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Versions: 00 01 02 03 draft-ietf-cose-x509

Network Working Group                                          J. Schaad
Internet-Draft                                            August Cellars
Intended status: Informational                              May 23, 2017
Expires: November 24, 2017

  CBOR Object Signing and Encryption (COSE): Headers for carrying and
                     referencing X.509 certificates


   This document defines the headers and usage for referring to and
   transporting X.509 certificates in the CBOR Encoded Message (COSE)

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 http://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 November 24, 2017.

Copyright Notice

   Copyright (c) 2017 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

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   (http://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 Headers  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Hash Algorithm Identifiers  . . . . . . . . . . . . . . . . .   6
     3.1.  SHA-2 256-bit Hash  . . . . . . . . . . . . . . . . . . .   6
     3.2.  SHA-2 256-bit Hash trucated to 64 bits  . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  COSE Header Parameter Registry  . . . . . . . . . . . . .   6
     4.2.  COSE Algorithm Registry . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In the process of writing RFCXXXX [I-D.ietf-cose-msg] discussions
   where held on the question of X.509 certificates [RFC5280]  and if
   there were needed.  At the time there were no use cases presented
   that appeared to have a sufficient set of support to include these
   headers.  Since that time a number of cases where X.509 certificate
   support is necessary have been defined.  This document provides a set
   of headers that will allow applications to transport and refer to
   X.509 certificates in a consistent manner.

   Some of the constrainted 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
   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 managment to be used
   which is well understood.

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1.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   When the words appear in lower case, their natural language meaning
   is used.

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) [I-D.ietf-ace-cbor-web-token].

   It is not necessarily expected that constrainted devices will fully
   support the evalaluation 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

   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.

   The header parameters defined in this document are:

   x5bag:  This header parameters 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.

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

      This header parameter 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

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

   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 will already have it.

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

      This header parameter 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

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

   x5t:  This header parameter provides the ability to identify an X.509
      certificate by a hash value.  The parameter is an array of two
      elements.  The first element is an algorithm identifier which is a
      signed 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
      parameter can be in either a protected or unprotected header bag.
      For interoperability, applications which use this header parameter
      MUST support the hash algorithm 'sha256', but can use other hash

   x5u:  This header parameter provides the ability to identify an X.509
      certificate by a URL.  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"

      *  Should we support a PEM type?  I cannot find a registered media
         type for one

      As this header element implies a trust relationship, the header
      parameter MUST be in the protected header bag.
      The URL provided MUST provide integrity protection.  For example,
      an HTTP or CoAP GET request to retrieve a certificate MUST use TLS
      [RFC5246] or DTLS.  If the certificate does not chain to an
      existing trust anchor, the identity of the server MUST be
      configured as trusted to provide new trust anchors.  This will
      normally be the situation when self-signed certificates are used.

   The header paramters used in the following locations:

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

   o  COSE_recipient object, in this object they identify the key used
      by the sender for static-static key agreement algorithms.  They
      would be used in place either XXXX or YYYY.

   | 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           | URL pointing to an X.509        |
   |         |       |               | certificate                     |

                        Table 1: X.509 COSE Headers

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

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3.  Hash Algorithm Identifiers

   The core COSE document did have a need for a standalone hash
   algorithm, and thus did not define any.  In this document, two hash
   algorithms are defined for use with the 'x5t' header parameter.

3.1.  SHA-2 256-bit Hash

   Define an algorithm identifier for SHA-256.

3.2.  SHA-2 256-bit Hash trucated to 64 bits

   This hash function uses the SHA-2 256-bit hash function as in the
   previous section, however it truncates the result to 64-bits for
   transmission.  The fact that it is a trucated hash means that there
   is now a high likelyhood that colisions will occur, thus this hash
   function cannot be used in situations where a unique items is
   required to be identified.  Luckly for the case of identifying a
   certificate that is not a requirement, the only requirement is that
   the number of potential certificates (and thus keys) to be tried is
   reduced to a small number.  (Hopefully that number is one, but it can
   not be assumed to be.)  After the set of certificates has been
   filtered down, the public key in each certificate will need to be
   tried for the operation in question.  The certificate can be
   validated either before or after it has been checked as working.  The
   trade-offs involved are:

   o  Certificate validation before using the key will imply that more
      network traffic may be required in order to fetch certificates and
      do revocation checking.

   o  Certificate validation after using the key means that bad keys can
      be used and, if not carefully checked, the result may be used
      prior to completing the certificate validation.  Using unvalidated
      keys can expose the device to more timing and oracle attacks as
      the attacker would be able to see if the key operation succeeded
      or failed as no network traffic to validate the certificate would

4.  IANA Considerations

4.1.  COSE Header Parameter Registry

   Put in the registrations.

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4.2.  COSE Algorithm Registry

   Put in the registrations.

5.  Security Considerations

   There are security considerations:

6.  References

6.1.  Normative References

              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-24 (work in progress), November 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

6.2.  Informative References

              Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", draft-ietf-ace-cbor-web-token-04
              (work in progress), April 2017.

              Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", draft-ietf-lamps-rfc5751-bis-06
              (work in progress), April 2017.

              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
              cose-ecdhe-06 (work in progress), April 2017.

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   [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,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com

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