Channel Bindings for SCRAM over TLS 1.3

Internet-Draft	Channel Bindings for SCRAM over TLS 1.3	May 2020
Whited	Expires 3 November 2020	[Page]

Abstract

This document defines a channel binding type, tls-scram-exporter, that is compatible with SCRAM [RFC5802] and TLS 1.3 [RFC8446] in accordance with On Channel Binding [RFC5056].¶

Status of This Memo

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This Internet-Draft will expire on 3 November 2020.¶

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

1. Introduction

After problems were found with the channel binding types defined in [RFC5929] they were not defined for TLS 1.3 [RFC8446]. To facilitate channel binding with TLS 1.3, a new channel binding type using keying material obtained from [RFC5705] is needed.¶

1.1. Conventions and Terminology

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.¶

2. The 'tls-scram-exporter' Channel Binding Type

IANA will register the 'tls-scram-exporter' channel binding type to match the description below.¶

Description: The EKM value obtained from the current TLS connection.¶

The EKM is obtained using the keying material exporters for TLS as defined in [RFC5705] by supplying the following inputs. The definition of "client-first-message-bare" and "server-first-message" can be found in [RFC5802].¶

Label:: The ASCII string "EXPORTER-SCRAM-Channel-Binding" with no terminating NUL.¶
Context value:: client-first-message-bare + "," + server-first-message¶
Length:: 32 bytes.¶

When TLS renegotiation is enabled channel binding using the "tls-scram-exporter" type is not define and MUST NOT be supported.¶

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], [RFC5802], and [RFC8446] apply to this document.¶

4. IANA Considerations

4.1. Registration of Channel Binding Type

This document adds the following registration in the "Channel-Binding Types" registry:¶

Subject:: Registration of channel binding tls-scram-exporter¶
Channel binding unique prefix:: tls-scram-exporter¶
Channel binding type:: unique¶
Channel type:: TLS [RFC8446]¶
Published specification:: draft-whited-tls-channel-bindings-for-tls13-00¶
Channel binding is secret:: no¶
Description:: <See specification>¶
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 TLS WG (tls@ietf.org, failing that, ietf@ietf.org).¶
Note:: See the published specification for advice on the applicability of this channel binding type.¶

4.2. Registration of Channel Binding TLS Exporter Label

This document adds the following registration in the "TLS Exporter Labels" registry:¶

Value:: EXPORTER-SCRAM-Channel-Binding¶
DTLS-OK:: Y¶
Recommended:: N¶
Reference:: This document¶

5. References

5.1. Normative References

[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC5056]: Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007, <https://www.rfc-editor.org/info/rfc5056>.
[RFC5705]: Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC5802]: Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, "Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, DOI 10.17487/RFC5802, July 2010, <https://www.rfc-editor.org/info/rfc5802>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446]: Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.

5.2. Informative References

[RFC5929]: Altman, J., Williams, N., and L. Zhu, "Channel Bindings for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010, <https://www.rfc-editor.org/info/rfc5929>.