Token Binding Working Group                                   G. Mandyam
Internet-Draft                                              L. Lundblade
Intended status: Standards Track                                 J. Azen
Expires: December 13, 2018                    Qualcomm Technologies Inc.
                                                           June 11, 2018

                       Attested TLS Token Binding


   Token binding allows HTTP servers to bind bearer tokens to TLS
   connections.  In order to do this, clients or user agents must prove
   possession of a private key.  However, proof-of-possession of a
   private key becomes truly meaningful to a server when accompanied by
   an attestation statement.  This specification describes extensions to
   the existing token binding protocol to allow for attestation
   statements to be sent along with the related token binding messages.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 13, 2018.

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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
   2.  Attestation Enhancement to TLS Token Binding Message  . . . .   3
     2.1.  KeyStore Attestation  . . . . . . . . . . . . . . . . . .   3
       2.1.1.  Verification Procedures . . . . . . . . . . . . . . .   4
     2.2.  TPMv2 Attestation . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Verification Procedures . . . . . . . . . . . . . . .   5
   3.  Extension Support Negotiation . . . . . . . . . . . . . . . .   5
     3.1.  Negotiating Token Binding Protocol Extensions . . . . . .   6
   4.  Example - Platform Attestation for Anomaly Detection  . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [I-D.ietf-tokbind-protocol] and [I-D.ietf-tokbind-negotiation]
   describe a framework whereby servers can leverage cryptographically-
   bound authentication tokens to verify TLS connections.  This is
   useful for prevention of man-in-the-middle attacks on TLS sessions,
   and provides a mechanism by which identity federation systems can be
   leveraged by a relying party to verify a client based on proof-of-
   possession of a private key.

   Once the use of token binding is negotiated as part of the TLS
   handshake, an application layer message (the Token Binding message)
   may be sent from the client to the relying party whose primary
   purpose is to encapsulate a signature over a value associated with
   the current TLS session (Exported Key Material, i.e. EKM - see

   Proof-of-possession of a private key is useful to a relying party,
   but the associated signature in the Token Binding message does not
   provide an indication as to how the private key is stored and in what
   kind of environment the associated cryptographic operation takes
   place.  This information may be required by a relying party in order
   to satisfy requirements regarding client platform integrity.
   Therefore, attestations are sometimes required by relying parties in
   order for them to accept signatures from clients.  As per the
   definition in [I-D.birkholz-tuda], "remote attestation describes the
   attempt to determine the integrity and trustworthiness of an endpoint

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   -- the attestee -- over a network to another endpoint -- the verifier
   -- without direct access."  Attestation statements are therefore
   widely used in any server verification operation that leverages
   client cryptography.

   TLS token binding can therefore be enhanced with remote attestation
   statements.  The attestation statement can be used to augment Token
   Binding message.  This could be used by a relying party for several
   different purpose, including (1) to determine whether to accept token
   binding messages from the associated client, or (2) require an
   additional mechanism for binding the TLS connection to an
   authentication operation by the client.

2.  Attestation Enhancement to TLS Token Binding Message

   The attestation statement can be processed 'in-band' as part of the
   Token Binding Message itself.  This document leverages the
   TokenBinding.extensions field of the Token Binding Message as
   described in Section 3.4 of [I-D.ietf-tokbind-protocol], where the
   extension data conforms to the guidelines of Section 6.3 of the same
   document.  The extension data takes the form of a CBOR (compact
   binary object representation) Data Definition Language construct,
   i.e. CDDL.

             extension_data = {attestation}
             attestation = (
               attestation_type:  tstr,
               attestation_data:  bstr,

   The attestation data is determined according to the attestation type.
   In this document, the following types are defined: "KeyStore" (where
   the corresponding attestation data defined in [Keystore]) and "TPMv2"
   (where the corresponding attestation data defined in [TPMv2]).
   Additional attestation types may be accepted by the token binding
   implementation (for instance, see Section 8 of [webauthn]).

2.1.  KeyStore Attestation

   KeyStore attestation is relevant to the Android operating system.
   The Android Keystore mechanism allows for an application (such as a
   browser implementing the Token Binding stack) to create a key pair,
   export the public key, and protect the private key in a hardware-
   backed keystore.  The Android Keystore can then be used to verify a
   keypair using the Keystore Attestation mechanism, which involves
   signing a payload according to a public key that chains to a root

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   certificate signed by an attestation root key that is specific to the
   device manufacturer.

   KeyStore attestation provides a signature over a payload generated by
   the application.  Since in this case the application is the Token
   Binding stack resident on the device, the payload is the Exported Key
   Material (EKM) corresponding to the current TLS connection (see
   Section 3.3 of [I-D.ietf-tokbind-protocol]).  Then the attestation
   takes the form of a signature accompanies by a chain of DER-encoded
   x.509 certificates:

             attestation_data = (
               sig:  bytes,
               x5c: [credCert: bytes, *(caCert: bytes)]

2.1.1.  Verification Procedures

   The steps at the server for verifying a Token Binding KeyStore
   Attestation are:

   o  Extract EKM for current TLS connection.

   o  Verify that attestation_data is in the expected CBOR format.

   o  Parse the first certificate listed in x5c and extract the public
      key, algorithm and challenge.  If the challenge does not match the
      EKM then the attestation is invalid.

   o  If the challenge matches the EKM, verify the sig with respect to
      the extracted public key and algorithm from the previous step.

   o  Verify the rest of the certificate chain up to the root.  The root
      certificate must match the expected root for the device.

2.2.  TPMv2 Attestation

   Version 2 of the Trusted Computing Group's Trusted Platform Module
   (TPM) specification provides for an attestation generated within the
   context of a TPM.  The attestation then is defined as

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             attestation_data = (
               tpmt_sig:  bytes,
               tpms_attest:  bytes,
               x5c: [credCert: bytes, *(caCert: bytes)]

   The tpmt_sig is generated over a tpms_attest structure signed with
   respect to the certificate chain provided in the x5c array.  It is
   derived from the TPMT_SIGNATURE data structure defined in
   Section 11.3.4 of [TPMv2]. tpms_attest is derived from the
   TPMS_ATTEST data structure in Section 10.2.8 of [TPMv2], specifically
   with the extraData field being set to a SHA-256 hash of the EKM.

2.2.1.  Verification Procedures

   The steps for verifying a Token Binding TPMv2 Attestation are:

   o  Extract EKM for current TLS connection.

   o  Verify that attestation_data is in the expected CBOR format.

   o  Parse the first certificate listed in x5c and extract the public

   o  Verify the tpms_attest structure,which includes

      *  Verify that the type field is set to TPM_ST_ATTEST_CERTIFY.

      *  Verify that extraData is equivalent to the EKM.

      *  Verify that magic is set to the expected TPM_GENERATED_VALUE
         for the expected command sequence used to generate the

      *  Verification of additonal TPMS_ATTEST data fields is optional.

   o  Verify tpmt_sig with respect to the public key provided in the
      first certifcate in x5c, using the algorithm as specified in the
      sigAlg field (see Sections 11.3.4, and 9.29 of [TPMv2]).

3.  Extension Support Negotiation

   Even if the client supports a Token Binding extension, it may not be
   desirable to send the extension if the server does not support it.
   The benefits of client-suppression of an extension could include
   saving of bits "over the wire" or simplified processing of the Token
   Binding message at the server.  Currently, extension support is not

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   communicated as part of the Token Binding extensions to TLS (see

   It is proposed that the Client and Server Hello extensions defined in
   Sections 3 and 4 of [I-D.ietf-tokbind-negotiation] be extended so
   that endpoints can communicate their support for specific
   TokenBinding.extensions.  With reference to Section 3, it is
   recommended that the "token_binding" TLS extension be augmented by
   the client to include supported TokenBinding.extensions as follows:

      enum {
          attestation(0), (255)
      } TokenBindingExtensions;

      struct {
          TB_ProtocolVersion token_binding_version;
          TokenBindingKeyParameters key_parameters_list<1..2^8-1>;
          TokenBindingExtensions supported_extensions_list<1..2^8-1>
      } TokenBindingParameters;

   The "supported_extensions_list" contains the list of identifiers of
   all token binding message extensions supported by the client.  A
   server supporting token binding extensions will respond in the server
   hello with an appropriate "token_binding" extension that includes a
   "supported_extensions_list".  This list must be a subset of the the
   extensions provided in the client hello.

3.1.  Negotiating Token Binding Protocol Extensions

   The negotation described in Section 4 of
   [I-D.ietf-tokbind-negotiation] still applies.  In addition, a client
   can receive a "supported_extensions_list" from the server as part of
   the server hello.  The client must terminate the handshake if the
   "supported_extensions_list" received from the server is not a subset
   of the "supported_extensions_list" sent by the client in the client
   hello.  If the server hello list of supported extensions is a subset
   of the client supported extensions, then the client must only send
   those extensions specified in the server hello in the Token Binding
   protocol.  If the server hello does not include a
   "supported_extensions_list", then the client must not send any
   extensions along with the Token Binding Message.

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4.  Example - Platform Attestation for Anomaly Detection

   An example of where a platform-based attestation is useful can be for
   remote attestation based on client traffic anomaly detection.  Many
   network infrastructure deployments employ network traffic monitors
   for anomalous pattern detection.  Examples of anomalous patterns
   detectable in the TLS handshake could be unexpected cipher suite
   negotiation for a given source/destination pairing.  In this case, it
   may be desirable for a client-enhanced attestation reflecting for
   instance that an expected offered cipher suite in the client hello
   message is present or the originating browser integrity is intact
   (e.g. through a hash over the browser application package).  If the
   network traffic monitor can interpret the atttestation included in
   the token binding message, then it can verify the attestation and
   potentially emit alerts based on an unexpected attestation.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  References

6.1.  Normative References

              Birkholz, H., Vigano, C., and C. Bormann, "Concise data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-greevenbosch-appsawg-
              cbor-cddl-11 (work in progress), July 2017.

              Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper,
              N., and J. Hodges, "Token Binding over HTTP", draft-ietf-
              tokbind-https-12 (work in progress), January 2018.

              Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
              "Transport Layer Security (TLS) Extension for Token
              Binding Protocol Negotiation", draft-ietf-tokbind-
              negotiation-10 (work in progress), October 2017.

              Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
              Hodges, "The Token Binding Protocol Version 1.0", draft-
              ietf-tokbind-protocol-16 (work in progress), October 2017.

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              Google Inc., "Verifying hardware-backed key pairs with Key

   [TPMv2]    The Trusted Computing Group, "Trusted Platform Module
              Library, Part 2: Structures", September 2016,

              The Worldwide Web Consortium, "Web Authentication: An API
              for accessing Scoped Credentials",

6.2.  Informative References

              Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
              "Time-Based Uni-Directional Attestation", draft-birkholz-
              tuda-02 (work in progress), July 2016.

Authors' Addresses

   Giridhar Mandyam
   Qualcomm Technologies Inc.
   5775 Morehouse Drive
   San Diego, California  92121

   Phone: +1 858 651 7200

   Laurence Lundblade
   Qualcomm Technologies Inc.
   5775 Morehouse Drive
   San Diego, California  92121

   Phone: +1 858 658 3584

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   Jon Azen
   Qualcomm Technologies Inc.
   5775 Morehouse Drive
   San Diego, California  92121

   Phone: +1 858 651 9476

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