Concise Identities
draft-birkholz-core-coid-01

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CoRE Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Informational                                C. Bormann
Expires: July 6, 2019                            Universitaet Bremen TZI
                                                             M. Pritikin
                                                                   Cisco
                                                            R. Moskowitz
                                                                  Huawei
                                                        January 02, 2019

                           Concise Identities
                      draft-birkholz-core-coid-01

Abstract

   There is an increased demand of trustworthy claim sets -- a set of
   system entity characteristics tied to an entity via signatures -- in
   order to provide information.  Claim sets represented via CBOR Web
   Tokens (CWT) can compose a variety of evidence suitable for
   constrained-node networks and to support secure device automation.
   This document focuses on sets of identifiers and attributes that are
   tied to a system entity and are typically used to compose identities
   appropriate for Constrained RESTful Environment (CoRE) authentication
   needs.

Status of This Memo

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   This Internet-Draft will expire on July 6, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Claims in a Concise Identity  . . . . . . . . . . . . . . . .   4
     2.1.  iss: CWT issuer . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  sub: CWT subject  . . . . . . . . . . . . . . . . . . . .   5
     2.3.  aud: CWT audience . . . . . . . . . . . . . . . . . . . .   5
     2.4.  exp: CWT expiration time  . . . . . . . . . . . . . . . .   5
     2.5.  nbf: CWT start of validity  . . . . . . . . . . . . . . .   5
     2.6.  iat: CWT time of issue  . . . . . . . . . . . . . . . . .   5
     2.7.  cti: CWT ID . . . . . . . . . . . . . . . . . . . . . . .   6
     2.8.  cnf: CWT proof-of-possession key claim  . . . . . . . . .   6
   3.  Signature Envelope  . . . . . . . . . . . . . . . . . . . . .   6
   4.  Processing Rules  . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Common Terminology on Identity Documents . . . . . .   8
     A.1.  Terms Specified in IEEE 802.1AR . . . . . . . . . . . . .   8
     A.2.  Terms Specified in RFC 4949 . . . . . . . . . . . . . . .   8
     A.3.  Terms specified in ISO/IEC 9594-8:2017  . . . . . . . . .   8
   Appendix B.  Concise Identities and Trust Relationships . . . . .   9
   Appendix C.  Concise Identity (CoID) CDDL Data Definition based
                on RFC 5280  . . . . . . . . . . . . . . . . . . . .  10
   Appendix D.  Concise Secure Device Identifier (CoDeID) based on
                IEEE 802.1AR-2018  . . . . . . . . . . . . . . . . .  10
     D.1.  The Intended Use of DevIDs  . . . . . . . . . . . . . . .  10
     D.2.  DevID Flavors . . . . . . . . . . . . . . . . . . . . . .  11
     D.3.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  12
     D.4.  Concise DevID CDDL data definition (sans COSE header) . .  12
   Appendix E.  Attic  . . . . . . . . . . . . . . . . . . . . . . .  14
     E.1.  Examples of claims taken from IEEE 802.1AR identifiers  .  14
       E.1.1.  7.2.1 version . . . . . . . . . . . . . . . . . . . .  14
       E.1.2.  7.2.2 serialNumber  . . . . . . . . . . . . . . . . .  14
       E.1.3.  7.2.3 signature . . . . . . . . . . . . . . . . . . .  14

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       E.1.4.  7.2.4 issuer Name . . . . . . . . . . . . . . . . . .  15
       E.1.5.  7.2.5 authoritykeyidentifier  . . . . . . . . . . . .  15
       E.1.6.  7.2.7.1 notBefore . . . . . . . . . . . . . . . . . .  15
       E.1.7.  7.2.7.2 notAfter  . . . . . . . . . . . . . . . . . .  15
       E.1.8.  7.2.8 subject . . . . . . . . . . . . . . . . . . . .  15
       E.1.9.  7.2.10 subjectPublicKeyInfo . . . . . . . . . . . . .  15
       E.1.10. 7.2.11 signatureAlgorithm . . . . . . . . . . . . . .  15
       E.1.11. 7.2.12 signatureValue . . . . . . . . . . . . . . . .  15
     E.2.  Examples of claims taken from X.509 certificates  . . . .  16
       E.2.1.  2.5.29.35 - Authority Key Identifier  . . . . . . . .  16
       E.2.2.  2.5.29.14 - Subject Key Identifier  . . . . . . . . .  16
       E.2.3.  2.5.29.15 - Key Usage . . . . . . . . . . . . . . . .  16
       E.2.4.  2.5.29.37 - Extended key usage  . . . . . . . . . . .  16
       E.2.5.  1.3.6.1.5.5.7.1.1 - Authority Information Access  . .  16
       E.2.6.  1.3.6.1.4.1.311.20.2 - Certificate Template Name
               Domain Controller (Microsoft) . . . . . . . . . . . .  16
   Appendix F.  Graveyard  . . . . . . . . . . . . . . . . . . . . .  16
     F.1.  7.2.9 subjectAltName  . . . . . . . . . . . . . . . . . .  17
     F.2.  7.2.13 extensions . . . . . . . . . . . . . . . . . . . .  17
     F.3.  2.5.29.31 - CRL Distribution Points . . . . . . . . . . .  17
     F.4.  2.5.29.17 - Subject Alternative Name  . . . . . . . . . .  17
     F.5.  2.5.29.19 - Basic Constraints . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   X.509 certificates [RFC5280] and Secure Device Identifier
   [IEEE-802.1AR] are ASN.1 encoded Identity Documents and intended to
   be tied to a system entity uniquely identified via these Identity
   Documents.  An Identity Document - in general, a public-key
   certificate - can be conveyed to other system entities in order to
   prove or authenticate the identity of the owner of the Identity
   Document.  Trust in the proof can be established by mutual trust of
   the provider and assessor of the identity in a third party
   verification (TVP) provided, for example, by a certificate authority
   (CA) or its subsidiaries (sub CA).

   The evidence a certificate comprises is typically composed of a set
   of claims that is signed using secret keys issued by a (sub) CA.  The
   core set of claims included in a certificate - its attributes - are
   well defined in the X.509v3 specifications and IEEE 802.1AR.

   This document summarizes the core set of attributes and provides a
   corresponding list of claims using concise integer labels to be used
   in claim sets for CBOR Web Tokens (CWT) [RFC8392].  A resulting
   Concise Identity (CoID) is able to represent a signed set of claims
   that composes an Identity as defined in [RFC4949].

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   The objective of using CWT as a basis for the signed claim sets
   defined in this document is to gain more flexibility and at the same
   time more rigorously defined semantics for the signed claim sets.  In
   addition, the benefits of using CBOR, COSE, and the corresponding CWT
   structure accrue, including more compact encoding and a simpler
   implementation in contrast to classical ASN.1 (DER/BER/PEM)
   structures and the X.509 complexity and uncertainty that has accreted
   since X.509 was released 29 years ago.  One area where both the
   compactness and the definiteness are highly desirable is in
   Constrained-Node Networks [RFC7228], which may also make use of the
   Constrained Application Protocol (CoAP, [RFC7252]); however, the area
   of application of Concise Identities is not limited to constrained-
   node networks.

   The present version of this document is a strawman that attempts to
   indicate the direction the work is intended to take.  Not all
   inspirations this version takes from X.509 maybe need to be taken.

1.1.  Terminology

   This document uses terminology from [RFC8392] and therefore also
   [RFC7519], as well as from [RFC8152].  Specifically, we note:

   Assertion:

   Claim:  A piece of information asserted about a subject.  A claim is
      represented as a name/value pair consisting of a Claim Name and a
      Claim Value.

   Claims are grouped into claims sets (represented here by a CWT),
   which need to be interpreted as a whole.  Note that this usage is a
   bit different from idiomatic English usage, where a claim would stand
   on its own.

   (Note that the current version of this draft is not very explicit
   about the relationship of identities and identifiers.  To be done in
   next version.)

2.  Claims in a Concise Identity

   A Concise Identity (CoID) is a CBOR Web Token [RFC8392] with certain
   claims present.  It can be signed in a number of ways, including a
   COSE_Sign1 data object [RFC8152].

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2.1.  iss: CWT issuer

   Optional: identifies the principal that is the claimant for the
   claims in the CoID ([RFC8392] Section 3.1.1, cf. Section 4.1.1 in
   [RFC7519]).

   o  Note that this is a StringOrURI (if it contains a ":" it needs to
      be a URI)

   o  For the "string" case (no ":"), there is no way to extract
      meaningful components from the string

   o  Make it a URI if it needs to be structured (not for routine
      retrieval, unless specified so by an application)

   o  If this URI looks like an HTTP or HTTPS URI then something
      retrievable by humans should exist there.

   o  Alternatively, some arithmetic can be applied to the URI (extract
      origin, add /.well-known/...) to find relevant information.

2.2.  sub: CWT subject

   Optional: identifies the principal that is the subject for the claims
   in the CoID ([RFC8392] Section 3.1.2, cf. Section 4.1.2 in
   [RFC7519]).

2.3.  aud: CWT audience

   Optional: identifies the recipients that the CoID is intended for
   ([RFC8392] Section 3.1.4, cf. Section 4.1.4 in [RFC7519]).

2.4.  exp: CWT expiration time

   Optional: the time on or after which the CoID must no longer be
   accepted for processing ([RFC8392] Section 3.1.4, cf. Section 4.1.4
   in [RFC7519]).

2.5.  nbf: CWT start of validity

   Optional: the time before which the CoID must not be accepted for
   processing ([RFC8392] Section 3.1.5, cf. Section 4.1.5 in [RFC7519]).

2.6.  iat: CWT time of issue

   Optional: the creation time of the CoID ([RFC8392] Section 3.1.6, cf.
   Section 4.1.6 in [RFC7519]).

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2.7.  cti: CWT ID

   The "cti" (CWT ID) claim provides a unique identifier for the CoID
   ([RFC8392] Section 3.1.7, cf. "jti" in Section 4.1.7 in [RFC7519]).

   CWT IDs are intended to be unique within an application, so they need
   to be either coordinated between issuers or based on sufficient
   randomness (e.g., 112 bits or more).

2.8.  cnf: CWT proof-of-possession key claim

   The "cnf" claim identifies the key that can be used by the subject
   for proof-of-possession and provides parameters to identify the CWT
   Confirmation Method ([I-D.ietf-ace-cwt-proof-of-possession]
   Section 3.1).

3.  Signature Envelope

   The signature envelope [TBD: need not actually be envelope, may be
   detached, too] carries additional information, e.g., the signature,
   as well as the identification of the signature algorithm employed
   (COSE: alg).  Additional information may pertain to the signature (as
   opposed to the claims being signed), e.g., a key id (COSE: kid) may
   be given in the header of the signature.

4.  Processing Rules

   (TBD: This should contain some discussion of the processing rules
   that apply for CoIDs.  Some of this will just be pointers to
   [I-D.ietf-oauth-jwt-bcp].)

5.  IANA Considerations

   This document makes no requests of IANA

6.  Security Considerations

7.  References

7.1.  Normative References

   [I-D.ietf-ace-cwt-proof-of-possession]
              Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
              possession-05 (work in progress), November 2018.

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   [I-D.ietf-oauth-jwt-bcp]
              Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", draft-ietf-oauth-jwt-bcp-04 (work in
              progress), November 2018.

   [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>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [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>.

7.2.  Informative References

   [IEEE-802.1AR]
              "IEEE Standard for Local and Metropolitan Area Networks -
              Secure Device Identity", IEEE standard,
              DOI 10.1109/ieeestd.2018.8423794, n.d..

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

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Appendix A.  Common Terminology on Identity Documents

   To illustrate the purpose and intent of Identity Documents,
   typically, terms, such as certificates, certificate chains/paths and
   trust anchors, are used.  To provide more context and for the
   convenience of the reader, three sources of definitions are
   highlighted in this section.

A.1.  Terms Specified in IEEE 802.1AR

   1.  a certificate is "a digitally signed object that binds
       information identifying an entity that possesses a secret private
       key to the corresponding public key."

   2.  a certificate chain is "an ordered list of intermediate
       certificates that links an end entity certificate ([...] a DevID
       certificate) to a trust anchor."

   3.  a trust anchor is "a Certificate Authority that is trusted and
       for which the trusting party holds information, usually in the
       form of a self-signed certificate issued by the trust anchor".

A.2.  Terms Specified in RFC 4949

   1.  a public-key certificate is "a digital certificate that binds a
       system entity's identifier to a public key value, and possibly to
       additional, secondary data items; i.e., a digitally signed data
       structure that attests to the ownership of a public key".

   2.  a certification path is "a linked sequence of one or more public-
       key certificates [...] that enables a certificate user to verify
       the signature on the last certificate in the path, and thus
       enables the user to obtain (from that last certificate) a
       certified public key, or certified attributes, of the system
       entity that is the subject of that last certificate".

   3.  a trust anchor is "a CA that is the subject of a trust anchor
       certificate or otherwise establishes a trust anchor key".
       Correspondingly, a trust anchor has a trust anchor certificate
       that "is a public-key certificate that is used to provide the
       first public key in a certification path".

A.3.  Terms specified in ISO/IEC 9594-8:2017

   1.  a public-key certificate is "the public key of an entity,
       together with some other information, rendered unforgeable by
       digital signature with the private key of the certification
       authority (CA) that issued it".

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   2.  a certification path is "an ordered list of one or more public-
       key certificates, starting with a public-key certificate signed
       by the trust anchor, and ending with the end-entity public-key
       certificate to be validated.  All intermediate public-key
       certificates, if any, are certification authority (CA)
       certificates in which the subject of the preceding public-key
       certificate is the issuer of the following public-key
       certificate".

   3.  a trust anchor is "an entity that is trusted by a relying party
       and used for validating public-key certificates".

Appendix B.  Concise Identities and Trust Relationships

   Following the terminology highlighted above, Concise Identities are
   signed CBOR Web Tokens that compose public-key Identity Documents
   based on asymmetric key pairs, potentially including additional
   assertions: claims that are secondary data items.

   In the context of certification paths, the "last certificate" in the
   certification path is the Identity Document that resides on the
   system component, which presents its Identity Document to relying
   partyies in order to be authenticated.  The "first certificate" in
   the certification path resides on the trust anchor.

   In order to be able to rely on the trust put into the Identity
   Document presented to relying parties, these have to put trust into
   two assumptions first:

   o  the corresponding trust anchor (certificate) is trusted.  In
      consequence, the consumer of the Identity Document requires a
      basis for decision whether to rely on the trust put in the trust
      anchor certificate, or not (e.g. via policies or a known
      certification paths).

   o  the secret key included in the system component that is presenting
      its Identity Document is protected.  In consequence, the secret
      key has to be stored in a shielded location.  Type and quality of
      the protection or shielding or even its location are assertions
      that can be included as secondary data items in the Identity
      Document.

   In summary, a path of trust relationships between a system
   component's Identity Document and a trusted authority's Identity
   Document is required to enable transitive trust in the system
   component that presents the Identity Document.

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Appendix C.  Concise Identity (CoID) CDDL Data Definition based on RFC
             5280

   COSE MUST be used to sign this CoID template flavor.

   "signatureAlgorithm" and "signature" are not part of the CoID map but
   of the COSE envelope.

   CoID = { version: uint .range 1..3 ; (8)
            issuer: text, ; iss(1)
            subject: text / bytes, ; sub(2)
            notAfter: uint, ; exp(4)
            notBefore: uint ; nbf(5)
            serialNumber: uint,; (7)
            subjectPublicKeyInfo: [ algorithm: COSE-Algorithm-Value,
                                    subjectPublicKey: bytes,
                                  ], ;(9)
            ? extensions: [ + [ extension-id: uint / registeredID,
                                extension-value: any,
                                ? criticality: bool,
                              ]
                          ], ;(0)
          }

   COSE-Algorithm-Value = uint .size 0..2 / nint .size 0..2
   registeredID = [ + uint ] ; OID

   extensions = 0
   issuer = 1
   subject = 2
   notAfter = 4
   notBefore = 5
   serialNumber = 7
   version = 8
   subjectPublicKeyInfo = 9

Appendix D.  Concise Secure Device Identifier (CoDeID) based on IEEE
             802.1AR-2018

   This section illustrates the context and background of Secure Device
   Identifiers.

D.1.  The Intended Use of DevIDs

   IEEE 802.1AR Secure Device Identifier are a specific subset of X.509
   Identity Documents that are intended to "authenticate a device's
   identity", where the corresponding Identity Document is
   "cryptographically bound to that device".  In this context,

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   "cryptographically bound" means that the Identity Document is
   "constructed using cryptographic operations to combine a secret with
   other arbitrary data objects such that it may be proven that the
   result could only be created by an entity having knowledge of the
   secret."

   While the intent of using X.509 Identity Documents as Device
   Identifiers starts to blur the line between authentication and
   authorization, the specification of IEEE 802.1AR Identity Documents
   provides a meaningful subset of assertions that can be used to
   identify one or more system components.  The following CDDL data
   definition maps the semantics of an RFC 5280 Public Key
   Infrastructure Certificate Profile, which provides the basis for the
   Secure Device Identifier semantics.  Both are mapped to a CWT
   representation.

D.2.  DevID Flavors

   In order to provide consistent semantics for the claims as defined
   below, understanding the distinction of IDevIDs (mandatory
   representation capabilities) and LDevIDs (recommended representation
   capabilities) is of the essence.

   Both flavors of Secure Device Identifiers share most of their
   assertion semantics (claim sets).

   IDevIDs are the _initially_ Secure Device Identifiers that "are
   normally created during manufacturing or initial provisioning" and
   are "installed on the device by the manufacturer".  IDevIDs are
   intended to be globally unique and to be stored in a way that
   protects it from modification (typically, a shielded location).  It
   is important to note that a potential segregation of a manufacturer
   into separate supply chain/tree entities is not covered by the
   802.1AR specification.

   LDevIDs are the _local significant_ Secure Device Identifiers that
   are intended to be "unique in the local administrative domain in
   which the device is used".  In essence, LDevIDs "can be created at
   any time [after IDevID provisioning], in accordance with local
   policies".  An "LDevID is bound to the device in a way that makes it
   infeasible for it to be forged or transferred to a device with a
   different IDevID without knowledge of the private key used to effect
   the cryptographic binding".

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D.3.  Privacy

   The exposition iof IDevID Identity Documents enables global unique
   identification of a system component.  To mitigate the obvious
   privacy LDevIDs may also be used as the sole identifier (by disabling
   the IDevID) to assure the privacy of the user of a DevID and the
   equipment in which it is installed.

D.4.  Concise DevID CDDL data definition (sans COSE header)

   COSE MUST be used to sign this DevID flavor, if represented via CoID.

   "signature" and "signatureValue" are not part of the CoID map but of
   the COSE envelope.

   "AlgorithmIdentifier" and corresponding "algorithm" and "parameters"
   should be part of the COSE envelope.

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CoDeID = { version: 3, ;(8)
           serialNumber: uint,(7)
           issuer: text, ; iss(1)
           notAfter: uint, ; exp(4)
           notBefore: uint ; nbf(5)
           subject: text / URI, ; sub(2)
           subjectPublicKeyInfo: [ algorithm: COSE-Algorithm-Value,
                                   subjectPublicKey: bytes,
                                 ], ;(9)
           signatureAlgorithm: COSE-Algorithm-Value ; 802.1ar-2018 states
                                                    ; this must be identical
                                                    ; to cose sig-alg (rm?)
           authorityKeyidentifier: bytes, ; all, non-critical,
           ? subjectKeyIdentifier; bytes, ; only intermediates, non-critical
           ? keyUsage : [ bitmask: bytes .size 1,
                          ? criticality: bool,
                        ]
           ? subjectAltName: text / iPAddress / registeredID,
           ? HardwareModuleName: [ hwType: registeredID,
                                   hwSerialNum: bytes,
                                 ],
           ? extensions: [ + [ extension-id: uint,
                               extension-value: any,
                               ? criticality: bool,
                             ],
         }

COSE-Algorithm-Value = uint .size 0..2 / nint .size 0..2
iPAddress = bytes .size 4 / bytes .size 16
registeredID = [ + uint ] ; OID

extensions = 0
issuer = 1
subject = 2
notAfter = 4
notBefore = 5
serialNumber = 7
version = 8
subjectPublicKeyInfo = 9
signatureAlgorithm = 10
authorityKeyidentifier = 11
subjectKeyIdentifier = 12
keyUsage = 13 ; could move to COSE header?
subjectAltName = 14
HardwareModuleName = 15

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Appendix E.  Attic

   Notes and previous content that will be pruned in next versions.

E.1.  Examples of claims taken from IEEE 802.1AR identifiers

   This appendix briefly discusses common fields in a X.509 certificate
   or an IEEE 802.1AR Secure Device Identifier and relates them to
   claims in a CoID.

   The original purpose of X.509 was only to sign the association
   between a name and a public key.  In principle, if something else
   needs to be signed as well, CMS [RFC5652] is required.  This
   principle has not been strictly upheld over time; this is
   demonstrated by the growth of various extensions to X.509
   certificates that might or might not be interpreted to carry various
   additional claims.

   This document details only the claim sets for CBOR Web Tokens that
   are necessary for authentication.  The plausible integration or
   replacement of ASN.1 formats in enrollment procotols, [D]TLS
   handshakes and similar are not in scope of this document.

   Subsections in this appendix are marked by the ASN.1 Object
   Identifier (OID) typically used for the X.509 item.  [TODO: Make this
   true; there are still some section numbers.]

E.1.1.  7.2.1 version

   The version field is typically not employed usefully in an X.509
   certificate, except possibly in legacy applications that accept
   original (pre-v3) X.509 certificates.

   Generally, the point of versioning is to deliberately inhibit
   interoperability (due to semantic meaning changes).  CoIDs do not
   employ versioning.  Where future work requires semantic changes,
   these will be expressed by making alternate kinds of claims.

E.1.2.  7.2.2 serialNumber

   Covered by cti claim.

E.1.3.  7.2.3 signature

   The signature, as well as the identification of the signature
   algorithm, are provided by the COSE container (e.g., COSE_Sign1) used
   to sign the CoID's CWT.

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E.1.4.  7.2.4 issuer Name

   Covered by iss claim.

E.1.5.  7.2.5 authoritykeyidentifier

   Covered by COSE kid in signature, if needed.

E.1.6.  7.2.7.1 notBefore

   Covered by nbf claim.

E.1.7.  7.2.7.2 notAfter

   Covered by exp claim.

   For Secured Device identifiers, this claim is typically left out.

   o  get a new one whenver you think you need it ("normal path")

   o  nonced ocsp? might benefit from a more lightweight freshness
      verification of existing signed assertion - exploration required!

   o  (first party only verfiable freshness may be cheaper than third-
      party verifiable?)

E.1.8.  7.2.8 subject

   Covered by sub claim.

   Note that if claim sets need to be made about multiple subjects, the
   favored approach in CoID is to create multiple CoIDs, one each per
   subject.

E.1.9.  7.2.10 subjectPublicKeyInfo

   Covered by cnf claim.

E.1.10.  7.2.11 signatureAlgorithm

   In COSE_Sign1 envelope.

E.1.11.  7.2.12 signatureValue

   In COSE_Sign1 envelope.

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E.2.  Examples of claims taken from X.509 certificates

   Most claims in X.509 certificates take the form of certificate
   extensions.  This section reviews a few common (and maybe not so
   common) certificate extensions and assesses their usefulness in
   signed claim sets.

E.2.1.  2.5.29.35 - Authority Key Identifier

   Used in certificate chaining.  Can be mapped to COSE "kid" of the
   issuer.

E.2.2.  2.5.29.14 - Subject Key Identifier

   Used in certificate chaining.  Can be mapped to COSE "kid" in the
   "cnf" (see Section 3.4 of [I-D.ietf-ace-cwt-proof-of-possession]).

E.2.3.  2.5.29.15 - Key Usage

   Usage information for a key claim that is included in the signed
   claims.  Can be mapped to COSE "key_ops" [TBD: Explain details].

E.2.4.  2.5.29.37 - Extended key usage

   Can include additional usage information such as 1.3.6.1.5.5.7.3.1
   for TLS server certificates or 1.3.6.1.5.5.7.3.2 for TLS client
   certificates.

E.2.5.  1.3.6.1.5.5.7.1.1 - Authority Information Access

   More information about the signer.  May include a pointer to signers
   higher up in the certificate chain (1.3.6.1.5.5.7.48.2), typically in
   the form of a URI to their certificate.

E.2.6.  1.3.6.1.4.1.311.20.2 - Certificate Template Name Domain
        Controller (Microsoft)

   This is an example for many ill-defined extensions that are on some
   arcs of the OID space somewhere.

   E.g., the UCS-2 string (ASN.1 BMPString) "IPSECIntermediateOffline"

Appendix F.  Graveyard

   Items and Content that was already discarded.

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F.1.  7.2.9 subjectAltName

   (See "sub").

F.2.  7.2.13 extensions

   Extensions are handled by adding CWT claims to the CWT.

F.3.  2.5.29.31 - CRL Distribution Points

   Usually URIs of places where a CRL germane to the certificate can be
   obtained.  Other forms of validating claim sets may be more
   appropriate than CRLs for the applications envisaged here.

   (Might be replaced by a more general freshness verification approach
   later.  For example one could define a generic "is this valid"
   request to an authority.)

F.4.  2.5.29.17 - Subject Alternative Name

   Additional names for the Subject.

   These may be an "OtherName", i.e. a mistery blob "defined by" an
   ASN.1 OID such as 1.3.6.1.4.1.9.21.2.3, or one out of a few formats
   such as URIs (which may, then, turn out not to be really URIs).
   Naming subjects obviously is a major issue that needs attention.

F.5.  2.5.29.19 - Basic Constraints

   Can identify the key claim as that for a CA, and can limit the length
   of a certificate path.  Empty in all the examples analyzed.

   Any application space can define new fields / claims as appropriate
   and use them.  There is no need for the underlying structure to
   define an additional extension method for this.  Instead, they can
   use the registry as defined in Section 9.1 of [RFC8392].>

Acknowledgements

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   Darmstadt  64295
   Germany

   Email: henk.birkholz@sit.fraunhofer.de

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   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

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

   Max Pritikin
   Cisco

   Email: pritikin@cisco.com

   Robert Moskowitz
   Huawei
   Oak Park, MI  48237

   Email: rgm@labs.htt-consult.com

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