Internet Engineering Task Force (IETF) S. Whited
Request for Comments: 9266
Category: Standards Track
Channel Bindings for TLS 1.3
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
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 https://www.rfc-editor.org/info/rfc9266
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Table of Contents 1.
Conventions and Terminology 2.
The 'tls-exporter' Channel Binding Type 3.
TLS 1.3 with SCRAM or GSS-API over SASL 4.
Security Considerations 4.1.
Uniqueness of Channel Bindings 4.2.
Use with Legacy TLS 5.
IANA Considerations 5.1.
Registration of Channel Binding Type 5.2.
Registration of Channel Binding TLS Exporter Label 6.
Normative References 6.2.
The "tls-unique" channel binding type defined in [RFC5929
] was found
to be susceptible 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, so the specification of channel bindings for TLS 1.3 was
deferred. Appendix C.5 of [RFC8446
] notes the lack of channel
bindings for TLS 1.3; this document defines such channel bindings and
fills that gap. Furthermore, this document updates [RFC5929
adding an additional unique channel binding type, "tls-exporter",
that replaces some usage of "tls-unique".
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
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
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 Section 7.5 of
], by supplying the following inputs:
Label: The ASCII string "EXPORTER-Channel-Binding" with no
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 [RFC7627
] is in
use, and it is always true for TLS 1.3 (see Appendix D of [RFC8446
3. TLS 1.3 with SCRAM or GSS-API over SASL
The specifications for Salted Challenge Response Authentication
Mechanism (SCRAM) [RFC5802
] and Generic Security Service
Application Program Interface (GSS-API) over Simple Authentication
and Security Layer (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 [RFC7677
] 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
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
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 derived data MUST NOT
be used for any purpose other than channel
bindings as described in [RFC5056
]. In particular, implementations MUST NOT
use channel binding as a secret key to protect privileged
The Security Considerations sections of [RFC5056
] apply to this document.
4.1. Uniqueness of Channel Bindings
The definition of channel bindings in [RFC5056
] 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 [RFC5056
The channel binding type defined by this document provides only the
weaker type of uniqueness, as per [RFC5056
]; 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.
4.2. 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 [RFC7627
] and the Security Considerations
section of [RFC5705
] (TLS 1.3 always provides unique master secrets,
as discussed in Appendix D of [RFC8446
When TLS renegotiation is enabled on a connection, the "tls-exporter"
channel binding type is not defined for that connection, and
implementations MUST NOT
In general, users wishing to take advantage of channel binding should
upgrade to TLS 1.3 or later.
5. IANA Considerations
5.1. Registration of Channel Binding Type
IANA has registered tls-exporter in the "Channel-Binding Types"
Channel-binding unique prefix: tls-exporter
Channel-binding type: unique
Channel type: TLS [RFC8446
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
Owner/Change controller name and email address: IESG
Expert reviewer name and contact information: IETF KITTEN WG
<firstname.lastname@example.org> or IETF TLS WG <email@example.com>
Note: See the published specification for advice on the
applicability of this channel binding type.
5.2. 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)
Reference: RFC 9266
6.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056
, DOI 10.17487/RFC5056
, November 2007,
] 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
] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801
, DOI 10.17487/RFC5801
July 2010, <https://www.rfc-editor.org/info/rfc5801
] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802
, July 2010,
] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929
, DOI 10.17487/RFC5929
, July 2010,
] Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
Authentication and Security Layer (SASL) Mechanisms", RFC 7677
, DOI 10.17487/RFC7677
, November 2015,
] 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
] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446
, DOI 10.17487/RFC8446
, August 2018,
6.2. Informative References
] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246
, August 2008,
] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension", RFC 7627
, DOI 10.17487/RFC7627
, September 2015,
Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
A., and P. Strub, "Triple Handshakes Considered Harmful:
Breaking and Fixing Authentication over TLS", March 2014,
United States of America