CBOR Object Signing and Encryption (COSE): Header parameters for carrying and referencing X.509 certificates
draft-ietf-cose-x509-07

Document Type Active Internet-Draft (cose WG)
Author Jim Schaad 
Last updated 2020-10-22 (latest revision 2020-09-17)
Replaces draft-schaad-cose-x509
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Network Working Group                                          J. Schaad
Internet-Draft                                            August Cellars
Intended status: Informational                         17 September 2020
Expires: 21 March 2021

    CBOR Object Signing and Encryption (COSE): Header parameters for
              carrying and referencing X.509 certificates
                        draft-ietf-cose-x509-07

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

   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 21 March 2021.

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Copyright Notice

   Copyright (c) 2020 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
   2.  X.509 COSE Header Parameters  . . . . . . . . . . . . . . . .   3
   3.  X.509 certificates and static-static ECDH . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  COSE Header Parameter Registry  . . . . . . . . . . . . .   8
     4.2.  COSE Header Algorithm Parameter Registry  . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   In the process of writing [RFC8152] the working group discussed X.509
   certificates [RFC5280] ad decided that no use cases wher prestented
   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, in the 6TiSCH
   environment [I-D.richardson-enrollment-roadmap], describes a device
   enrollment solution that relies on the presence in the device of a
   factory-installed certificate.  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 third scenario is the use of COSE as

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   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.

   Example COSE messages for the various header parameters defined below
   can be found at https://github.com/cose-wg/Examples.

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 of X.509 certificates: it is perfectly
   reasonable for a constrained device to be provisioned with a
   certificate which it can then provide 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 use a
   public key for that thumbprint, but without performing the
   certificate evaluation or even having the entire certificate.

   Certificates obtained from any of these methods MUST still be
   validated.  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 to certificates of
   a chain length of one as well as longer chains.  If the application
   cannot establish trust in the certificate, that certificate cannot be
   used.

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   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.  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 be used as a signal to
      modify the set of trust anchors.  As the contents of this header
      parameter are untrusted input, the header parameter can be in
      either the protected or unprotected header bucket.

      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
         bstr.

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

   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 attribute.

      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 be used as a signal to
      modify the set of trust anchors.  As the contents of this header
      parameter are untrusted input, the header parameter can be in
      either the protected or unprotected header bucket.

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      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
         bstr.

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

   x5t:  This header parameter 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 algorithm is registered in the "COSE Algorithms" registry The
      second element is a binary string containing the hash value.

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

      For interoperability, applications which use this header parameter
      MUST support the hash algorithm 'SHA-256', but can use other hash
      algorithms.

      RFC Editor please remove the following 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].  The referenced resource can be
      any of the following media types:

      *  application/pkix-cert [RFC2585]

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

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

      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.
      In particular, self-signed certificates MUST NOT be trusted
      without an out-of-band confirmation.

   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.

         +=========+=======+===============+=====================+
         | Name    | Label | 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 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 ]

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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
      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,   |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

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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.

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, 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.

   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).

6.  References

6.1.  Normative References

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   [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

   [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>.

   [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-
              dtls13-38, 29 May 2020,
              <https://tools.ietf.org/html/draft-ietf-tls-dtls13-38>.

   [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>.

   [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>.

   [I-D.selander-ace-cose-ecdhe]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", Work in Progress,
              Internet-Draft, draft-selander-ace-cose-ecdhe-14, 11
              September 2019, <https://tools.ietf.org/html/draft-
              selander-ace-cose-ecdhe-14>.

   [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>.

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

   [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>.

   [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>.

   [I-D.richardson-enrollment-roadmap]
              Richardson, M., "Device Enrollment in IETF protocols -- A
              Roadmap", Work in Progress, Internet-Draft, draft-
              richardson-enrollment-roadmap-02, 23 May 2018,
              <https://tools.ietf.org/html/draft-richardson-enrollment-
              roadmap-02>.

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com

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