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Atlanta GA USA sam@samwhited.com https://blog.samwhited.com/

Internet Transport Layer Security 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 , , , and .

Introduction The "unique" channel binding types defined in were found to be vulnerable to the "triple handshake vulnerability" without the extended master secret extension defined in . Because of this they were not defined for TLS 1.3 (see section C.5). To facilitate channel binding with TLS 1.3, a new channel binding type is needed.

Conventions and Terminology Throughout this document the acronym "EKM" is used to refer to Exported Keying Material as defined in . 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 when, and only when, they appear in all capitals, as shown here.

The 'tls-exporter' Channel Binding Type 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 and section 7.5 by supplying the following inputs:

Label:

The ASCII string "EXPORTER-Channel-Binding" with no terminating NUL.

Context value:

Empty context value.

Length:

32 bytes.

In previous versions of TLS the "tls-unique" channel binding type was defined as the default channel binding if no mechanism was defined for negotiating a different channel binding. Because "tls-unique" is not defined for TLS 1.3, the default channel binding mechanism for TLS versions 1.3 and greater MUST be "tls-exporter".

Security Considerations 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 , , and apply to this document.

Use with Legacy TLS 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 . 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 .

IANA Considerations

Registration of Channel Binding Type This document adds the following registration in the "Channel-Binding Types" registry:

Subject:

Registration of channel binding tls-exporter

Channel binding unique prefix:

tls-exporter

Channel binding type:

unique

Channel type:

TLS

Published specification:

&docName;

Channel binding is secret:

no

Description:

The EKM value obtained from the current TLS connection.

Intended usage:

COMMON

Person and email address to contact for further information:

Sam Whited <sam@samwhited.com>.

Owner/Change controller name and email address:

IESG.

Expert reviewer name and contact information:

IETF KITTEN or TLS WG (kitten@ietf.org or tls@ietf.org, failing that, ietf@ietf.org).

Note:

See the published specification for advice on the applicability of this channel binding type.

Registration of Channel Binding TLS Exporter Label This document adds the following registration in the "TLS Exporter Labels" registry:

Value:

EXPORTER-Channel-Binding

DTLS-OK:

Y

Recommended:

Y

Reference:

This document

References Normative References Informative References