Network Working Group O. Friel Internet-Draft Cisco Intended status: Standards Track D. Harkins Expires: 27 November 2022 Hewlett-Packard Enterprise 26 May 2022 Bootstrapped TLS Authentication draft-friel-tls-eap-dpp-05 Abstract This document defines a TLS extension that enables a server to prove to a client that it has knowledge of the public key of a key pair where the client has knowledge of the private key of the key pair. Unlike standard TLS key exchanges, the public key is never exchanged in TLS protocol messages. Proof of knowledge of the public key is used by the client to bootstrap trust in the server. The use case outlined in this document is to establish trust in an EAP server. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 27 November 2022. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. Friel & Harkins Expires 27 November 2022 [Page 1] Internet-Draft TLS-POK May 2022 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Bootstrap Key Pair . . . . . . . . . . . . . . . . . . . 3 1.3. Alignment with Wi-Fi Alliance Device Provisioning Profile . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Bootstrapping in TLS 1.3 . . . . . . . . . . . . . . . . . . 4 2.1. External PSK Derivation . . . . . . . . . . . . . . . . . 4 2.2. Changes to TLS 1.3 Handshake . . . . . . . . . . . . . . 5 3. Using TLS Bootstrapping in EAP . . . . . . . . . . . . . . . 7 4. Summary of Work . . . . . . . . . . . . . . . . . . . . . . . 8 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction On-boarding of devices with no, or limited, user interface can be difficult. Typically, a credential is needed to access the network and network connectivity is needed to obtain a credential. This poses a catch-22. If trust in the integrity of a device's public key can be obtained in an out-of-band fashion, a device can be authenticated and provisioned with a usable credential for network access. While this authentication can be strong, the device's authentication of the network is somewhat weaker. [duckling] presents a functional security model to address this asymmetry. There are on-boarding protocols, such as [DPP], to address this use case but they have drawbacks. [DPP] for instance does not support wired network access. This document describes an on-boarding protocol, which we refer to as TLS Proof of Knowledge or TLS-POK. Friel & Harkins Expires 27 November 2022 [Page 2] Internet-Draft TLS-POK May 2022 1.1. Terminology The following terminology is used throughout this document. * BSK: Bootstrap Key which is an elliptic curve public private key pair. * DPP: Device Provisioning Protocol * EPSK: External Pre-Shared Key * PSK: Pre-Shared Key 1.2. Bootstrap Key Pair The mechanism for on-boarding of devices defined in this document relies on bootstrap key pairs. A client device has an associated elliptic curve (EC) bootstrap key pair (BSK). The BSK may be static and baked into device firmware at manufacturing time, or may be dynamic and generated at on-boarding time by the device. If the BSK public key, specifically the ASN.1 SEQUENCE SubjectPublicKeyInfo from [RFC5280], can be shared in a trustworthy manner with a TLS server, a form of "origin entity authentication" (the step from which all subsequent authentication proceeds) can be obtained. The exact mechanism by which the server gains knowledge of the BSK public key is out of scope of this specification, but possible mechanisms include scanning a QR code to obtain a base64 encoding of the ASN.1-formatted public key or uploading of a Bill of Materials (BOM) which includes the public key. If the QR code is physically attached to the client device, or the BOM is associated with the device, the assumption is that the public key obtained in this bootstrapping method belongs to the client. In this model, physical possession of the device implies legitimate ownership. The server may have knowledge of multiple BSK public keys corresponding to multiple devices, and existing TLS mechanisms are leveraged that enable the server to identity a specific bootstrap public key corresponding to a specific device. Friel & Harkins Expires 27 November 2022 [Page 3] Internet-Draft TLS-POK May 2022 Using the process defined herein, the client proves to the server that it has possession of the private analog to its public bootstrapping key. Provided that the mechanism in which the server obtained the BSK public key is trustworthy, a commensurate amount of authenticity of the resulting connection can be obtained. The server also proves that it knows the client's public key which, if the client does not gratuitously expose its public key, can be used to obtain a modicum of correctness, that the client is connecting to the correct network (see [duckling]). 1.3. Alignment with Wi-Fi Alliance Device Provisioning Profile The definition of the BSK public key aligns with that given in [DPP]. This, for example, enables the QR code format as defined in [DPP] to be reused for TLS-POK. Therefore, a device that supports both wired LAN and Wi-Fi LAN connections can have a single QR code printed on its label, and the bootstrap key can be used for DPP if the device bootstraps against a Wi-Fi network, or TLS-POK if the device bootstraps against a wired network. Similarly, a common bootstrap public key format could be imported in a BOM into a server that handles devices connecting over both wired and Wi-Fi networks. Any bootstrapping method defined for, or used by, [DPP] is compatible with TLS-POK. 2. Bootstrapping in TLS 1.3 Bootstrapping in TLS 1.3 leverages Certificate-Based Authentication with an External Pre-Shared Key [RFC8773]. The External PSK (EPSK) is derived from the BSK public key, and the EPSK is imported using [I-D.ietf-tls-external-psk-importer]. This BSK MUST be from a cryptosystem suitable for doing ECDSA. The TLS PSK handshake gives the client proof that the server knows the BSK public key. Certificate based authentication of the client by the server is carried out using the BSK, giving the server proof that the client knows the BSK private key. This satisfies the proof of ownership requirements outlined in Section 1. 2.1. External PSK Derivation An [I-D.ietf-tls-external-psk-importer] EPSK is made of of the tuple of (Base Key, External Identity, Hash). The EPSK is derived from the BSK public key using [RFC5869] with the hash algorithm from the ciphersuite: Friel & Harkins Expires 27 November 2022 [Page 4] Internet-Draft TLS-POK May 2022 epsk = HKDF-Expand(HKDF-Extract(<>, bskey), "tls13-imported-bsk", L) epskid = HKDF-Expand(HKDF-Extract(<>, bskey), "tls13-bspsk-identity", L) where: - epsk is the EPSK Base Key - epskid is the EPSK External Identity - <> is a NULL salt - bskey is the DER-encoded ASN.1 subjectPublicKeyInfo representation of the BSK public key - L is the length of the digest of the underlying hash algorithm The [I-D.ietf-tls-external-psk-importer] ImportedIdentity structure is defined as: struct { opaque external_identity<1...2^16-1>; opaque context<0..2^16-1>; uint16 target_protocol; uint16 target_kdf; } ImportedIdentity; and is created using the following values: external_identity = epskid context = "tls13-bsk" target_protocol = TLS1.3(0x0304) target_kdf = HKDF_SHA256(0x0001) The EPSK and ImportedIdentity are used in the TLS handshake as specified in [I-D.ietf-tls-external-psk-importer]. A performance versus storage tradeoff a server can choose is to precompute the identity of every bootstrapped key with every hash algorithm that it uses in TLS and use that to quickly lookup the bootstrap key and generate the PSK. Servers that choose not to employ this optimization will have to do a runtime check with every bootstrap key it holds against the identity the client provides. 2.2. Changes to TLS 1.3 Handshake The client includes the "tls_cert_with_extern_psk" extension in the ClientHello, per [RFC8773]. The client identifies the BSK by inserting the serialized content of ImportedIdentity into the PskIdentity.identity in the PSK extension, per [I-D.ietf-tls-external-psk-importer]. The server looks up the client's EPSK key in its database using the mechanisms documented in Friel & Harkins Expires 27 November 2022 [Page 5] Internet-Draft TLS-POK May 2022 [I-D.ietf-tls-external-psk-importer]. If no match is found, the server SHALL terminate the TLS handshake with an alert. If the server found the matching BSK, it includes the "tls_cert_with_extern_psk" extension in the ServerHello message, and the corresponding EPSK identity in the "pre_shared_key" extension. When these extensions have been successfully negotiated, the TLS 1.3 key schedule SHALL include both the EPSK in the Early Secret derivation and an (EC)DHE shared secret value in the Handshake Secret derivation. After successful negotiation of these extensions, the full TLS 1.3 handshake is performed with the additional caveat that the client authenticates with a raw public key (its BSK) per [RFC7250]. The BSK is always an elliptic curve key pair, therefore the ClientCertTypeExtension SHALL always indicate RawPublicKey and the type of the client's Certificate SHALL be ECDSA and contain the client's BSK public key as a DER-encoded ASN.1 subjectPublicKeyInfo SEQUENCE. When the server processes the client's Certificate it MUST ensure that it is identical to the BSK public key that it used to generate the EPSK and ImportedIdentity for this handshake. When clients use the [duckling] form of authentication, they MAY forgo the checking of the server's certificate in the CertificateVerify and rely on the integrity of the bootstrapping method employed to distribute its key in order to validate trust in the authenticated TLS connection. The handshake is shown in Figure 1. Friel & Harkins Expires 27 November 2022 [Page 6] Internet-Draft TLS-POK May 2022 Client Server -------- -------- ClientHello + cert_with_extern_psk + client_cert_type=RawPublicKey + key_share + pre_shared_key --------> ServerHello + cert_with_extern_psk + client_cert_type=RawPublicKey + key_share + pre_shared_key {EncryptedExtensions} {CertificateRequest} {Certificate} {CertificateVerify} <-------- {Finished} {Certificate} {CertificateVerify} {Finished} --------> [Application Data] <-------> [Application Data] Figure 1: TLS 1.3 TLS-POK Handshake 3. Using TLS Bootstrapping in EAP Enterprise deployments typically require an 802.1X/EAP-based authentication to obtain network access. Protocols like [RFC7030] can be used to enroll devices into a Certification Authority to allow them to authenticate using 802.1X/EAP. But this creates a Catch-22 where a certificate is needed for network access and network access is needed to obtain certificate. Devices whose bootstrapping key can been obtained in an out-of-band fashion can perform an EAP-TLS-based exchange, for instance [RFC7170], and authenticate the TLS exchange using the bootstrapping extensions defined in Section 2. This network connectivity can then be used to perform an enrollment protocol (such as provided by [RFC7170]) to obtain a credential for subsequent network connectivity and certificate lifecycle maintenance. Upon "link up", an Authenticator on an 802.1X-protected port will issue an EAP Identify request to the newly connected peer. For unprovisioned devices that desire to take advantage of TLS-POK, there is no initial realm in which to construct an NAI (see [RFC4282]) so the initial EAP Identity response SHOULD contain simply the name "TLS-POK" in order to indicate to the Authenticator that an EAP method that supports TLS-POK SHOULD be started. Friel & Harkins Expires 27 November 2022 [Page 7] Internet-Draft TLS-POK May 2022 Authenticating Peer Authenticator ------------------- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (TLS-POK) -> <- EAP-Request/ EAP-Type=TEAP (TLS Start) . . . 4. Summary of Work The protocol outlined here can be broadly broken up into 4 distinct areas: * TLS extensions to transport the bootstrap public key identifier * Use of the TLS 1.3 extension for certificate-based authentication with an external PSK * The client's use of a raw public key in its certificate * TEAP extensions to leverage the new TLS-POK handshake for trust establishment This document captures all 4 areas. 5. IANA Considerations None. 6. Security Considerations Bootstrap and trust establishment by the TLS server is based on proof of knowledge of the client's bootstrap public key, a non-public datum. The TLS server obtains proof that the client knows its bootstrap public key and, in addition, also possesses its corresponding private analog. Trust on the part of the client is based on validation of the server certificate and the TLS 1.3 handshake. In addition, the client assumes that knowledge of its public bootstrapping key is not widely disseminated and therefore any device that proves knowledge of its Friel & Harkins Expires 27 November 2022 [Page 8] Internet-Draft TLS-POK May 2022 bootstrapping key is the appropriate device from which to receive provisioning, for instance via [RFC7170]. [duckling] describes a security model for this type of "imprinting". An attack on the bootstrapping method which substitutes the public key of a corrupted device for the public key of an honest device can result in the TLS sever on-boarding and trusting the corrupted device. If an adversary has knowledge of the bootstrap public key, the adversary may be able to make the client bootstrap against the adversary's network. For example, if an adversary intercepts and scans QR labels on clients, and the adversary can force the client to connect to its server, then the adversary can complete the TLS-POK handshake with the client and the client will connect to the adversary's server. Since physical possession implies ownership, there is nothing to prevent a stolen device from being on-boarded. 7. References 7.1. Normative References [I-D.ietf-tls-external-psk-importer] Benjamin, D. and C. A. Wood, "Importing External PSKs for TLS", Work in Progress, Internet-Draft, draft-ietf-tls- external-psk-importer-08, 22 April 2022, . [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, . [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010, . [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., Weiler, S., and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, June 2014, . Friel & Harkins Expires 27 November 2022 [Page 9] Internet-Draft TLS-POK May 2022 [RFC8773] Housley, R., "TLS 1.3 Extension for Certificate-Based Authentication with an External Pre-Shared Key", RFC 8773, DOI 10.17487/RFC8773, March 2020, . 7.2. Informative References [DPP] Wi-Fi Alliance, "Device Provisioning Profile", 2020. [duckling] Stajano, F. and E. Rescorla, "The Ressurecting Ducking: Security Issues for Ad-Hoc Wireless Networks", 1999. [RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The Network Access Identifier", RFC 4282, DOI 10.17487/RFC4282, December 2005, . [RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., "Enrollment over Secure Transport", RFC 7030, DOI 10.17487/RFC7030, October 2013, . [RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna, "Tunnel Extensible Authentication Protocol (TEAP) Version 1", RFC 7170, DOI 10.17487/RFC7170, May 2014, . Authors' Addresses Owen Friel Cisco Email: ofriel@cisco.com Dan Harkins Hewlett-Packard Enterprise Email: daniel.harkins@hpe.com Friel & Harkins Expires 27 November 2022 [Page 10]