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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group O. Friel 3 Internet-Draft Cisco 4 Intended status: Standards Track D. Harkins 5 Expires: August 8, 2022 Hewlett-Packard Enterprise 6 February 04, 2022 8 Bootstrapped TLS Authentication 9 draft-friel-tls-eap-dpp-04 11 Abstract 13 This document defines a TLS extension that enables a server to prove 14 to a client that it has knowledge of the public key of a key pair 15 where the client has knowledge of the private key of the key pair. 16 Unlike standard TLS key exchanges, the public key is never exchanged 17 in TLS protocol messages. Proof of knowledge of the public key is 18 used by the client to bootstrap trust in the server. The use case 19 outlined in this document is to establish trust in an EAP server. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on August 8, 2022. 38 Copyright Notice 40 Copyright (c) 2022 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (https://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 56 1.1. Bootstrap Key Pair . . . . . . . . . . . . . . . . . . . 2 57 1.2. Alignment with Wi-Fi Alliance Device Provisioning Profile 3 58 2. Bootstrapping in TLS 1.3 . . . . . . . . . . . . . . . . . . 4 59 2.1. Bootstrap Extended PSK . . . . . . . . . . . . . . . . . 4 60 2.2. Changes to TLS 1.3 Handshake . . . . . . . . . . . . . . 5 61 3. Using TLS Bootstrapping in EAP . . . . . . . . . . . . . . . 6 62 4. Summary of Work . . . . . . . . . . . . . . . . . . . . . . . 7 63 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 64 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 65 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 66 7.1. Normative References . . . . . . . . . . . . . . . . . . 8 67 7.2. Informative References . . . . . . . . . . . . . . . . . 8 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 70 1. Introduction 72 On-boarding of devices with no, or limited, user interface can be 73 difficult. Typically, a credential is needed to access the network 74 and network connectivity is needed to obtain a credential. This 75 poses a catch-22. 77 If trust in the integrity of a device's public key can be obtained in 78 an out-of-band fashion, a device can be authenticated and provisioned 79 with a usable credential for network access. While this 80 authentication can be strong, the device's authentication of the 81 network is somewhat weaker. [duckling] presents a functional 82 security model to address this asymmetry. 84 There are on-boarding protocols, such as [DPP], to address this use 85 case but they have drawbacks. [DPP] for instance does not support 86 wired network access. This document describes an on-boarding 87 protocol, which we refer to as TLS Proof of Knowledge or TLS-POK. 89 1.1. Bootstrap Key Pair 91 The mechanism for on-boarding of devices defined in this document 92 relies on bootstrap key pairs. A client device has an associated 93 elliptic curve (EC) key pair. The key pair may be static and baked 94 into device firmware at manufacturing time, or may be dynamic and 95 generated at on-boarding time by the device. If this public key, 96 specifically the ASN.1 SEQUENCE SubjectPublicKeyInfo from [RFC5280], 97 can be shared in a trustworthy manner with a TLS server, a form of 98 "origin entity authentication" (the step from which all subsequent 99 authentication proceeds) can be obtained. 101 The exact mechanism by which the server gains knowledge of the public 102 key is out of scope of this specification, but possible mechanisms 103 include scanning a QR code to obtain a base64 encoding of the 104 ASN.1-formatted public key or uploading of a Bill of Materials (BOM) 105 which includes the public key. If the QR code is physically attached 106 to the client device, or the BOM is associated with the device, the 107 assumption is that the public key obtained in this bootstrapping 108 method belongs to the client. In this model, physical possession of 109 the device implies legitimate ownership. 111 The server may have knowledge of multiple bootstrap public keys 112 corresponding to multiple devices, and TLS extensions are defined in 113 this document that enable the server to identity a specific bootstrap 114 public key correspinding to a specific device. 116 Using the process defined herein, the client proves to the server 117 that it has possession of the private analog to its public 118 bootstrapping key. Provided that the mechanism in which the server 119 obtained the bootstrapping key is trustworthy, a commensurate amount 120 of authenticity of the resulting connection can be obtained. The 121 server also proves that it knows the client's public key which, if 122 the client does not gratuitously expose its public key, can be used 123 to obtain a modicum of correctness, that the client is connecting to 124 the correct network (see [duckling]). 126 1.2. Alignment with Wi-Fi Alliance Device Provisioning Profile 128 The definition of the boostrap public key aligns with that given in 129 [DPP]. This, for example, enables the QR code format as defined in 130 [DPP] to be reused for TLS-POK. Therefore, a device that supports 131 both wired LAN and Wi-Fi LAN connections can have a single QR code 132 printed on its label, and the bootstrap key can be used for DPP if 133 the device bootstraps against a Wi-Fi network, or TLS-POK if the 134 device bootstraps against a wired network. Similarly, a common 135 bootstrap public key format could be imported in a BOM into a server 136 that handles devices connecting over both wired and Wi-Fi networks. 138 Any bootstrapping method defined for, or used by, [DPP] is compatible 139 with TLS-POK. 141 2. Bootstrapping in TLS 1.3 143 The bootstrapping modifications introduce an extension to identify a 144 "bootstrapping" key which is converted into an external PSK and used 145 directly in the TLS 1.3 handshake. This key MUST be from a 146 cryptosystem suitable for doing ECDSA. 148 2.1. Bootstrap Extended PSK 150 This document defines the "bskey" extended PSK type by expanding on 151 the work in [extensible-psks]. 153 enum { 154 bskey(TBD), (255) 155 } ExtendedPskIdentityType; 157 A bskey PSK is a varient of an external PSK which, in this case, is 158 derived from a public key. 160 The PSKIdentity of a bskey extended PSK is encoded with a string 161 derived from the DER-encoded ASN.1 subjectPublicKeyInfo 162 representation of the bootstrapping public key. 164 struct { 165 opaque identity<1..2^32-1> 166 } BootstrapPSKIdentity; 168 Both the bskey PSK and the BootstrapPSKIdentity are computed using 169 [RFC5869] with the hash algorithm from the ciphersuite: 171 bskeypsk = HKDF-Expand(HKDF-Extract(<>, bskey), 172 "tls13-extended-psk-bskey", L) 173 identity = HKDF-Expand(HKDF-Extract(<>, bskey), 174 "tls13-psk-identity-bskey", L) 175 where: 176 - <> is a NULL salt 177 - bskey is the DER-encoded ASN.1 subjectPublicKeyInfo 178 representation of the bootstrapping key 179 - L is the length of the digest of the underlying hash 180 algorithm 182 A performance versus storage tradeoff a server can choose is to 183 precompute the identity of every bootstrapped key with every hash 184 algorithm that it uses in TLS and use that to quickly lookup the 185 bootstrap key and generate the PSK. Servers that choose not to 186 employ this optimization will have to do a runtime check with every 187 bootstrap key it holds against the identity the client provides. 189 2.2. Changes to TLS 1.3 Handshake 191 The client includes the "tls_cert_with_extern_psk" extension in the 192 ClientHello, per [RFC8773], and identifies the bootstrapping key 193 using the BootstrapPSKIdentity extension. The server looks up the 194 client's bootstrapping key in its database by checking the hash of 195 each entry with the value received in the ClientHello. If no match 196 is found, the server SHALL terminate the TLS handshake with an alert. 198 If the server found the matching bootstrap key, it generates the 199 bskeypsk and includes the "tls_cert_with_extern_psk" extension in the 200 ServerHello message. When these extensions have been successfully 201 negotiated, the TLS 1.3 key schedule SHALL include both the bskeypsk 202 in the Early Secret derivation and an (EC)DHE shared secret value in 203 the Handshake Secret derivation. 205 After successful negotiation of these extensions, the full TLS 1.3 206 handshake is performed with the additional caveat that the client 207 authenticates with a raw public key (its bootstrapping key) per 208 [RFC7250]. The bootstrapping key is always an elliptic curve public 209 key, therefore the ClientCertTypeExtension SHALL always indicate 210 RawPublicKey and the type of the client's Certificate SHALL be ECDSA 211 and contain the client's bootstrapping key as a DER-encoded ASN.1 212 subjectPublicKeyInfo SEQUENCE. 214 When the server processes the client's Certificate it MUST ensure 215 that it is identical to the bootstrapping public key that it used to 216 generate an external PSK and PSKIdentifier for this handshake. 218 When clients use the [duckling] form of authentication, they MAY 219 forgo the checking of the server's certificate in the 220 CertificateVerify and rely on the integrity of the bootstrapping 221 method employed to distribute its key in order to validate trust in 222 the authenticated TLS connection. 224 The handshake is shown in Figure 1. 226 Client Server 227 -------- -------- 228 ClientHello 229 + bskey_id 230 + cert_with_extern_psk 231 + client_cert_type=RawPublicKey 232 + key_share --------> 233 ServerHello 234 + bskey_id 235 + cert_with_extern_psk 236 + client_cert_type=RawPublicKey 237 + key_share 238 {EncryptedExtensions} 239 {CertificateRequest} 240 {Certificate} 241 {CertificateVerify} 242 <-------- {Finished} 243 {Certificate} 244 {CertificateVerify} 245 {Finished} --------> 246 [Application Data] <-------> [Application Data] 248 Figure 1: TLS 1.3 TLS-POK Handshake 250 3. Using TLS Bootstrapping in EAP 252 Enterprise deployments typically require an 802.1X/EAP-based 253 authentication to obtain network access. Protocols like [RFC7030] 254 can be used to enroll devices into a Certification Authority to allow 255 them to authenticate using 802.1X/EAP. But this creates a Catch-22 256 where a certificate is needed for network access and network access 257 is needed to obtain certificate. 259 Devices whose bootstrapping key can been obtained in an out-of-band 260 fashion can perform an EAP-TLS-based exchange, for instance 261 [RFC7170], and authenticate the TLS exchange using the bootstrapping 262 extensions defined in Section 2. This network connectivity can then 263 be used to perform an enrollment protocol (such as provided by 264 [RFC7170]) to obtain a credential for subsequent network connectivity 265 and certificate lifecycle maintenance. 267 Upon "link up", an Authenticator on an 802.1X-protected port will 268 issue an EAP Identify request to the newly connected peer. For 269 unprovisioned devices that desire to take advantage of TLS-POK, there 270 is no initial realm in which to construct an NAI (see [RFC4282]) so 271 the initial EAP Identity response SHOULD contain simply the name 272 "TLS-POK" in order to indicate to the Authenticator that an EAP 273 method that supports TLS-POK SHOULD be started. 275 Authenticating Peer Authenticator 276 ------------------- ------------- 277 <- EAP-Request/ 278 Identity 280 EAP-Response/ 281 Identity (TLS-POK) -> 283 <- EAP-Request/ 284 EAP-Type=TEAP 285 (TLS Start) 286 . 287 . 288 . 290 4. Summary of Work 292 The protocol outlined here can be broadly broken up into 4 distinct 293 areas: 295 o TLS extensions to transport the bootstrap public key identifier 297 o Use of the TLS 1.3 extension for certificate-based authentication 298 with an external PSK 300 o The client's use of a raw public key in its certificate 302 o TEAP extensions to leverage the new TLS-POK handshake for trust 303 establishment 305 This document captures all 4 areas. 307 5. IANA Considerations 309 IANA will allocated an ExtensionPSKIdentityType for the bskey type 310 from the TLS 1.3 repository created by [extensible-psks] and replace 311 TBD in this document with that number. 313 6. Security Considerations 315 Bootstrap and trust establishment by the TLS server is based on proof 316 of knowledge of the client's bootstrap public key, a non-public 317 datum. The TLS server obtains proof that the client knows its 318 bootstrap public key and, in addition, also possesses its 319 corresponding private analog. 321 Trust on the part of the client is based on validation of the server 322 certificate and the TLS 1.3 handshake. In addition, the client 323 assumes that knowledge of its public bootstrapping key is not widely 324 disseminated and therefore any device that proves knowledge of it's 325 bootstrapping key is the appropriate device from which to receive 326 provisioning, for instance via [RFC7170]. [duckling] describes a 327 security model for this type of "imprinting". 329 An attack on the bootstrapping method which substitutes the public 330 key of a corrupted device for the public key of an honest device can 331 result in the TLS sever on-boarding and trusting the corrupted 332 device. 334 If an adversary has knowledge of the bootstrap public key, the 335 adversary may be able to make the client bootstrap against the 336 adversary's network. For example, if an adversary intercepts and 337 scans QR labels on clients, and the adversary can force the client to 338 connect to its server, then the adversary can complete the TLS-POK 339 handshake with the client and the client will connect to the 340 adversary's server. Since physical possession implies ownership, 341 there is nothing to prevent a stolen device from being on-boarded. 343 7. References 345 7.1. Normative References 347 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand 348 Key Derivation Function (HKDF)", RFC 5869, 349 DOI 10.17487/RFC5869, May 2010, 350 . 352 [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., 353 Weiler, S., and T. Kivinen, "Using Raw Public Keys in 354 Transport Layer Security (TLS) and Datagram Transport 355 Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, 356 June 2014, . 358 [RFC8773] Housley, R., "TLS 1.3 Extension for Certificate-Based 359 Authentication with an External Pre-Shared Key", RFC 8773, 360 DOI 10.17487/RFC8773, March 2020, 361 . 363 7.2. Informative References 365 [DPP] Wi-Fi Alliance, "Device Provisioning Profile", 2020. 367 [duckling] 368 Stajano, F. and E. Rescorla, "The Ressurecting Ducking: 369 Security Issues for Ad-Hoc Wireless Networks", 1999. 371 [extensible-psks] 372 Wood, C. and R. Anderson, "Extensible Pre-Shared Key Types 373 for TLS", n.d., . 376 [RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The 377 Network Access Identifier", RFC 4282, 378 DOI 10.17487/RFC4282, December 2005, 379 . 381 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 382 Housley, R., and W. Polk, "Internet X.509 Public Key 383 Infrastructure Certificate and Certificate Revocation List 384 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 385 . 387 [RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., 388 "Enrollment over Secure Transport", RFC 7030, 389 DOI 10.17487/RFC7030, October 2013, 390 . 392 [RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna, 393 "Tunnel Extensible Authentication Protocol (TEAP) Version 394 1", RFC 7170, DOI 10.17487/RFC7170, May 2014, 395 . 397 Authors' Addresses 399 Owen Friel 400 Cisco 402 Email: ofriel@cisco.com 404 Dan Harkins 405 Hewlett-Packard Enterprise 407 Email: daniel.harkins@hpe.com