Attested TLS Token Binding
draft-mandyam-tokbind-attest-06

Token Binding Working Group                                   G. Mandyam
Internet-Draft                                              L. Lundblade
Intended status: Standards Track                                 J. Azen
Expires: January 25, 2019                     Qualcomm Technologies Inc.
                                                           July 24, 2018

                       Attested TLS Token Binding
                    draft-mandyam-tokbind-attest-06

Abstract

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

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Attestation Enhancement to TLS Token Binding Message  . . . .   3
     2.1.  KeyStore Attestation  . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Verification Procedures . . . . . . . . . . . . . . .   4
     2.2.  TPMv2 Attestation . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Verification Procedures . . . . . . . . . . . . . . .   5
   3.  Extension Support Negotiation . . . . . . . . . . . . . . . .   6
     3.1.  Negotiating Token Binding Protocol Extensions . . . . . .   7
   4.  Example - Platform Attestation for Anomaly Detection  . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  TLS Extensions Registry . . . . . . . . . . . . . . . . .   8
     5.2.  Token Binding Extensions for Attestation  . . . . . . . .   8
   6.  Security and Privacy Considerations . . . . . . . . . . . . .   9
     6.1.  Attestation Privacy Considerations  . . . . . . . . . . .   9
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   [I-D.ietf-tokbind-protocol] and [I-D.ietf-tokbind-negotiation]
   describe a framework whereby servers can leverage cryptographically-
   bound authentication tokens in part to create uniquely-identifiable
   TLS bindings that can span multiple connections between a client and
   a server.  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.  The payload used for the signature is
   the token binding public key (see [I-D.ietf-tokbind-protocol]).  Use
   of the token binding public key allows for generation of the
   attestation signature once over the lifetime of the public key.

   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

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   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
   -- 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 value of the extension, as required by this same
   section, is assigned per attestation type.  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]).

   The attestation data will likely include a signature over a challenge
   (depenting on the attestation type).  The challenge can be used to
   prevent replay of the attestation.  However since the attestation is
   itself part of the token binding message (which has its own anti-
   replay protection mechanism), the attestation signature need only be

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   generated over a known payload associated with the TLS token binding
   session - the token binding public key.  As a result, the token
   binding client only needs to send the attestation once during the
   lifetime of the token binding public key.  In other words, if an
   attestation is included in the token binding message, it should only
   be sent in the initial token binding message following the creation
   of the token binding key pair.

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

   The octet value of the token binding extension that serves as
   identifiaction for the Keystore attestation type is requested to be
   0.

   KeyStore attestation provides a signature over a payload generated by
   the application.  The payload is a SHA-256 hash of the token binding
   public key 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, a signature-generation algorithmic
   identifier corresponding to the COSE algorithm registry
   ([cose_iana]), and a chain of DER-encoded x.509 certificates:

             attestation_data = (
               alg: int,
               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  Retrieve token binding public key for the current TLS connection,
      and compute is SHA-256 hash.

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   o  Verify that attestation_data is in the expected CBOR format.

   o  Parse the first certificate listed in x5c and extract the public
      key and challenge.  If the challenge does not match the SHA-256
      hash of the token binding public key then the attestation is
      invalid.

   o  If the challenge matches the expected hash of the token binding
      public key, 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

             attestation_data = (
               alg: int,
               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, and the
   algorithmic identifier corresponding to the COSE algorithm registry
   ([cose_iana]).  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 token
   binding public key.

   The octet value of the token binding extension that serves as
   identifiaction for the TPMv2 attestation type is requested to be 1.

2.2.1.  Verification Procedures

   The steps for verifying a Token Binding TPMv2 Attestation are:

   o  Extract the token binding public key for the current TLS
      connection.

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   o  Verify that attestation_data is in the expected CBOR format.

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

   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 SHA-256 hash of the
         token binding public key for the current TLS connection.

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

      *  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, 11.2.1.5 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
   communicated as part of the Token Binding extensions to TLS (see
   [I-D.ietf-tokbind-negotiation]).

   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:

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      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.

   Since a TLS extension cannot itself be extended, the "token_binding"
   TLS extension cannot be reused.  Therefore it is proposed that a new
   TLS extension be defined - "token_binding_with_extensions".  This TLS
   extension codepoint is identical to the existing "token_binding"
   extension except for the additional data structures defined above.

3.1.  Negotiating Token Binding Protocol Extensions

   The negotation described in Section 4 of
   [I-D.ietf-tokbind-negotiation] still applies, except now the
   "token_binding_with_extensions" codepoint would be used if the client
   supports any token binding extension.  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.

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

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   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 the following requests to IANA.

5.1.  TLS Extensions Registry

   This document proposes an update of the TLS "ExtensionType Values"
   registry.  The following addition to the registry is requested:

   Value: TBD

   Extension name: token_binding_with_extensions

   Reference: this document

   Recommended: Yes

5.2.  Token Binding Extensions for Attestation

   This document proposes two extensions conformant with Section 6.3 of
   [I-D.ietf-tokbind-protocol], with the following specifics:

   Androoid Keystore Attestation:

   o  Value: 0

   o  Description: Android Keystore Attestation

   o  Specification: This document

   TPM v2 Attestation:

   o  Value: 1

   o  Description: TPMv2 Attestation

   o  Specification: This document

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6.  Security and Privacy Considerations

   The security and privacy considerations provided in Section 7 of
   [I-D.ietf-tokbind-protocol] are applicable to the attestation
   extensions proposed in this document.  Additional considerations are
   provided in this section.

6.1.  Attestation Privacy Considerations

   The root signing key for the certificate chain used in verifying an
   attestation can be unique to the device.  As a result, this can be
   used to track a device and/or end user.  This potential privacy issue
   can be mitigated by the use of batch keys as an alternative to unique
   keys, or by generation of origin-specific attestation keys.

   The attestation data may also contain device-specific identifiers, or
   information that can be used to fingerprint a device.  Sensitive
   information can be excluded from the attestation data when this is a
   concern.

7.  Acknowledgments

   Thanks to Andrei Popov for his detailed review and recommendations.

8.  References

8.1.  Normative References

   [cose_iana]
              Internet Assigned Numbers Authority, "COSE Algorithms",
              <https://www.iana.org/assignments/cose/
              cose.xhtml#algorithms>.

   [I-D.greevenbosch-appsawg-cbor-cddl]
              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.

   [I-D.ietf-tokbind-https]
              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.

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   [I-D.ietf-tokbind-negotiation]
              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.

   [I-D.ietf-tokbind-protocol]
              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.

   [Keystore]
              Google Inc., "Verifying hardware-backed key pairs with Key
              Attestation",
              <https://developer.android.com/training/articles/
              security-key-attestation>.

   [TPMv2]    The Trusted Computing Group, "Trusted Platform Module
              Library, Part 2: Structures", September 2016,
              <http://www.trustedcomputinggroup.org/wp-content/uploads/
              TPM-Rev-2.0-Part-2-Structures-01.38.pdf>.

   [webauthn]
              The Worldwide Web Consortium, "Web Authentication: An API
              for accessing Scoped Credentials",
              <https://www.w3.org/TR/webauthn/>.

8.2.  Informative References

   [I-D.birkholz-tuda]
              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
   USA

   Phone: +1 858 651 7200
   Email: mandyam@qti.qualcomm.com

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

   Phone: +1 858 658 3584
   Email: llundbla@qti.qualcomm.com

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

   Phone: +1 858 651 9476
   Email: jazen@qti.qualcomm.com

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