5Internet Engineering Task Force (IETF) S. Whited
6Request for Comments: 9266 July 2022
7Updates: 5801, 5802, 5929, 7677
8Category: Standards Track
12 Channel Bindings for TLS 1.3
16 This document defines a channel binding type, tls-exporter, that is
17 compatible with TLS 1.3 in accordance with RFC 5056, "On the Use of
18 Channel Bindings to Secure Channels". Furthermore, it updates the
19 default channel binding to the new binding for versions of TLS
20 greater than 1.2. This document updates RFCs 5801, 5802, 5929, and
25 This is an Internet Standards Track document.
27 This document is a product of the Internet Engineering Task Force
28 (IETF). It represents the consensus of the IETF community. It has
29 received public review and has been approved for publication by the
30 Internet Engineering Steering Group (IESG). Further information on
31 Internet Standards is available in Section 2 of RFC 7841.
33 Information about the current status of this document, any errata,
34 and how to provide feedback on it may be obtained at
35 https://www.rfc-editor.org/info/rfc9266.
39 Copyright (c) 2022 IETF Trust and the persons identified as the
40 document authors. All rights reserved.
42 This document is subject to BCP 78 and the IETF Trust's Legal
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48 include Revised BSD License text as described in Section 4.e of the
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50 in the Revised BSD License.
55 1.1. Conventions and Terminology
56 2. The 'tls-exporter' Channel Binding Type
57 3. TLS 1.3 with SCRAM or GSS-API over SASL
58 4. Security Considerations
59 4.1. Uniqueness of Channel Bindings
60 4.2. Use with Legacy TLS
61 5. IANA Considerations
62 5.1. Registration of Channel Binding Type
63 5.2. Registration of Channel Binding TLS Exporter Label
65 6.1. Normative References
66 6.2. Informative References
71 The "tls-unique" channel binding type defined in [RFC5929] was found
72 to be susceptible to the "triple handshake vulnerability"
73 [TRIPLE-HANDSHAKE] without the extended master secret extension
74 defined in [RFC7627]. While TLS 1.3 uses a complete transcript hash
75 akin to the extended master secret procedures, the safety of channel
76 bindings with TLS 1.3 was not analyzed as part of the core protocol
77 work, so the specification of channel bindings for TLS 1.3 was
78 deferred. Appendix C.5 of [RFC8446] notes the lack of channel
79 bindings for TLS 1.3; this document defines such channel bindings and
80 fills that gap. Furthermore, this document updates [RFC5929] by
81 adding an additional unique channel binding type, "tls-exporter",
82 that replaces some usage of "tls-unique".
841.1. Conventions and Terminology
86 Throughout this document, the acronym "EKM" is used to refer to
87 "Exported Keying Material" as defined in [RFC5705].
89 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
90 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
91 "OPTIONAL" in this document are to be interpreted as described in
92 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
93 capitals, as shown here.
97 Channel binding mechanisms are not useful until TLS implementations
98 expose the required data. To facilitate this, "tls-exporter" uses
99 Exported Keying Material (EKM), which is already widely exposed by
100 TLS implementations. The EKM is obtained using the keying material
101 exporters for TLS, as defined in [RFC5705] and Section 7.5 of
102 [RFC8446], by supplying the following inputs:
104 Label: The ASCII string "EXPORTER-Channel-Binding" with no
107 Context value: Zero-length string.
111 This channel binding mechanism is defined only when the TLS handshake
112 results in unique master secrets. This is true of TLS versions prior
113 to 1.3 when the extended master secret extension of [RFC7627] is in
114 use, and it is always true for TLS 1.3 (see Appendix D of [RFC8446]).
1163. TLS 1.3 with SCRAM or GSS-API over SASL
118 The specifications for Salted Challenge Response Authentication
119 Mechanism (SCRAM) [RFC5802] [RFC7677] and Generic Security Service
120 Application Program Interface (GSS-API) over Simple Authentication
121 and Security Layer (SASL) [RFC5801] define "tls-unique" as the
123 defined for TLS 1.3 (and greater), this document updates [RFC5801],
124 [RFC5802], and [RFC7677] to use "tls-exporter" as the default channel
125 binding over TLS 1.3 (and greater). Note that this document does not
126 change the default channel binding for SCRAM mechanisms over TLS 1.2
127 [RFC5246], which is still "tls-unique" (also note that RFC 5246 has
128 been obsoleted by RFC 8446).
130 Additionally, this document updates the aforementioned documents to
131 make "tls-exporter" the mandatory-to-implement channel binding if any
132 channel bindings are implemented for TLS 1.3. Implementations that
133 support channel binding over TLS 1.3 MUST implement "tls-exporter".
1354. Security Considerations
137 The channel binding type defined in this document is constructed so
138 that disclosure of the channel binding data does not leak secret
139 information about the TLS channel and does not affect the security of
142 The derived data MUST NOT be used for any purpose other than channel
143 bindings as described in [RFC5056]. In particular, implementations
144 MUST NOT use channel binding as a secret key to protect privileged
147 The Security Considerations sections of [RFC5056], [RFC5705], and
148 [RFC8446] apply to this document.
1504.1. Uniqueness of Channel Bindings
152 The definition of channel bindings in [RFC5056] defines the concept
153 of a "unique" channel binding as being one that is unique to the
154 channel endpoints and unique over time, that is, a value that is
155 unique to a specific instance of the lower-layer security protocol.
156 When TLS is the lower-layer security protocol, as for the channel
157 binding type defined in this document, this concept of uniqueness
158 corresponds to uniquely identifying the specific TLS connection.
160 However, a stronger form of uniqueness is possible, which would
161 entail uniquely identifying not just the lower-layer protocol but
162 also the upper-layer application or authentication protocol that is
163 consuming the channel binding. The distinction is relevant only when
164 there are multiple instances of an authentication protocol, or
165 multiple distinct authentication protocols, that run atop the same
166 lower-layer protocol. Such a situation is rare; most consumers of
167 channel bindings establish an instance of the lower-layer secure
168 protocol, run a single application or authentication protocol as the
169 upper-layer protocol, then terminate both upper and lower-layer
170 protocols. In this situation, the stronger form of uniqueness is
171 trivially achieved, given that the channel binding value is unique in
172 the sense of [RFC5056].
174 The channel binding type defined by this document provides only the
175 weaker type of uniqueness, as per [RFC5056]; it does not achieve the
176 stronger uniqueness per the upper-layer protocol instance described
177 above. This stronger form of uniqueness would be useful in that it
178 provides protection against cross-protocol attacks for the multiple
179 authentication protocols running over the same instance of the lower-
180 layer protocol, and it provides protection against replay attacks
181 that seek to replay a message from one instance of an authentication
182 protocol in a different instance of the same authentication protocol,
183 again running over the same instance of the lower-layer protocol.
184 Both of these properties are highly desirable when performing formal
185 analysis of upper-layer protocols; if these properties are not
186 provided, such formal analysis is essentially impossible. In some
187 cases, one or both of these properties may already be provided by
188 specific upper-layer protocols, but that is dependent on the
189 mechanism(s) in question, and formal analysis requires that the
190 property is provided in a generic manner across all potential upper-
191 layer protocols that exist or might exist in the future.
193 Accordingly, applications that make use of the channel binding type
194 defined in this document MUST NOT use the channel binding for more
195 than one authentication mechanism instance on a given TLS connection.
196 Such applications MUST immediately close the TLS connection after the
197 conclusion of the upper-layer protocol.
1994.2. Use with Legacy TLS
202 versions below 1.3, extra precaution must be taken to ensure that the
203 chosen cipher suites always result in unique master secrets. For
204 more information, see [RFC7627] and the Security Considerations
205 section of [RFC5705] (TLS 1.3 always provides unique master secrets,
206 as discussed in Appendix D of [RFC8446]).
208 When TLS renegotiation is enabled on a connection, the "tls-exporter"
209 channel binding type is not defined for that connection, and
210 implementations MUST NOT support it.
212 In general, users wishing to take advantage of channel binding should
213 upgrade to TLS 1.3 or later.
2155. IANA Considerations
2175.1. Registration of Channel Binding Type
219 IANA has registered tls-exporter in the "Channel-Binding Types"
222 Channel-binding unique prefix: tls-exporter
224 Channel-binding type: unique
226 Channel type: TLS [RFC8446]
228 Published specification: RFC 9266
230 Channel-binding is secret: no
232 Description: The EKM value obtained from the current TLS connection.
234 Intended usage: COMMON
236 Person and email address to contact for further information: Sam
237 Whited <sam@samwhited.com>
239 Owner/Change controller name and email address: IESG
241 Expert reviewer name and contact information: IETF KITTEN WG
242 <kitten@ietf.org> or IETF TLS WG <tls@ietf.org>
244 Note: See the published specification for advice on the
245 applicability of this channel binding type.
2475.2. Registration of Channel Binding TLS Exporter Label
249 IANA has added the following registration in the "TLS Exporter
250 Labels" registry under the "Transport Layer Security (TLS)
251 Parameters" registry:
253 Value: EXPORTER-Channel-Binding
2636.1. Normative References
265 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
266 Requirement Levels", BCP 14, RFC 2119,
267 DOI 10.17487/RFC2119, March 1997,
268 <https://www.rfc-editor.org/info/rfc2119>.
270 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
271 Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
272 <https://www.rfc-editor.org/info/rfc5056>.
274 [RFC5705] Rescorla, E., "Keying Material Exporters for Transport
275 Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
276 March 2010, <https://www.rfc-editor.org/info/rfc5705>.
278 [RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
279 Service Application Program Interface (GSS-API) Mechanisms
280 in Simple Authentication and Security Layer (SASL): The
281 GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
282 July 2010, <https://www.rfc-editor.org/info/rfc5801>.
284 [RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
285 "Salted Challenge Response Authentication Mechanism
286 (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
287 DOI 10.17487/RFC5802, July 2010,
288 <https://www.rfc-editor.org/info/rfc5802>.
290 [RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
291 for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
292 <https://www.rfc-editor.org/info/rfc5929>.
294 [RFC7677] Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
295 Authentication and Security Layer (SASL) Mechanisms",
296 RFC 7677, DOI 10.17487/RFC7677, November 2015,
297 <https://www.rfc-editor.org/info/rfc7677>.
299 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
300 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
301 May 2017, <https://www.rfc-editor.org/info/rfc8174>.
303 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
304 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
305 <https://www.rfc-editor.org/info/rfc8446>.
3076.2. Informative References
309 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
310 (TLS) Protocol Version 1.2", RFC 5246,
311 DOI 10.17487/RFC5246, August 2008,
312 <https://www.rfc-editor.org/info/rfc5246>.
314 [RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
315 Langley, A., and M. Ray, "Transport Layer Security (TLS)
316 Session Hash and Extended Master Secret Extension",
317 RFC 7627, DOI 10.17487/RFC7627, September 2015,
318 <https://www.rfc-editor.org/info/rfc7627>.
321 Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
322 A., and P. Strub, "Triple Handshakes Considered Harmful:
323 Breaking and Fixing Authentication over TLS", March 2014,
324 <https://www.mitls.org/pages/attacks/3SHAKE>.
330 United States of America
331 Email: sam@samwhited.com
332 URI: https://blog.samwhited.com/