect secret information.¶
The Security Considerations sections of [RFC5056], [RFC5705], and [RFC8446] apply to this document.¶
While it is possible to use this channel binding mechanism with TLS versions below 1.3, extra precaution must be taken to ensure that the chosen cipher suites always result in unique master secrets. For more information see the Security Considerations section of [RFC5705].¶
When TLS renegotiation is enabled the "tls-exporter" channel binding type is not defined and implementations MUST NOT support it.¶
In general, users wishing to take advantage of channel binding should upgrade to TLS 1.3 or later.¶
The derived data MUST NOT be used for any purpose other than channel bindings as described in [RFC5056].¶
This document adds the following registration in the "Channel-Binding Types" registry:¶