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Whited 3 Internet-Draft 4 March 2022 4 Updates: 5801, 5802, 5929, 7677, 8446 (if 5 approved) 6 Intended status: Standards Track 7 Expires: 5 September 2022 9 Channel Bindings for TLS 1.3 10 draft-ietf-kitten-tls-channel-bindings-for-tls13-15 12 Abstract 14 This document defines a channel binding type, tls-exporter, that is 15 compatible with TLS 1.3 in accordance with RFC 5056, On Channel 16 Binding. Furthermore, it updates the default channel binding to the 17 new binding for versions of TLS greater than 1.2. This document 18 updates RFC5801, RFC5802, RFC5929, RFC7677, and RFC8446. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at https://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on 5 September 2022. 37 Copyright Notice 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 43 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 44 license-info) in effect on the date of publication of this document. 45 Please review these documents carefully, as they describe your rights 46 and restrictions with respect to this document. Code Components 47 extracted from this document must include Revised BSD License text as 48 described in Section 4.e of the Trust Legal Provisions and are 49 provided without warranty as described in the Revised BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 1.1. Conventions and Terminology . . . . . . . . . . . . . . . 2 55 2. The 'tls-exporter' Channel Binding Type . . . . . . . . . . . 3 56 3. TLS 1.3 with SCRAM or GSS-API over SASL . . . . . . . . . . . 3 57 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 58 4.1. Uniqueness of Channel Bindings . . . . . . . . . . . . . 4 59 4.2. Use with Legacy TLS . . . . . . . . . . . . . . . . . . . 5 60 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 61 5.1. Registration of Channel Binding Type . . . . . . . . . . 5 62 5.2. Registration of Channel Binding TLS Exporter Label . . . 6 63 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 64 6.1. Normative References . . . . . . . . . . . . . . . . . . 6 65 6.2. Informative References . . . . . . . . . . . . . . . . . 7 66 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 68 1. Introduction 70 The "tls-unique" channel binding type defined in [RFC5929] was found 71 to be vulnerable to the "triple handshake vulnerability" 72 [TRIPLE-HANDSHAKE] without the extended master secret extension 73 defined in [RFC7627]. While TLS 1.3 uses a complete transcript hash 74 akin to the extended master secret procedures, the safety of channel 75 bindings with TLS 1.3 was not analyzed as part of the core protocol 76 work, and so the specification of channel bindings for TLS 1.3 was 77 deferred. [RFC8446] section C.5 notes the lack of channel bindings 78 for TLS 1.3; as this document defines such channel bindings, it 79 updates [RFC8446] to note that this gap has been filled. 80 Furthermore, this document updates [RFC5929] by adding an additional 81 unique channel binding type, "tls-exporter", that replaces some usage 82 of "tls-unique". 84 1.1. 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 BCP 92 14 [RFC2119] [RFC8174] when, and only when, they appear in all 93 capitals, as shown here. 95 2. The 'tls-exporter' Channel Binding Type 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 TLS 100 implementations. The EKM is obtained using the keying material 101 exporters for TLS as defined in [RFC5705] and [RFC8446] section 7.5 102 by supplying the following inputs: 104 Label: The ASCII string "EXPORTER-Channel-Binding" with no 105 terminating NUL. 107 Context value: Zero-length string. 109 Length: 32 bytes. 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 is always true for TLS 1.3 (see [RFC8446] appendix D). 116 3. TLS 1.3 with SCRAM or GSS-API over SASL 118 SCRAM ([RFC5802], and [RFC7677]) and GSS-API over SASL [RFC5801] 119 define "tls-unique" as the default channel binding to use over TLS. 120 As "tls-unique" is not defined for TLS 1.3 (and greater), this 121 document updates [RFC5801], [RFC5802], and [RFC7677] to use "tls- 122 exporter" as the default channel binding over TLS 1.3 (and greater). 123 Note that this document does not change the default channel binding 124 for SCRAM mechanisms over TLS 1.2 [RFC5246], which is still "tls- 125 unique". 127 Additionally, this document updates the aforementioned documents to 128 make "tls-exporter" the mandatory to implement channel binding if any 129 channel bindings are implemented for TLS 1.3. Implementations that 130 support channel binding over TLS 1.3 MUST implement "tls-exporter". 132 4. Security Considerations 134 The channel binding type defined in this document is constructed so 135 that disclosure of the channel binding data does not leak secret 136 information about the TLS channel and does not affect the security of 137 the TLS channel. 139 The derived data MUST NOT be used for any purpose other than channel 140 bindings as described in [RFC5056]. In particular, implementations 141 MUST NOT use channel binding as a secret key to protect privileged 142 information. 144 The Security Considerations sections of [RFC5056], [RFC5705], and 145 [RFC8446] apply to this document. 147 4.1. Uniqueness of Channel Bindings 149 The definition of channel bindings in [RFC5056] defines the concept 150 of a "unique" channel binding as being one that is unique to the 151 channel endpoints and unique over time, that is, a value that is 152 unique to a specific instance of the lower layer security protocol. 153 When TLS is the lower layer security protocol, as for the channel 154 binding type defined in this document, this concept of uniqueness 155 corresponds to uniquely identifying the specific TLS connection. 157 However, a stronger form of uniqueness is possible, which would 158 entail uniquely identifying not just the lower layer protocol but 159 also the upper layer application or authentication protocol that is 160 consuming the channel binding. The distinction is relevant only when 161 there are multiple instances of an authentication protocol, or 162 multiple distinct authentication protocols, that run atop the same 163 lower layer protocol. Such a situation is rare -- most consumers of 164 channel bindings establish an instance of the lower layer secure 165 protocol, run a single application or authentication protocol as the 166 upper layer protocol, then terminate both upper and lower layer 167 protocols. In this situation the stronger form of uniqueness is 168 trivially achieved, given that the channel binding value is unique in 169 the sense of [RFC5056]. 171 The channel binding type defined by this document provides only the 172 weaker type of uniqueness, as per [RFC5056]; it does not achieve the 173 stronger uniqueness per upper layer protocol instance described 174 above. This stronger form of uniqueness would be useful in that it 175 provides protection against cross-protocol attacks for the multiple 176 authentication protocols running over the same instance of the lower 177 layer protocol, and it provides protection against replay attacks 178 that seek to replay a message from one instance of an authentication 179 protocol in a different instance of the same authentication protocol, 180 again running over the same instance of the lower layer protocol. 181 Both of these properties are highly desirable when performing formal 182 analysis of upper layer protocols; if these properties are not 183 provided, such formal analysis is essentially impossible. In some 184 cases one or both of these properties may already be provided by 185 specific upper layer protocols, but that is dependent on the 186 mechanism(s) in question, and formal analysis requires that the 187 property is provided in a generic manner, across all potential upper 188 layer protocols that exist or might exist in the future. 190 Accordingly, applications that make use of the channel binding type 191 defined in this document MUST NOT use the channel binding for more 192 than one authentication mechanism instance on a given TLS connection. 193 Such applications MUST immediately close the TLS connection after the 194 conclusion of the upper layer protocol. 196 4.2. Use with Legacy TLS 198 While it is possible to use this channel binding mechanism with TLS 199 versions below 1.3, extra precaution must be taken to ensure that the 200 chosen cipher suites always result in unique master secrets. For 201 more information see [RFC7627] and the Security Considerations 202 section of [RFC5705] (TLS 1.3 always provides unique master secrets, 203 as discussed in Appendix D of [RFC8446].) 205 When TLS renegotiation is enabled on a connection the "tls-exporter" 206 channel binding type is not defined for that connection and 207 implementations MUST NOT support it. 209 In general, users wishing to take advantage of channel binding should 210 upgrade to TLS 1.3 or later. 212 5. IANA Considerations 214 5.1. Registration of Channel Binding Type 216 This document adds the following registration in the "Channel-Binding 217 Types" registry: 219 Subject: Registration of channel binding tls-exporter 221 Channel binding unique prefix: tls-exporter 223 Channel binding type: unique 225 Channel type: TLS [RFC8446] 227 Published specification: draft-ietf-kitten-tls-channel-bindings-for- 228 tls13-15 230 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 . 239 Owner/Change controller name and email address: IESG. 241 Expert reviewer name and contact information: IETF KITTEN or TLS WG 242 (kitten@ietf.org or tls@ietf.org, failing that, ietf@ietf.org). 244 Note: See the published specification for advice on the 245 applicability of this channel binding type. 247 5.2. Registration of Channel Binding TLS Exporter Label 249 This document adds the following registration in the "TLS Exporter 250 Labels" registry, which is part of the "Transport Layer Security 251 (TLS) Parameters" group: 253 Value: EXPORTER-Channel-Binding 255 DTLS-OK: Y 257 Recommended: Y 259 Reference: This document 261 6. References 263 6.1. 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 . 270 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 271 Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007, 272 . 274 [RFC5705] Rescorla, E., "Keying Material Exporters for Transport 275 Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, 276 March 2010, . 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, . 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 . 290 [RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings 291 for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010, 292 . 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 . 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, . 303 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 304 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 305 . 307 6.2. 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 . 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 . 320 [TRIPLE-HANDSHAKE] 321 Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti, 322 A., and P. Strub, "Password Storage", March 2014, 323 . 325 Author's Address 327 Sam Whited 328 Atlanta, GA 329 United States of America 330 Email: sam@samwhited.com 331 URI: https://blog.samwhited.com/