CBOR Object Signing and Encryption (COSE): Headers for carrying and referencing X.509 certificates
draft-ietf-cose-x509-03

Document Type Active Internet-Draft (cose WG)
Last updated 2019-08-18
Replaces draft-schaad-cose-x509
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Network Working Group                                          J. Schaad
Internet-Draft                                            August Cellars
Intended status: Informational                           August 18, 2019
Expires: February 19, 2020

  CBOR Object Signing and Encryption (COSE): Headers for carrying and
                     referencing X.509 certificates
                        draft-ietf-cose-x509-03

Abstract

   The CBOR Signing And Encrypted Message (COSE) structure uses
   references to keys in general.  For some algorithms, additional
   properties are defined which carry parts of 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

   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 February 19, 2020.

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Copyright Notice

   Copyright (c) 2019 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/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
     1.2.  Open Questions  . . . . . . . . . . . . . . . . . . . . .   3
   2.  X.509 COSE Headers  . . . . . . . . . . . . . . . . . . . . .   3
   3.  X.509 certificates and static-static ECDH . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  COSE Header Parameter Registry  . . . . . . . . . . . . .   7
     4.2.  COSE Header Algorithm Parameter Registry  . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   In the process of writing [RFC8152] discussions where held on the
   question of X.509 certificates [RFC5280] and if there was a needed to
   provide for them.  At the time there were no use cases presented that
   appeared to have a sufficient need for these attributes.  Since that
   time a number of cases where X.509 certificate support is necessary
   have been defined.  This document provides a set of attributes that
   will allow applications to transport and refer to X.509 certificates
   in a consistent manner.

   Some of the constrained device situations are being used where an
   X.509 PKI is already installed.  One of these situations is the 6tish
   environment for enrollment of devices where the certificates are
   installed at the factory.  The [I-D.selander-ace-cose-ecdhe] draft
   was also written with the idea that long term certificates could be
   used to provide for authentication of devices and uses them to
   establish session keys.  A final scenario is the use of COSE as a

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   messaging application where long term existence of keys can be used
   along with a central authentication authority.  The use of
   certificates in this scenario allows for key management to be used
   which is well understood.

   Example COSE messages for the various headers defined below can be
   found at https://github.com/cose-wg/Examples.  THIS IS NOT YET DONE
   BUT SHOULD BE COMING NOT LONG AFTER THE F2F MEETING.

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.

1.2.  Open Questions

      Should we define an extended key usage?

      Revocation info?

2.  X.509 COSE Headers

   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 will fully
   support the evaluation and processing of X.509 certificates, it is
   perfectly reasonable for a certificate to be assigned to a device
   which it can then provide to a relying party along with a signature
   or encrypted message, the relying party not being a constrained
   device.

   Certificates obtained from any of these methods MUST still be
   validated.  This validation can be done via the PKIX rules in
   [RFC5280] or by using a different trust structure, such as a trusted
   certificate distributer for self-signed certificates.  The PKIX
   validation includes matching against the trust anchors configured for
   the application.  These rules apply to certificates of a chain length
   of one as well as longer chains.  If the application cannot establish
   a trust in the certificate, then it cannot be used.

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   The header attributes defined in this document are:

   x5bag:    This header attributes contains a bag of X.509
             certificates.  The set of certificates in this header are
             unordered and may contain self-signed certificates.  The
             certificate bag can contain certificates which are
             completely extraneous to the message.  (An example of this
             would be to carry a certificate with a key agreement key
             usage in a signed message.)  As the certificates are
             unordered, the party evaluating the signature will need to
             do the necessary path building.  Certificates needed for
             any particular chain to be built may be absent from the
             bag.

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

             This header attribute allows for a single 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 bstr.

             *  If multiple certificates are conveyed, a CBOR array of
                bstrs is used.  Each certificate being in its own bstr.

   x5chain:  This header attribute 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 will already have it.  This means that the relying
             party is still required to do path building, but that a
             candidate path is proposed in this attribute.

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

             This header attribute allows for a single 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 bstr.

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             *  If multiple certificates are conveyed, a CBOR array of
                bstrs is used.  Each certificate being in it's own slot.

   x5t:      This header attribute provides the ability to identify an
             X.509 certificate by a hash value.  The attribute is an
             array of two elements.  The first element is an algorithm
             identifier which is an integer or a string containing the
             hash algorithm identifier.  The second element is a binary
             string containing the hash value.

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

             For interoperability, applications which use this header
             attribute MUST support the hash algorithm 'sha256', but can
             use other hash algorithms.

   x5u:      This header attribute provides the ability to identify an
             X.509 certificate by a URI.  The referenced resource can be
             any of the following media types:

             *  application/pkix-cert [RFC2585]

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

             As this header attribute implies a trust relationship, the
             attribute MUST be in the protected attributes.

             The URI provided MUST provide integrity protection and
             server authentication.  For example, an HTTP or CoAP GET
             request to retrieve a certificate MUST use TLS [RFC8446] or
             DTLS [I-D.ietf-tls-dtls13].  If the certificate does not
             chain to an existing trust anchor, the certificate MUST NOT
             be trusted unless the server is configured as trusted to
             provide new trust anchors.  This will normally be the
             situation when self-signed certificates are used.

   The header attributes are used in the following locations:

   *  COSE_Signature and COSE_Sign0 objects, in these objects they
      identify the key that was used for generating signature.

   *  COSE_recipient objects, in this location they may be used to
      identify the certificate for the recipient of the message.

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         +---------+-------+---------------+---------------------+
         | Name    | Value | value type    | description         |
         +=========+=======+===============+=====================+
         | x5bag   | TBD4  | COSE_X509     | An unordered bag of |
         |         |       |               | X.509 certificates  |
         +---------+-------+---------------+---------------------+
         | x5chain | TBD3  | COSE_X509     | An ordered chain of |
         |         |       |               | X.509 certificates  |
         +---------+-------+---------------+---------------------+
         | x5t     | TBD1  | COSE_CertHash | Hash of an X.509    |
         |         |       |               | certificate         |
         +---------+-------+---------------+---------------------+
         | x5u     | TBD2  | uri           | URI pointing to an  |
         |         |       |               | X.509 certificate   |
         +---------+-------+---------------+---------------------+

                        Table 1: X.509 COSE Headers

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

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

3.  X.509 certificates and static-static ECDH

   The header attributes 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 senders
   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 attribute contains the chain of
                    certificates starting with the sender's key exchange
                    certificate.  The structure is the same as 'x5bag'.

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

   x5u-sender:      This header attribute contains a URI for the
                    sender's key exchange certificate.  The structure
                    and processing are the same as 'x5u'.

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   +---------------+-----+-------------+-------------------+-----------+
   | Name          |Value|Type         | Algorithm         |Description|
   +===============+=====+=============+===================+===========+
   | x5t-sender    |TBD  |COSE_CertHash| ECDH-SS+HKDF-256, |Thumbprint |
   |               |     |             | ECDH-SS+HKDF-512, |for the    |
   |               |     |             | ECDH-SS+A128KW,   |senders    |
   |               |     |             | ECDH- SS+AES192KW, |X.509      |
   |               |     |             | ECDH-SS+AES256KW  |certificate|
   +---------------+-----+-------------+-------------------+-----------+
   | x5u-sender    |TBD  |uri          | ECDH-SS+HKDF-256, |URI for the|
   |               |     |             | ECDH-SS+HKDF-512, |senders    |
   |               |     |             | ECDH-SS+A128KW,   |X.509      |
   |               |     |             | ECDH- SS+AES192KW, |certificate|
   |               |     |             | ECDH-SS+AES256KW  |           |
   +---------------+-----+-------------+-------------------+-----------+
   | x5chain-sender|TBD  |COSE_X509    | ECDH-SS+HKDF-256, |static key |
   |               |     |             | ECDH-SS+HKDF-512, |X.509      |
   |               |     |             | ECDH-SS+A128KW,   |certificate|
   |               |     |             | ECDH- SS+AES192KW, |chain      |
   |               |     |             | ECDH-SS+AES256KW  |           |
   +---------------+-----+-------------+-------------------+-----------+

                   Table 2: Static ECDH Algorithm Values

4.  IANA Considerations

4.1.  COSE Header Parameter Registry

   IANA is requested to register the new COSE Header items in Table 1 in
   the "COSE Header Parameters" registry.

4.2.  COSE Header Algorithm Parameter Registry

   IANA is requested to register the new COSE Header items in Table 2 in
   the "COSE Header Algorithm Parameters" registry.

5.  Security Considerations

   Establishing trust in a certificate is a vital part of processing.
   Trust cannot be assumed whenever a new self-signed certificate
   appears on the client, instead a well defined 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

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   certificates during path building.  Doing the network access can
   consume resources dealing with power and network bandwidth.  On the
   other hand, an oracle can potentially be built based on if the
   network resources are only accessed if the signature validation
   passes.  In any event, both the signature and certificate validation
   MUST be checked before acting on any requests.

   As called out in the COSE algorithms document
   [I-D.ietf-cose-rfc8152bis-algs] basic checking on the keys in a
   certificate needs to be performed prior to using them.  These can
   include validating that points are on curves for elliptical curve
   algorithms and that sizes of keys are acceptable for RSA.  The use of
   unvalidated keys can lead either to loss of security or excessive
   consumption of resources.

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>.

   [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>.

   [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>.

6.2.  Informative References

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-03
              (work in progress), June 10, 2019,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-algs-03>.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-32 (work in progress), July 8,

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              2019, <https://www.ietf.org/archive/id/
              draft-ietf-tls-dtls13-32>.

   [I-D.selander-ace-cose-ecdhe]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
              cose-ecdhe-13 (work in progress), March 11, 2019,
              <https://www.ietf.org/archive/id/draft-selander-ace-cose-
              ecdhe-13>.

   [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>.

   [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>.

   [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>.

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com

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