Channel Bindings for TLS 1.3

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Table of Contents

1. Introduction

The "tls-unique" channel binding type defined in [RFC5929] was found to be vulnerable to the "triple handshake vulnerability" [TRIPLE-HANDSHAKE] without the extended master secret extension defined in [RFC7627]. While TLS 1.3 uses a complete transcript hash akin to the extended master secret procedures, the safety of channel bindings with TLS 1.3 was not analyzed as part of the core protocol work, and so the specification of channel bindings for TLS 1.3 was deferred. [RFC8446] section C.5 notes the lack of channel bindings for TLS 1.3; as this document defines such channel bindings, it updates [RFC8446] to note that this gap has been filled. Furthermore, this document updates [RFC5929] by adding an additional unique channel binding type, "tls-exporter", that replaces some usage of "tls-unique".¶

2. 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 [RFC5705] and [RFC8446] section 7.5 by supplying the following inputs:¶

Label:

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

Context value:

Zero-length string.¶

Length:

32 bytes.¶

This channel binding mechanism is defined only when TLS cipher negotiation results in unique master secrets, which is true of TLS 1.3 which always behaves as if it were using the extended master secret fix required by previous versions of TLS (see [RFC8446] appendix D).¶

3. TLS 1.3 with SCRAM or GSS-API over SASL

SCRAM [RFC5802] and GSS-API over SASL [RFC5801] define "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 [RFC5801] and [RFC5802] to use "tls-exporter" as the default channel binding over TLS 1.3 (and greater).¶

4. Security Considerations

The channel binding type defined in this document is constructed so that disclosure of the channel binding data does not leak secret information about the TLS channel and does not affect the security of the TLS channel.¶

The Security Considerations sections of [RFC5056], [RFC5705], and [RFC8446] apply to this document.¶

5. IANA Considerations

Author's Address