RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                         J. "O'Donoghue"
Expires: December 3, 2020                     Qualcomm Technologies Inc.
                                                           N. Cam-Winget
                                                           Cisco Systems
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                           June 01, 2020

               A CBOR Tag for Unprotected CWT Claims Sets


   CBOR Web Token (CWT, RFC 8392) Claims Sets sometimes do not need the
   protection afforded by wrapping them into COSE, as is required for a
   true CWT.  This specification defines a CBOR tag for such unprotected
   CWT Claims Sets (UCCS) and discusses conditions for its proper use.

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 3, 2020.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivation and Requirements . . . . . . . . . . . . . . . . .   3
   3.  Characteristics of a Secure Channel . . . . . . . . . . . . .   3
     3.1.  UCCS and Remote ATtestation procedureS (RATS) . . . . . .   4
     3.2.  Privacy Preserving Channels . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in
   a CBOR Object Signing and Encryption (COSE, [RFC8152]) envelope.
   COSE provides - amongst other things - the integrity protection
   mandated by RFC 8392 and optional encryption for CWTs.  Under the
   right circumstances, though, a signature providing proof for
   authenticity and integrity can be provided through the transfer
   protocol and thus omitted from the information in a CWT without
   compromising the intended goal of authenticity and integrity.  If a
   mutually Secured Channel is established between two remote peers, and
   if that Secure Channel provides the correct properties, it is
   possible to omit the protection provided by COSE, creating a use case
   for unprotected CWT Claims Sets.  Similarly, if there is one-way
   authentication, the party that did not authenticate may be in a
   position to send authentication information through this channel that
   allows the already authenticated party to authenticate the other

   This specification allocates a CBOR tag to mark Unprotected CWT
   Claims Sets (UCCS) as such and discusses conditions for its proper
   use in the scope of Remote ATtestation procedureS (RATS).

   This specification does not change [RFC8392]: A true CWT does not
   make use of the tag allocated here; the UCCS tag is an alternative to
   using COSE protection and a CWT tag.  Consequently, in a well-defined
   scope, it might be acceptable to strip a CWT of its COSE container an

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   replace the CWT Claims Set's CWT CBOR tag with a UCCS CBOR tag for
   further processing - or vice versa.

1.1.  Terminology

   The term Claim is used as in [RFC8725].

   The terms Claim Key, Claim Value, and CWT Claims Set are used as in

   UCCS:  Unprotected CWT Claims Set; a CBOR map of Claims as defined by
      the CWT Claims Registry that are composed of pairs of Claim Keys
      and Claim Values.

   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.

2.  Motivation and Requirements

   Use cases involving the conveyance of claims, in particular, remote
   attestations [I-D.ietf-rats-architecture] require a standardized data
   schema and format that can be trasferred and transported using
   different communication channels.  As these are Claims, [RFC8392] are
   a suitable format but how these Claims are secured depends on the
   deployment, the security capabilities of the device, as well as their
   software stack.  For example, a Claim may be securely stored and
   conveyed using the device's trusted execution environment or
   especially in some resource constrained environments the same process
   that provides the secure communication transport is also the delegate
   to compose the Claim to be conveyed.  Whether it is a transfer or
   transport, a Secure Channel is presumed to be used for conveying such
   UCCS.  The following section further describes the requirements and
   scenarios in which UCCS can be used.

3.  Characteristics of a Secure Channel

   A Secure Channel for the conveyance of UCCS needs to provide the
   security properties that would otherwise be provided by COSE for a
   CWT.  In this regard, UCCS is similar in security considerations to
   JWTs [RFC8725] using the algorithm "none".  RFC 8725 states: "if a
   JWT is cryptographically protected end-to-end by a transport layer,
   such as TLS using cryptographically current algorithms, there may be
   no need to apply another layer of cryptographic protections to the
   JWT.  In such cases, the use of the "none" algorithm can be perfectly
   acceptable.".  Analogously, the considerations discussed in Sections

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   2.1, 3.1, and 3.2 of RFC 8725 apply to the use of UCCS as elaborated
   on in this document.

   Secure Channels are often set up in a handshake protocol that
   mutually derives a session key, where the handshake protocol
   establishes the authenticity of one of both ends of the
   communication.  The session key can then be used to provide
   confidentiality and integrity of the transfer of information inside
   the Secure Channel.  A well-known example of a such a Secure Channel
   setup protocol is the TLS [RFC8446] handshake; the TLS record
   protocol can then be used for secure conveyance.

   As UCCS were initially created for use in Remote ATtestation
   procedureS (RATS) Secure Channels, the following subsection provides
   a discussion of their use in these channels.  Where other
   environments are intended to be used to convey UCCS, similar
   considerations need to be documented before UCCS can be used.

3.1.  UCCS and Remote ATtestation procedureS (RATS)

   Secure Channels can be transient in nature.  For the purposes of this
   specification, the mechanisms used to establish a Secure Channel are
   out of scope.  As a minimum requirement in the scope of RATS Claims,
   however, the Verifier must authenticate the Attester as part of the
   Secure Channel establishment.

   If only authenticity/integrity for a Claim is required, a Secure
   Channel MUST be established to, at minimum, provide integrity of the
   communication.  Further, the provider of the UCCS SHOULD be
   authenticated by the reciever to ensure the channel is truly secured
   and the sender is validated.  If confidentiality is also required,
   the receiving side SHOULD also be authenticated.

   The extent to which a Secure Channel can provide assurances that UCCS
   originate from a trustworthy attesting environment depends on the
   characteristics of both the cryptographic mechanisms used to
   establish the channel and the characteristics of the attesting
   environment itself.  A Secure Channel established or maintained using
   weak cryptography may not provide the assurance required by a relying
   party of the authenticity and integrity of the UCCS.

   Where the security assurance required of an attesting environment by
   a relying party requires it, the attesting environment may be
   implemented using techniques designed to provide enhanced protection
   from an attacker wishing to tamper with or forge UCCS.  A possible
   approach might be to implement the attesting environment in a
   hardened environment such as a TEE [I-D.ietf-teep-architecture] or a
   TPM [TPM2].

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   As with EATs nested in other EATs (Section of
   [I-D.ietf-rats-eat]), the Secure Channel does not endorse fully
   formed CWTs transferred through it.  Effectively, the COSE envelope
   of a CWT shields the CWT Claims Set from the endorsement of the
   Secure Channel.  (Note that EAT might add a nested UCCS Claim, and
   this statement does not apply to UCCS nested into UCCS, only to fully
   formed CWTs)

3.2.  Privacy Preserving Channels

   A Secure Channel which preserves the privacy of the Attester may
   provide security properties equivalent to COSE, but only inside the
   life-span of the session established.  In general, a Verifier cannot
   correlate UCCS received in different sessions from the same attesting
   environment based on the cryptographic mechanisms used when a privacy
   preserving Secure Channel is employed.

   In the case of a Remote Attestation, the attester must consider
   whether any UCCS it returns over a privacy preserving Secure Channel
   compromises the privacy in unacceptable ways.  As an example, the use
   of the EAT UEID [I-D.ietf-rats-eat] Claim in UCCS over a privacy
   preserving Secure Channel allows a verifier to correlate UCCS from a
   single attesting environment across many Secure Channel sessions.
   This may be acceptable in some use-cases (e.g. if the attesting
   environment is a physical sensor in a factory) and unacceptable in
   others (e.g. if the attesting environment is a device belonging to a

4.  IANA Considerations

   In the registry [IANA.cbor-tags], IANA is requested to allocate the
   tag in Table 1 from the FCFS space, with the present document as the
   specification reference.

       |    Tag | Data Item | Semantics                            |
       | TBD601 | map       | Unprotected CWT Claims Set [RFCthis] |

                         Table 1: Values for Tags

5.  Security Considerations

   The security considerations of [RFC7049] and [RFC8392] apply.

   {#secchan} discusses security considerations for Secure Channels, in
   which UCCS might be used.  This documents provides the CBOR tag

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   definition for UCCS and a discussion on security consideration for
   the use of UCCS in Remote ATtestation procedureS (RATS).  Uses of
   UCCS outside the scope of RATS are not covered by this document.  The
   UCCS specification - and the use of the UCCS CBOR tag,
   correspondingly - is not intended for use in a scope where a scope-
   specific security consideration discussion has not been conducted,
   vetted and approved for that use.

6.  References

6.1.  Normative References

              IANA, "Concise Binary Object Representation (CBOR) Tags",

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

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,

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   [TPM2]     "Trusted Platform Module Library Specification, Family
              "2.0", Level 00, Revision 01.59 ed., Trusted Computing
              Group", 2019.

6.2.  Informative References

              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture",
              draft-ietf-rats-architecture-04 (work in progress), May

              Mandyam, G., Lundblade, L., Ballesteros, M., and J.
              O'Donoghue, "The Entity Attestation Token (EAT)", draft-
              ietf-rats-eat-03 (work in progress), February 2020.

              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", draft-ietf-teep-architecture-08 (work in
              progress), April 2020.

Appendix A.  Example

   The example CWT Claims Set from Appendix A.1 of [RFC8392] can be
   turned into an UCCS by enclosing it with a tag number TBD601:

        / iss / 1: "coap://as.example.com",
        / sub / 2: "erikw",
        / aud / 3: "coap://light.example.com",
        / exp / 4: 1444064944,
        / nbf / 5: 1443944944,
        / iat / 6: 1443944944,
        / cti / 7: h'0b71'

Authors' Addresses

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   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   Darmstadt  64295

   Email: henk.birkholz@sit.fraunhofer.de

   Jeremy O'Donoghue
   Qualcomm Technologies Inc.
   279 Farnborough Road
   Farnborough  GU14 7LS
   United Kingdom

   Email: jodonogh@qti.qualcomm.com

   Nancy Cam-Winget
   Cisco Systems
   3550 Cisco Way
   San Jose, CA  95134

   Email: ncamwing@cisco.com

   Carsten Bormann
   Universitaet Bremen TZI
   Bibliothekstrasse 1
   Bremen  28369

   Phone: +49-421-218-63921
   Email: cabo@tzi.de

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