Bootstrapped TLS Authentication
draft-friel-tls-eap-dpp-03

Document Type Active Internet-Draft (individual)
Authors Owen Friel  , Dan Harkins 
Last updated 2021-07-09 (latest revision 2021-02-21)
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Network Working Group                                           O. Friel
Internet-Draft                                                     Cisco
Intended status: Standards Track                              D. Harkins
Expires: January 10, 2022                     Hewlett-Packard Enterprise
                                                           July 09, 2021

                    Bootstrapped TLS Authentication
                       draft-friel-tls-eap-dpp-03

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

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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Bootstrap Key Pair  . . . . . . . . . . . . . . . . . . .   2
     1.2.  Alignment with Wi-Fi Alliance Device Provisioning Profile   3
   2.  Bootstrapping in TLS 1.3  . . . . . . . . . . . . . . . . . .   4
     2.1.  Bootstrap Extended PSK  . . . . . . . . . . . . . . . . .   4
     2.2.  Changes to TLS 1.3 Handshake  . . . . . . . . . . . . . .   5
   3.  Using TLS Bootstrapping in EAP  . . . . . . . . . . . . . . .   6
   4.  Summary of Work . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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.

1.1.  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) key pair.  The key pair 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 this public key,
   specifically the ASN.1 SEQUENCE SubjectPublicKeyInfo from [RFC5280],

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   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 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 upload of a Bill of Materials (BOM).
   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 bootstrap public keys
   corresponding to multiple devices, and TLS extensions are defined in
   this document that enable the server to identity a specific bootstrap
   public key correspinding to a specific device.

   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 bootstrapping 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]).

1.2.  Alignment with Wi-Fi Alliance Device Provisioning Profile

   The definition of the boostrap 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.

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2.  Bootstrapping in TLS 1.3

   The bootstrapping modifications introduce an extension to identify a
   "bootstrapping" key which is converted into an external PSK and used
   directly in the TLS 1.3 handshake.  This key MUST be from a
   cryptosystem suitable for doing ECDSA.

2.1.  Bootstrap Extended PSK

   This document defines the "bskey" extended PSK type by expanding on
   the work in [extensible-psks].

    enum {
      bskey(TBD), (255)
    } ExtendedPskIdentityType;

   A bskey PSK is a varient of an external PSK which, in this case, is
   derived from a public key.

   The PSKIdentity of a bskey extended PSK is encoded with a string
   derived from the DER-encoded ASN.1 subjectPublicKeyInfo
   representation of the bootstrapping public key.

    struct {
      opaque identity<1..2^32-1>
    } BootstrapPSKIdentity;

   Both the bskey PSK and the BootstrapPSKIdentity are computed using
   [RFC5869] with the hash algorithm from the ciphersuite:

   bskeypsk = HKDF-Expand(HKDF-Extract(<>, bskey),
                          "tls13-extended-psk-bskey", L)
   identity = HKDF-Expand(HKDF-Extract(<>, bskey),
                          "tls13-psk-identity-bskey", L)
   where:
     - <> is a NULL salt
     - bskey is the DER-encoded ASN.1 subjectPublicKeyInfo
       representation of the bootstrapping key
     - L is the length of the digest of the underlying hash
       algorithm

   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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2.2.  Changes to TLS 1.3 Handshake

   The client includes the "tls_cert_with_extern_psk" extension in the
   ClientHello, per [RFC8773], and identifies the bootstrapping key
   using the BootstrapPSKIdentity extension.  The server looks up the
   client's bootstrapping key in its database by checking the hash of
   each entry with the value received in the ClientHello.  If no match
   is found, the server SHALL terminate the TLS handshake with an alert.

   [[ TODO: should we define an explicit unknown_bsk_identity alert,
   similar to unknown_psk_identity ]]

   If the server found the matching bootstrap key, it generates the
   bskeypsk and includes the "tls_cert_with_extern_psk" extension in the
   ServerHello message.  When these extensions have been successfully
   negotiated, the TLS 1.3 key schedule SHALL include both the bskeypsk
   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 bootstrapping key) per
   [RFC7250].  The bootstrapping key is always an elliptic curve public
   key, therefore the ClientCertTypeExtension SHALL always indicate
   RawPublicKey and the type of the client's Certificate SHALL be ECDSA
   and contain the client's bootstrapping key as a DER-encoded ASN.1
   subjectPublicKeyInfo SEQUENCE.

   [[DISCUSS: since the bskey identity is being negotiated we already
   know what the client cert type will be, the ClientCertTypeExtension
   is superfluous.  Should it be removed from this spec?]]

   When the server processes the client's Certificate it MUST ensure
   that it is identical to the bootstrapping public key that it used to
   generate an external PSK and PSKIdentifier 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.

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            Client                                            Server
            --------                                          --------
            ClientHello
            + bskey_id
            + cert_with_extern_psk
            + client_cert_type=RawPublicKey
            + key_share                -------->
                                                           ServerHello
                                                            + bskey_id
                                                + cert_with_extern_psk
                                       + client_cert_type=RawPublicKey
                                                           + key_share
                                                 {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.

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

       EAP-Response/
       Identity (TLS-POK) ->

                               <- EAP-Request/
                               EAP-Type=TEAP
                              (TLS Start)
                          .
                          .
                          .

4.  Summary of Work

   [TODO: agree with WG chairs where this work lives and where it should
   be documented.]

   The protocol outlined here can be broadly broken up into 4 distinct
   areas:

   o  TLS extensions to transport the bootstrap public key identifier

   o  Use of the TLS 1.3 extension for certificate-based authentication
      with an external PSK

   o  The client's use of a raw public key in its certificate

   o  TEAP extensions to leverage the new TLS-POK handshake for trust
      establishment

   This document captures all 4 areas, but it may be more appropriate to
   merge into an existing document.

5.  IANA Considerations

   IANA will allocated an ExtensionPSKIdentityType for the bskey type
   from the TLS 1.3 repository created by [extensible-psks] and replace
   TBD in this document with that number.

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

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   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 it is
   the appropriate device from which to receive provisioning, for
   instance via [RFC7170].

   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

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [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, <https://www.rfc-editor.org/info/rfc7250>.

   [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,
              <https://www.rfc-editor.org/info/rfc8773>.

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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.

   [extensible-psks]
              Wood, C. and R. Anderson, "Extensible Pre-Shared Key Types
              for TLS", n.d., <https://chris-wood.github.io/draft-tls-
              extensible-psks/draft-group-tls-extensible-psks.html>.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282,
              DOI 10.17487/RFC4282, December 2005,
              <https://www.rfc-editor.org/info/rfc4282>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [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,
              <https://www.rfc-editor.org/info/rfc7170>.

Authors' Addresses

   Owen Friel
   Cisco

   Email: ofriel@cisco.com

   Dan Harkins
   Hewlett-Packard Enterprise

   Email: daniel.harkins@hpe.com

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