CBOR Object Signing and Encryption (COSE): Header parameters for carrying and referencing X.509 certificates
draft-ietf-cose-x509-09

Network Working Group                                          J. Schaad
Internet-Draft                                            August Cellars
Intended status: Standards Track                             25 May 2022
Expires: 26 November 2022

    CBOR Object Signing and Encryption (COSE): Header parameters for
              carrying and referencing X.509 certificates
                        draft-ietf-cose-x509-09

Abstract

   The CBOR Signing And Encrypted Message (COSE) structure uses
   references to keys in general.  For some algorithms, additional
   properties are defined which carry parameters relating to keys as
   needed.  The COSE Key structure is used for transporting keys outside
   of COSE messages.  This document extends the way that keys can be
   identified and transported by providing attributes that refer to or
   contain X.509 certificates.

Contributing to this document

   This note is to be removed before publishing as an RFC.

   The source for this draft is being maintained in GitHub.  Suggested
   changes should be submitted as pull requests at https://github.com/
   cose-wg/X509.  Instructions are on that page as well.  Editorial
   changes can be managed in GitHub, but any substantial issues need to
   be discussed on the COSE mailing list.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 November 2022.

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Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
   2.  X.509 COSE Header Parameters  . . . . . . . . . . . . . . . .   3
   3.  X.509 certificates and static-static ECDH . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  COSE Header Parameter Registry  . . . . . . . . . . . . .   9
     4.2.  COSE Header Algorithm Parameter Registry  . . . . . . . .   9
     4.3.  Media Type application/cose-x509  . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   In the process of writing [RFC8152], the working group discussed
   X.509 certificates [RFC5280] and decided that no use cases were
   presented that showed a need to support certificates.  Since that
   time, a number of cases have been defined in which X.509 certificate
   support is necessary, and by implication, applications will need a
   documented and consistent way to handle such certificates.  This
   document defines a set of attributes that will allow applications to
   transport and refer to X.509 certificates in a consistent manner.

   In some of these cases, a constrained device is being deployed in the
   context of an existing X.509 PKI: for example,
   [I-D.ietf-anima-constrained-voucher] describes a device enrollment
   solution that relies on the presence of a factory-installed
   certificate on the device.  The [I-D.ietf-lake-edhoc] draft was also
   written with the idea that long term certificates could be used to

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   provide for authentication of devices, and uses them to establish
   session keys.  Another possible scenario is the use of COSE as the
   basis for a secure messaging application.  This scenario assumes the
   presence of long term keys and a central authentication authority.
   Basing such an application on public key certificates allows it to
   make use of well established key management disciplines.

1.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.  X.509 COSE Header Parameters

   The use of X.509 certificates allows for an existing trust
   infrastructure to be used with COSE.  This includes the full suite of
   enrollment protocols, trust anchors, trust chaining and revocation
   checking that have been defined over time by the IETF and other
   organizations.  The key structures that have been defined in COSE
   currently do not support all of these properties, although some may
   be found in COSE Web Tokens (CWT) [RFC8392].

   It is not necessarily expected that constrained devices themselves
   will evaluate and process X.509 certificates: it is perfectly
   reasonable for a constrained device to be provisioned with a
   certificate that it subsequently provides to a relying party - along
   with a signature or encrypted message - on the assumption that the
   relying party is not a constrained device, and is capable of
   performing the required certificate evaluation and processing.  It is
   also reasonable that a constrained device would have the hash of a
   certificate associated with a public key and be configured to use a
   public key for that thumbprint, but without performing the
   certificate evaluation or even having the entire certificate.  In any
   case, there still needs to be an entity that is responsible for
   handling the possible certificate revocation.

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   Parties that intend to rely on the assertions made by a certificate
   obtained from any of these methods still need to validate it.  This
   validation can be done according to the PKIX rules in [RFC5280] or by
   using a different trust structure, such as a trusted certificate
   distributor for self-signed certificates.  The PKIX validation
   includes matching against the trust anchors configured for the
   application.  These rules apply when the validation succeeds in a
   single step as well as when certificate chains need to be built.  If
   the application cannot establish trust in the certificate, the public
   key contained in the certificate cannot be used for cryptographic
   operations.

   The header parameters defined in this document are:

   x5bag:  This header parameter contains a bag of X.509 certificates.
      The set of certificates in this header parameter is unordered and
      may contain self-signed certificates.  Note that there could be
      duplicate certificates.  The certificate bag can contain
      certificates which are completely extraneous to the message.  (An
      example of this would be where a signed message is being used to
      transport a certificate containing a key agreement key.)  As the
      certificates are unordered, the party evaluating the signature
      will need to be capable of building the certificate path as
      necessary.  That party will also have to take into account that
      the bag may not contain the full set of certificates needed to
      build any particular chain.

      The trust mechanism MUST process any certificates in this
      parameter as untrusted input.  The presence of a self-signed
      certificate in the parameter MUST NOT cause the update of the set
      of trust anchors without some out-of-band confirmation.  As the
      contents of this header parameter are untrusted input, the header
      parameter can be in either the protected or unprotected header
      bucket.  Sending the header parameter in the unprotected header
      bucket allows an intermediary to remove or add certificates.

      The end-entity certificate MUST be integrity protected by COSE.
      This can e.g. be done by sending the header parameter in the
      protected header, sending a x5bag in the unprotected header
      combined with a x5t in the protected header, or including the end-
      entity certificate in the external_aad.

      This header parameter allows for a single X.509 certificate or a
      bag of X.509 certificates to be carried in the message.

      *  If a single certificate is conveyed, it is placed in a CBOR
         byte string.

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      *  If multiple certificates are conveyed, a CBOR array of byte
         strings is used, with each certificate being in its own byte
         string.

   x5chain:  This header parameter contains an ordered array of X.509
      certificates.  The certificates are to be ordered starting with
      the certificate containing the end-entity key followed by the
      certificate which signed it and so on.  There is no requirement
      for the entire chain to be present in the element if there is
      reason to believe that the relying party already has, or can
      locate the missing certificates.  This means that the relying
      party is still required to do path building, but that a candidate
      path is proposed in this header parameter.

      The trust mechanism MUST process any certificates in this
      parameter as untrusted input.  The presence of a self-signed
      certificate in the parameter MUST NOT cause the update of the set
      of trust anchors without some out-of-band confirmation.  As the
      contents of this header parameter are untrusted input, the header
      parameter can be in either the protected or unprotected header
      bucket.  Sending the header parameter in the unprotected header
      bucket allows an intermediary to remove or add certificates.

      The end-entity certificate MUST be integrity protected by COSE.
      This can e.g. be done by sending the header parameter in the
      protected header, sending a x5chain in the unprotected header
      combined with a x5t in the protected header, or including the end-
      entity certificate in the external_aad as.

      This header parameter allows for a single X.509 certificate or a
      chain of X.509 certificates to be carried in the message.

      *  If a single certificate is conveyed, it is placed in a CBOR
         byte string.

      *  If multiple certificates are conveyed, a CBOR array of byte
         strings is used, with each certificate being in its own byte
         string.

   x5t:  This header parameter identifies the end-entity X.509

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      certificate by a hash value (a thumbprint).  The 'x5t' header
      parameter is represented as an array of two elements.  The first
      element is an algorithm identifier which is an integer or a string
      containing the hash algorithm identifier corresponding to the
      Value column (integer or text string) of the algorithm registered
      in the "COSE Algorithms" registry
      https://www.iana.org/assignments/cose/cose.xhtml#algorithms.  The
      second element is a binary string containing the hash value
      computed over the DER encoded certificate.

      As this header parameter does not provide any trust, the header
      parameter can be in either a protected or unprotected header
      bucket.

      The identification of the end-entity certificate MUST be integrity
      protected by COSE.  This can be done by sending the header
      parameter in the protected header or including the end-entity
      certificate in the external_aad.

      The 'x5t' header parameter can be used alone or together with the
      'x5bag', 'x5chain', or 'x5u' header parameters to provide
      integrity protection of the end-entity certificate.

      For interoperability, applications which use this header parameter
      MUST support the hash algorithm 'SHA-256', but can use other hash
      algorithms.  This requirement allows for different implementations
      to be configured to use an interoperable algorithm, but does not
      preclude the use (by prior agreement) of other algorithms.

      RFC Editor please remove the following two paragraphs:

      During AD review, a question was raised about how effective the
      previous statement is in terms of dealing with a MTI algorithm.
      There needs to be some type of arrangement between the parties to
      agree that a specific hash algorithm is going to be used in
      computing the thumbprint.  Making it a MUST use would make that
      true, but it then means that agility is going to be very
      difficult.

      The worry is that while SHA-256 may be mandatory, if a sender
      supports SHA-256 but only sends SHA-512 then the recipient which
      only does SHA-256 would not be able to use the thumbprint.  In
      that case both applications would conform to the specification,
      but still not be able to inter-operate.

   x5u:  This header parameter provides the ability to identify an X.509
      certificate by a URI [RFC3986].  It contains a CBOR text string.
      The referenced resource can be any of the following media types:

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      *  application/pkix-cert [RFC2585]

      *  application/pkcs7-mime; smime-type="certs-only" [RFC8551]

      *  application/cose-x509 Section 4.3

      *  application/cose-x509; usage=chain Section 4.3

      When the application/cose-x509 media type is used, the data is a
      CBOR sequence of single-entry COSE_X509 structures (encoding
      "bstr").  If the parameter "usage" is set to "chain", this
      sequence indicates a certificate chain.

      The end-entity certificate MUST be integrity protected by COSE.
      This can e.g. be done by sending the x5u in the unprotected or
      protected header combined with a x5t in the protected header, or
      including the end-entity certificate in the external_aad.  As the
      end-entity certificate is integrity protected by COSE, the URI
      does not need to provide any protection.

      If a retrieved certificate does not chain to an existing trust
      anchor, that certificate MUST NOT be trusted unless the URI
      provided integrity protection and server authentication and the
      server is configured as trusted to provide new trust anchors or if
      an out-of-band confirmation can be received for trusting the
      retrieved certificate.  In case an HTTP or CoAP GET request is
      used to retrieve a certificate, TLS [RFC8446], DTLS
      [I-D.ietf-tls-dtls13] or OSCORE [RFC8613] SHOULD be used.

   The header parameters are used in the following locations:

   *  COSE_Signature and COSE_Sign1 objects: in these objects they
      identify the certificate to be used for validating the signature.

   *  COSE_recipient objects: in this location they identify the
      certificate for the recipient of the message.

   The labels assigned to each header parameter can be found in the
   following table.

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         +=========+=======+===============+=====================+
         | Name    | Label | Value Type    | Description         |
         +=========+=======+===============+=====================+
         | x5bag   | 32    | COSE_X509     | An unordered bag of |
         |         |       |               | X.509 certificates  |
         +---------+-------+---------------+---------------------+
         | x5chain | 33    | COSE_X509     | An ordered chain of |
         |         |       |               | X.509 certificates  |
         +---------+-------+---------------+---------------------+
         | x5t     | 34    | COSE_CertHash | Hash of an X.509    |
         |         |       |               | certificate         |
         +---------+-------+---------------+---------------------+
         | x5u     | 35    | uri           | URI pointing to an  |
         |         |       |               | X.509 certificate   |
         +---------+-------+---------------+---------------------+

                   Table 1: X.509 COSE Header Parameters

   Below is an equivalent CDDL [RFC8610] description of the text above.

   COSE_X509 = bstr / [ 2*certs: bstr ]
   COSE_CertHash = [ hashAlg: (int / tstr), hashValue: bstr ]

   The content of the bstr are the bytes of a DER encoded certificate.

3.  X.509 certificates and static-static ECDH

   The header parameters defined in the previous section are used to
   identify the recipient certificates for the ECDH key agreement
   algorithms.  In this section we define the algorithm specific
   parameters that are used for identifying or transporting the sender's
   key for static-static key agreement algorithms.

   These attributes are defined analogously to those in the previous
   section.  There is no definition for the certificate bag, as the same
   attribute would be used for both the sender and recipient
   certificates.

   x5chain-sender:  This header parameter contains the chain of
      certificates starting with the sender's key exchange certificate.
      The structure is the same as 'x5chain'.

   x5t-sender:  This header parameter contains the hash value for the
      sender's key exchange certificate.  The structure is the same as
      'x5t'.

   x5u-sender:  This header parameter contains a URI for the sender's

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      key exchange certificate.  The structure and processing are the
      same as 'x5u'.

   +===============+=====+=============+===================+===========+
   |Name           |Label|Type         | Algorithm         |Description|
   +===============+=====+=============+===================+===========+
   |x5t-sender     |TBD  |COSE_CertHash| ECDH-SS+HKDF-256, |Thumbprint |
   |               |     |             | ECDH-SS+HKDF-512, |for the    |
   |               |     |             | ECDH-SS+A128KW,   |sender's   |
   |               |     |             | ECDH-SS+A192KW,   |X.509      |
   |               |     |             | ECDH-SS+A256KW    |certificate|
   +---------------+-----+-------------+-------------------+-----------+
   |x5u-sender     |TBD  |uri          | ECDH-SS+HKDF-256, |URI for the|
   |               |     |             | ECDH-SS+HKDF-512, |sender's   |
   |               |     |             | ECDH-SS+A128KW,   |X.509      |
   |               |     |             | ECDH-SS+A192KW,   |certificate|
   |               |     |             | ECDH-SS+A256KW    |           |
   +---------------+-----+-------------+-------------------+-----------+
   |x5chain-sender |TBD  |COSE_X509    | ECDH-SS+HKDF-256, |static key |
   |               |     |             | ECDH-SS+HKDF-512, |X.509      |
   |               |     |             | ECDH-SS+A128KW,   |certificate|
   |               |     |             | ECDH-SS+A192KW,   |chain      |
   |               |     |             | ECDH-SS+A256KW    |           |
   +---------------+-----+-------------+-------------------+-----------+

                   Table 2: Static ECDH Algorithm Values

4.  IANA Considerations

4.1.  COSE Header Parameter Registry

   IANA is requested to register the new COSE Header parameters in
   Table 1 in the "COSE Header Parameters" registry.  The "Value
   Registry" field is empty for all of the items.  For each item, the
   'Reference' field points to this document.

4.2.  COSE Header Algorithm Parameter Registry

   IANA is requested to register the new COSE Header Algorithm
   parameters in Table 2 in the "COSE Header Algorithm Parameters"
   registry.  For each item, the 'Reference' field points to this
   document.

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4.3.  Media Type application/cose-x509

   When the application/cose-x509 media type is used, the data is a CBOR
   sequence of single-entry COSE_X509 structures (encoding "bstr").  If
   the parameter "usage" is set to "chain", this sequence indicates a
   certificate chain.

   IANA is requested to register the following media type [RFC6838]:

   Type name:  application

   Subtype name:  cose-x509

   Required parameters:  N/A

   Optional parameters:  usage

      *  Can be absent to provide no further information about the
         intended meaning of the order in the CBOR sequence of
         certificates.

      *  Can be set to "chain" to indicate that the sequence of data
         items is to be interpreted as a certificate chain.

   Encoding considerations:  binary

   Security considerations:  See the Security Considerations section of
      RFCthis.

   Interoperability considerations:  N/A

   Published specification:  RFCthis

   Applications that use this media type:  Applications that employ COSE
      and use X.509 as a certificate type.

   Fragment identifier considerations:  N/A

   Additional information:  Deprecated alias names for this type:  N/A

                            Magic number(s):  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person & email address to contact for further information:
   iesg@ietf.org

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   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  COSE WG

   Change controller:  IESG

   Provisional registration? (standards tree only):  no

5.  Security Considerations

   Establishing trust in a certificate is a vital part of processing.  A
   major component of establishing trust is determining what the set of
   trust anchors are for the process.  A new self-signed certificate
   appearing on the client cannot be a trigger to modify the set of
   trust anchors, because a well-defined trust-establishment process is
   required.  One common way for a new trust anchor to be added (or
   removed) from a device is by doing a new firmware upgrade.

   In constrained systems, there is a trade-off between the order of
   checking the signature and checking the certificate for validity.
   Validating certificates can require that network resources be
   accessed in order to get revocation information or retrieve
   certificates during path building.  The resulting network access can
   consume power and network bandwidth.  On the other hand, if the
   certificates are validated after the signature is validated, an
   oracle can potentially be built based on detecting the network
   resources which is only done if the signature validation passes.  In
   any event, both the signature and certificate validation MUST be
   completed successfully before acting on any requests.

   Unless it is known that the CA required proof-of-possession of the
   subject's private key to issue an end-entity certificate, the end-
   entity certificate MUST be integrity protected by COSE.  Without
   proof-of-possession, an attacker can trick the CA to issue an
   identity-misbinding certificate with someone else's "borrowed"
   public-key but with a different subject.  A MITM attacker can then
   perform an identity-misbinding attack by replacing the real end-
   entity certificate in COSE with such an identity-misbinding
   certificate.

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   End-entity X.509 certificates contain identities that a passive on-
   path attacker eavesdropping on the conversation can use to identify
   and track the subject.  COSE does not provide identity protection by
   itself and the x5t and x5u header parameters are just alternative
   permanent identifiers and can also be used to track the subject.  To
   provide identity protection, COSE can be sent inside another security
   protocol providing confidentiality.

   Before using the key in a certificate, the key MUST be checked
   against the algorithm to be used and any algorithm specific checks
   need to be made.  These checks can include validating that points are
   on curves for elliptical curve algorithms, and that sizes of RSA keys
   are of an acceptable size.  The use of unvalidated keys can lead
   either to loss of security or excessive consumption of resources (for
   example using a 200K RSA key).

   When processing the x5u header parameter the security considerations
   of [RFC3986] and specifically those defined in Section 7.1 of
   [RFC3986] also apply.

   Regardless of the source, certification path validation is an
   important part of establishing trust in a certificate.  Section 6 of
   [RFC5280] provides guidance for the path validation.  The security
   considerations of [RFC5280] are also important for the correct usage
   of this document.

   Protecting the integrity of the x5bag, x5chain and x5t contents by
   placing them in the protected header bucket can help mitigate some
   risks of a misbehaving certificate authority (cf.  Section 5.1 of
   [RFC2634]).

   The security of the algorithm used for 'x5t' does not affect the
   security of the system as this header parameter selects which
   certificate that is already present on the system should be used, but
   it does not provide any trust.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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   [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>.

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

   [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>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

6.2.  Informative References

   [I-D.ietf-anima-constrained-voucher]
              Richardson, M., Stok, P. V. D., Kampanakis, P., and E.
              Dijk, "Constrained Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", Work in Progress, Internet-Draft,
              draft-ietf-anima-constrained-voucher-17, 7 April 2022,
              <https://www.ietf.org/archive/id/draft-ietf-anima-
              constrained-voucher-17.txt>.

   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J. P., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
              Progress, Internet-Draft, draft-ietf-lake-edhoc-14, 18 May
              2022, <https://www.ietf.org/archive/id/draft-ietf-lake-
              edhoc-14.txt>.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-tls-
              dtls13-43.txt>.

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP",
              RFC 2585, DOI 10.17487/RFC2585, May 1999,
              <https://www.rfc-editor.org/info/rfc2585>.

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   [RFC2634]  Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
              RFC 2634, DOI 10.17487/RFC2634, June 1999,
              <https://www.rfc-editor.org/info/rfc2634>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [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,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", RFC 8551, DOI 10.17487/RFC8551,
              April 2019, <https://www.rfc-editor.org/info/rfc8551>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

Appendix A.  Acknowledgements

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com

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