Network Working Group                                           O. Friel
Internet-Draft                                                     Cisco
Intended status: Standards Track                              D. Harkins
Expires: 14 August 2023                       Hewlett-Packard Enterprise
                                                        10 February 2023

   Bootstrapped TLS Authentication with Proof of Knowledge (TLS-POK)


   This document defines a mechanism that enables a bootstrapping device
   to establish trust and mutually authenticate against a network.
   Bootstrapping devices have a public private key pair, and this
   mechanism enables a network server to prove to the device that it
   knows the public key, and the device to prove to the server that it
   knows the private key.  The mechanism leverages existing DPP and TLS
   standards and can be used in an EAP exchange.

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Bootstrapping Overview  . . . . . . . . . . . . . . . . .   3
     1.3.  EAP Network Access  . . . . . . . . . . . . . . . . . . .   4
   2.  Bootstrap Key Pair  . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Alignment with Wi-Fi Alliance Device Provisioning
           Profile . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Bootstrapping in TLS 1.3  . . . . . . . . . . . . . . . . . .   5
     3.1.  External PSK Derivation . . . . . . . . . . . . . . . . .   6
     3.2.  TLS 1.3 Handshake Details . . . . . . . . . . . . . . . .   7
   4.  Using TLS Bootstrapping in EAP  . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 a device has a public / private keypair, and 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.

   Device on-boarding protocols such as the Device Provisioning Profile
   [DPP], also referred to as Wi-Fi Easy Connect, address this use case
   but they have drawbacks.  [DPP] for instance does not support wired
   network access, and does not specify how the device's DPP keypair can

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   be used in a TLS handshake.  This document describes an on-boarding
   protocol that can be used for wired network access, which we refer to
   as TLS Proof of Knowledge or TLS-POK.

   This document does not address the problem of Wi-Fi network
   discovery, where a bootstrapping device detects multiple different
   Wi-Fi networks and needs a more robust and scalable mechanism than
   simple round-robin to determine the correct network to attach to.
   DPP addresses this issue.  Thus, the intention is that DPP is the
   recommended mechanism for bootstrapping against Wi-Fi networks, and
   TLS-POK is the recommended mechanism for bootstrapping against wired

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terminology is used throughout this document.

   *  802.1X: IEEE Port-Based Network Access Control

   *  BSK: Bootstrap Key which is an elliptic curve public private key

   *  DPP: Device Provisioning Protocol [DPP]

   *  EAP: Extensible Authentication Protocol [RFC3748]

   *  EPSK: External Pre-Shared Key

   *  EST: Enrollment over Secure Transport [RFC7030]

   *  PSK: Pre-Shared Key

   *  TEAP: Tunnel Extensible Authentication Protocol [RFC7170]

1.2.  Bootstrapping Overview

   A bootstrapping device holds a public / private key pair which we
   refer to as a Bootstrap Key (BSK).  The private key of the BSK is
   known only by the device.  The public key of the BSK is known by the
   device, is known by the owner or holder of the device, and is
   provisioned on the network by the network operator.  In order to
   establish trust and mutually authenticate, the network proves to the

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   device that it knows the public part of the BSK, and the device
   proves to the network that it knows the private part of the BSK.
   Once this trust has been established during bootstrapping, the
   network can provision the device with a credential that it uses for
   subsequent network access.  More details on the BSK are given in
   Section 2.

1.3.  EAP Network Access

   Enterprise deployments typically require an [IEEE802.1X]/EAP-based
   authentication to obtain network access.  Protocols like Enrollment
   over Secure Transport (EST) [RFC7030] can be used to enroll devices
   into a Certification Authority to allow them to authenticate using
   802.1X/EAP.  This creates a Catch-22 where a certificate is needed
   for network access and network access is needed to obtain

   Devices whose BSK public key can been obtained in an out-of-band
   fashion and provisioned on the network can perform an EAP-TLS-based
   exchange, for instance Tunnel Extensible Authentication Protocol
   (TEAP) [RFC7170], and authenticate the TLS exchange using the
   bootstrapping mechanisms defined in Section 3.  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.

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.

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   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.

   Using the process defined herein, the client proves to the server
   that it has possession of the private key of its BSK.  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 BSK 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]).

2.1.  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, or dynamically display a single QR code on a display, 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 into 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.

3.  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
   [RFC9258].  This BSK MUST be from a cryptosystem suitable for doing
   ECDSA.  As the BSK is an ASN.1 SEQUENCE SubjectPublicKeyInfo, the
   client presents a raw public key certificate as specified in Using
   Raw Public Keys in TLS and DTLS [RFC7250].

   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.

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3.1.  External PSK Derivation

   An [RFC9258] EPSK is made up 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:

   epsk   = HKDF-Expand(HKDF-Extract(<>, bskey),
                          "tls13-imported-bsk", L)
   epskid = HKDF-Expand(HKDF-Extract(<>, bskey),
                          "tls13-bspsk-identity", L)
     - 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

   The [RFC9258] 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 [RFC9258].

   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.

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3.2.  TLS 1.3 Handshake Details

   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 [RFC9258].  The client
   MUST also include the [RFC7250] "client_certificate_type" extension
   in the ClientHello and MUST specify type of RawPublicKey.

   Upon receipt of the ClientHello, the server looks up the client's
   EPSK key in its database using the mechanisms documented in
   [RFC9258].  The ImportedIdentity context value of "tls13-bsk" informs
   the server that the mechanisms specified in this document for
   deriving the EPSK and executing the TLS handshake MUST be used.  If
   no match is found, the server MUST 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 MUST include both the EPSK in the Early Secret
   derivation and an (EC)DHE shared secret value in the Handshake Secret

   After successful negotiation of these extensions, the full TLS 1.3
   handshake is performed with the additional caveat that the server
   MUST send a CertificateRequest message and client MUST authenticate
   with a raw public key (its BSK) per [RFC7250].  The BSK is always an
   elliptic curve key pair, therefore the type of the client's
   Certificate MUST be ECDSA and MUST contain the client's BSK public
   key as a DER-encoded ASN.1 subjectPublicKeyInfo SEQUENCE.

   Note that the client MUST NOT share its BSK public key with the
   server until after the client has completed processing of the
   ServerHello and verified the TLS key schedule.  The PSK proof has
   completed at this stage, and the server has proven to the client that
   is knows the BSK public key, and it is therefore safe for the client
   to send the BSK public key to the server in the Certificate message.
   If the PSK verification step fails when processing the ServerHello,
   the client terminates the TLS handshake and the BSK public key MUST
   NOT be shared with the server.

   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.

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   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.

            Client                                            Server
            --------                                          --------
            + cert_with_extern_psk
            + client_cert_type=RawPublicKey
            + key_share
            + pre_shared_key           -------->
                                                + cert_with_extern_psk
                                       + client_cert_type=RawPublicKey
                                                           + key_share
                                                      + pre_shared_key
                                       <--------            {Finished}
            {Finished}                 -------->
            [Application Data]         <------->    [Application Data]

                   Figure 1: TLS 1.3 TLS-POK Handshake

4.  Using TLS Bootstrapping in EAP

   Upon "link up", an Authenticator on an 802.1X-protected port will
   issue an EAP Identity 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
   "" in order to indicate to the Authenticator that
   an EAP method that supports TLS-POK SHOULD be started.

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      Authenticating Peer     Authenticator
      -------------------     -------------
                               <- EAP-Request/

       ( ->

                               <- EAP-Request/
                              (TLS Start)

       (TLS client_hello with
        and pre_shared_key) ->


   Both client and server have derived the EPSK and associated [RFC9258]
   ImportedIdentity from the BSK as described in Section 3.1.  When the
   client starts the TLS exchange in the EAP transaction, it includes
   the ImportedIdentity structure in the pre_shared_key extension in the
   ClientHello.  When the server received the ClientHello, it extracts
   the ImportedIdentity and looks up the EPSK and BSK public key.  As
   previously mentioned in Section 2, the exact mechanism by which the
   server has gained knowledge of or been provisioned with the BSK
   public key is outside the scope of this document.

   The server continues with the TLS handshake and uses the EPSK to
   prove that it knows the BSK public key.  When the client replies with
   its Certificate, CertificateVerify and Finished messages, the server
   MUST ensure that the public key in the Certificate message matches
   the BSK public key.

   Once the TLS handshake completes, the client and server have
   established mutual trust.  The server can then proceed to provision a
   credential onto the client using, for example, the mechanisms
   outlined in [RFC7170].

   The client can then use this provisioned credential for subsequent
   network authentication.  The BSK is only used during bootstrap, and
   it not used for any subsequent network access.

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5.  IANA Considerations

   This document registers the following arpa domain:

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 key.

   Trust on the part of the client is based on successful completion of
   the TLS 1.3 handshake using the EPSK derived from the BSK.  This
   proves to the client that the server knows its BSK public key.  In
   addition, the client assumes that knowledge of its BSK public key is
   not widely disseminated and therefore any server that proves
   knowledge of its BSK public key is the appropriate server 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

   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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [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, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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,

   [RFC9258]  Benjamin, D. and C. A. Wood, "Importing External Pre-
              Shared Keys (PSKs) for TLS 1.3", RFC 9258,
              DOI 10.17487/RFC9258, July 2022,

7.2.  Informative References

   [DPP]      Wi-Fi Alliance, "Device Provisioning Profile", 2020.

   [duckling] Stajano, F. and E. Rescorla, "The Ressurecting Duckling:
              Security Issues for Ad-Hoc Wireless Networks", 1999.

              IEEE, "Port-Based Network Access Control", 2010.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,

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   [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

   Dan Harkins
   Hewlett-Packard Enterprise

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