Atlanta GA United States of America sam@samwhited.com https://blog.samwhited.com/

sec kitten This document defines a channel binding type, tls-exporter, that is compatible with TLS 1.3 in accordance with RFC 5056, "On the Use of Channel Bindings to Secure Channels". Furthermore, it updates the default channel binding to the new binding for versions of TLS greater than 1.2. This document updates RFCs 5801, 5802, 5929, and 7677.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

• .  Introduction
  • .  Conventions and Terminology
• .  The 'tls-exporter' Channel Binding Type
• .  TLS 1.3 with SCRAM or GSS-API over SASL
• .  Security Considerations
  • .  Uniqueness of Channel Bindings
  • .  Use with Legacy TLS
• .  IANA Considerations
  • .  Registration of Channel Binding Type
  • .  Registration of Channel Binding TLS Exporter Label
• .  References
  • .  Normative References
  • .  Informative References
• Author's Address

Introduction The "tls-unique" channel binding type defined in was found to be susceptible to the "triple handshake vulnerability" without the extended master secret extension defined in . 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, so the specification of channel bindings for TLS 1.3 was deferred. notes the lack of channel bindings for TLS 1.3; this document defines such channel bindings and fills that gap. Furthermore, this document updates by adding an additional unique channel binding type, "tls-exporter", that replaces some usage of "tls-unique".

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 , 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 the TLS handshake results in unique master secrets. This is true of TLS versions prior to 1.3 when the extended master secret extension of is in use, and it is always true for TLS 1.3 (see ).

TLS 1.3 with SCRAM or GSS-API over SASL The specifications for Salted Challenge Response Authentication Mechanism (SCRAM) and Generic Security Service Application Program Interface (GSS-API) over Simple Authentication and Security Layer (SASL) 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 , , and to use "tls-exporter" as the default channel binding over TLS 1.3 (and greater). Note that this document does not change the default channel binding for SCRAM mechanisms over TLS 1.2 , which is still "tls-unique" (also note that RFC 5246 has been obsoleted by RFC 8446). Additionally, this document updates the aforementioned documents to make "tls-exporter" the mandatory-to-implement channel binding if any channel bindings are implemented for TLS 1.3. Implementations that support channel binding over TLS 1.3 MUST implement "tls-exporter".

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 derived data MUST NOT be used for any purpose other than channel bindings as described in . In particular, implementations MUST NOT use channel binding as a secret key to protect privileged information. The Security Considerations sections of , , and apply to this document.

Uniqueness of Channel Bindings The definition of channel bindings in defines the concept of a "unique" channel binding as being one that is unique to the channel endpoints and unique over time, that is, a value that is unique to a specific instance of the lower-layer security protocol. When TLS is the lower-layer security protocol, as for the channel binding type defined in this document, this concept of uniqueness corresponds to uniquely identifying the specific TLS connection. However, a stronger form of uniqueness is possible, which would entail uniquely identifying not just the lower-layer protocol but also the upper-layer application or authentication protocol that is consuming the channel binding. The distinction is relevant only when there are multiple instances of an authentication protocol, or multiple distinct authentication protocols, that run atop the same lower-layer protocol. Such a situation is rare; most consumers of channel bindings establish an instance of the lower-layer secure protocol, run a single application or authentication protocol as the upper-layer protocol, then terminate both upper and lower-layer protocols. In this situation, the stronger form of uniqueness is trivially achieved, given that the channel binding value is unique in the sense of . The channel binding type defined by this document provides only the weaker type of uniqueness, as per ; it does not achieve the stronger uniqueness per the upper-layer protocol instance described above. This stronger form of uniqueness would be useful in that it provides protection against cross-protocol attacks for the multiple authentication protocols running over the same instance of the lower-layer protocol, and it provides protection against replay attacks that seek to replay a message from one instance of an authentication protocol in a different instance of the same authentication protocol, again running over the same instance of the lower-layer protocol. Both of these properties are highly desirable when performing formal analysis of upper-layer protocols; if these properties are not provided, such formal analysis is essentially impossible. In some cases, one or both of these properties may already be provided by specific upper-layer protocols, but that is dependent on the mechanism(s) in question, and formal analysis requires that the property is provided in a generic manner across all potential upper-layer protocols that exist or might exist in the future. Accordingly, applications that make use of the channel binding type defined in this document MUST NOT use the channel binding for more than one authentication mechanism instance on a given TLS connection. Such applications MUST immediately close the TLS connection after the conclusion of the upper-layer protocol.

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 and the Security Considerations section of (TLS 1.3 always provides unique master secrets, as discussed in ). When TLS renegotiation is enabled on a connection, the "tls-exporter" channel binding type is not defined for that connection, and implementations MUST NOT support it. In general, users wishing to take advantage of channel binding should upgrade to TLS 1.3 or later.

IANA Considerations

Registration of Channel Binding Type IANA has registered tls-exporter in the "Channel-Binding Types" registry:

Channel-binding unique prefix:

tls-exporter

Channel-binding type:

unique

Channel type:

TLS

Published specification:

RFC 9266

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 WG <kitten@ietf.org> or IETF TLS WG <tls@ietf.org>

Note:

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

Registration of Channel Binding TLS Exporter Label IANA has added the following registration in the "TLS Exporter Labels" registry under the "Transport Layer Security (TLS) Parameters" registry:

Value:

EXPORTER-Channel-Binding

DTLS-OK:

Y

Recommended:

Y

Reference:

RFC 9266

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The concept of channel binding allows applications to establish that the two end-points of a secure channel at one network layer are the same as at a higher layer by binding authentication at the higher layer to the channel at the lower layer. This allows applications to delegate session protection to lower layers, which has various performance benefits. This document discusses and formalizes the concept of channel binding to secure channels. [STANDARDS-TRACK] A number of protocols wish to leverage Transport Layer Security (TLS) to perform key establishment but then use some of the keying material for their own purposes. This document describes a general mechanism for allowing that. [STANDARDS-TRACK] This document describes how to use a Generic Security Service Application Program Interface (GSS-API) mechanism in the Simple Authentication and Security Layer (SASL) framework. This is done by defining a new SASL mechanism family, called GS2. This mechanism family offers a number of improvements over the previous "SASL/ GSSAPI" mechanism: it is more general, uses fewer messages for the authentication phase in some cases, and supports negotiable use of channel binding. Only GSS-API mechanisms that support channel binding and mutual authentication are supported. [STANDARDS-TRACK] The secure authentication mechanism most widely deployed and used by Internet application protocols is the transmission of clear-text passwords over a channel protected by Transport Layer Security (TLS). There are some significant security concerns with that mechanism, which could be addressed by the use of a challenge response authentication mechanism protected by TLS. Unfortunately, the challenge response mechanisms presently on the standards track all fail to meet requirements necessary for widespread deployment, and have had success only in limited use. This specification describes a family of Simple Authentication and Security Layer (SASL; RFC 4422) authentication mechanisms called the Salted Challenge Response Authentication Mechanism (SCRAM), which addresses the security concerns and meets the deployability requirements. When used in combination with TLS or an equivalent security layer, a mechanism from this family could improve the status quo for application protocol authentication and provide a suitable choice for a mandatory-to-implement mechanism for future application protocol standards. [STANDARDS-TRACK] This document defines three channel binding types for Transport Layer Security (TLS), tls-unique, tls-server-end-point, and tls-unique-for-telnet, in accordance with RFC 5056 (On Channel Binding). Note that based on implementation experience, this document changes the original definition of 'tls-unique' channel binding type in the channel binding type IANA registry. [STANDARDS-TRACK] This document registers the Simple Authentication and Security Layer (SASL) mechanisms SCRAM-SHA-256 and SCRAM-SHA-256-PLUS, provides guidance for secure implementation of the original SCRAM-SHA-1-PLUS mechanism, and updates the SCRAM registration procedures of RFC 5802. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Informative References This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] The Transport Layer Security (TLS) master secret is not cryptographically bound to important session parameters such as the server certificate. Consequently, it is possible for an active attacker to set up two sessions, one with a client and another with a server, such that the master secrets on the two sessions are the same. Thereafter, any mechanism that relies on the master secret for authentication, including session resumption, becomes vulnerable to a man-in-the-middle attack, where the attacker can simply forward messages back and forth between the client and server. This specification defines a TLS extension that contextually binds the master secret to a log of the full handshake that computes it, thus preventing such attacks.

Author's Address

Atlanta GA United States of America sam@samwhited.com https://blog.samwhited.com/