CBOR Object Signing and Encryption (COSE): Header parameters for carrying and referencing X.509 certificates
draft-ietf-cose-x509-08

Network Working Group                                          J. Schaad
Internet-Draft                                            August Cellars
Intended status: Standards Track                        13 December 2020
Expires: 16 June 2021

    CBOR Object Signing and Encryption (COSE): Header parameters for
              carrying and referencing X.509 certificates
                        draft-ietf-cose-x509-08

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 16 June 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/
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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 . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  COSE Header Parameter Registry  . . . . . . . . . . . . .   8
     4.2.  COSE Header Algorithm Parameter Registry  . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

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, in the 6TiSCH
   environment, [I-D.richardson-enrollment-roadmap] 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 provide for authentication of devices, and uses them to
   establish session keys.  Another possible scenario is the use of COSE

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   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.

   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.

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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.  Note that there could be
      duplicating 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.

      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.

      *  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.

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      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.

      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 provides the ability to identify an X.509
      certificate by a hash value (a thumbprint).  The 'x5t' header
      parameter can be 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
      either the Value (integer) or Name (string) column of the
      algorithm registered in the "COSE Algorithms" registry.  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.

      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.

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      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:

      *  application/pkix-cert [RFC2585]

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

      As this header parameter implies a trust relationship between the
      party generating the x5u parameter and the party hosting the
      referred-to resource, this header parameter 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 retrieved 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 or if an out-of-band confirmation can be received
      for trusting the retrieved certificate.

   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   | 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 ]

   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,   |senders    |
   |               |     |             | ECDH-SS+A192KW,   |X.509      |
   |               |     |             | ECDH-SS+A256KW    |certificate|
   +---------------+-----+-------------+-------------------+-----------+
   |x5u-sender     |TBD  |uri          | ECDH-SS+HKDF-256, |URI for the|
   |               |     |             | ECDH-SS+HKDF-512, |senders    |
   |               |     |             | 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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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.

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

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

   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

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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>.

   [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>.

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-
              dtls13-39, 2 November 2020,
              <https://tools.ietf.org/html/draft-ietf-tls-dtls13-39>.

   [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.ietf-lake-edhoc]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", Work in Progress,
              Internet-Draft, draft-ietf-lake-edhoc-02, 2 November 2020,
              <https://tools.ietf.org/html/draft-ietf-lake-edhoc-02>.

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   [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>.

   [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-03, 7 October 2020,
              <https://tools.ietf.org/html/draft-richardson-enrollment-
              roadmap-03>.

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com

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