Channel Bindings for TLS 1.3

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Table of Contents

1. Introduction

The 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.¶

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:

Empty context value.¶

Length:

32 bytes.¶

When TLS renegotiation is enabled the "tls-exporter" channel binding type is not defined and implementations MUST NOT support it.¶

3. Security Considerations

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].¶

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

4. IANA Considerations

Author's Address