ACE                                                        M. Sahni, Ed.
Internet-Draft                                          S. Tripathi, Ed.
Intended status: Standards Track                      Palo Alto Networks
Expires: 16 November 2023                                    15 May 2023

         CoAP Transfer for the Certificate Management Protocol


   This document specifies the use of Constrained Application Protocol
   (CoAP) as a transfer mechanism for the Certificate Management
   Protocol (CMP).  CMP defines the interaction between various PKI
   entities for the purpose of certificate creation and management.
   CoAP is an HTTP-like client-server protocol used by various
   constrained devices in the IoT space.

Status of This Memo

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

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   This Internet-Draft will expire on 16 November 2023.

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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  CoAP Transfer Mechanism for CMP . . . . . . . . . . . . . . .   3
     2.1.  CoAP URI Format . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Discovery of CMP RA/CA  . . . . . . . . . . . . . . . . .   4
     2.3.  CoAP Request Format . . . . . . . . . . . . . . . . . . .   4
     2.4.  CoAP Block-Wise Transfer Mode . . . . . . . . . . . . . .   4
     2.5.  Multicast CoAP  . . . . . . . . . . . . . . . . . . . . .   5
     2.6.  Announcement PKIMessage . . . . . . . . . . . . . . . . .   5
   3.  Proxy Support . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Certificate Management Protocol (CMP) [RFC4210] is used by the
   PKI entities for the generation and management of certificates.  One
   of the requirements of Certificate Management Protocol is to be
   independent of the transport protocol in use.  CMP has mechanisms to
   take care of required transactions, error reporting and protection of

   The Constrained Application Protocol (CoAP) defined in [RFC7252],
   [RFC7959] and [RFC8323] is a client-server protocol like HTTP.  It is
   designed to be used by constrained devices over constrained networks.
   The recommended transport for CoAP is UDP, however [RFC8323]
   specifies the support of CoAP over TCP, TLS and Websockets.

   This document specifies the use of CoAP over UDP as a transport
   medium for the CMP version 2 [RFC4210], CMP version 3
   [I-D.ietf-lamps-cmp-updates] designated as CMP in this document and
   Lightweight CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile].
   This document, in general, follows the HTTP transfer for CMP
   specifications defined in [RFC6712] and specifies the requirements
   for using CoAP as a transfer mechanism for the CMP.

   This document also provides guidance on how to use a "CoAP-to-HTTP"
   proxy to ease adoption of CoAP transfer mechanism by enabling the
   interconnection with existing PKI entities already providing CMP over

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1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  CoAP Transfer Mechanism for CMP

   A CMP transaction consists of exchanging PKIMessages [RFC4210]
   between PKI End Entities (EEs), Registration Authorities (RAs), and
   Certification Authorities (CAs).  If the EEs are constrained devices
   then they may prefer, as a CMP client, the use of CoAP instead of
   HTTP as the transfer mechanism.  The RAs and CAs, in general, are not
   constrained and can support both CoAP and HTTP Client and Server
   implementations.  This section specifies how to use CoAP as the
   transfer mechanism for the Certificate Management Protocol.

2.1.  CoAP URI Format

   The CoAP URI format is described in section 6 of [RFC7252].  The CoAP
   endpoints MUST support use of the path prefix "/.well-known/" as
   defined in [RFC8615] and the registered name "cmp" to help with
   endpoint discovery and interoperability.  Optional path segments MAY
   be added after the registered application name (i.e. after "/.well-
   known/cmp") to provide distinction.  The path segment 'p' followed by
   an arbitraryLabel <name> could for example support the
   differentiation of specific CAs or certificate profiles.  Further
   path segments, e.g., as specified in the Lightweight CMP Profile [I-
   D.ietf-lamps-lightweight-cmp-profile], could indicate PKI management
   operations using an operationLabel <operation>.  A valid full CMP URI
   can look like this:


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2.2.  Discovery of CMP RA/CA

   The EEs can be configured with enough information to form the CMP
   server URI.  The minimum information that can be configured is the
   scheme i.e. "coap:" or "coaps:" and the authority portion of the URI,
   e.g. "".  If the port number is not specified in the
   authority, then the default ports numbers MUST be assumed for the
   "coap:" and the "coaps:" scheme URIs.  The default port for coap:
   scheme URIs is 5683 and the default port for coaps: scheme URIs is
   5684 [RFC7252].

   Optionally, in the environments where a Local Registration Authority
   (LRA) or a Local CA is deployed, EEs can also use the CoAP service
   discovery mechanism [RFC7252] to discover the URI of the Local RA or
   CA.  The CoAP CMP endpoints supporting service discovery MUST also
   support resource discovery in the CoRE Link Format as described in
   [RFC6690].  The Link MUST include the 'ct' attribute defined in
   section 7.2.1 of [RFC7252] with the value of "application/pkixcmp" as
   defined in the CoAP Content-Formats IANA registry.

2.3.  CoAP Request Format

   The CMP PKIMessages MUST be DER encoded and sent as the body of the
   CoAP POST request.  A CMP client MUST send each CoAP requests marked
   as a Confirmable message [RFC7252].  If the CoAP request is
   successful then the CMP RA or CA MUST return a Success 2.xx response
   code otherwise CMP RA or CA MUST return an appropriate Client Error
   4.xx or Server Error 5.xx response code.  A CMP RA or CA may choose
   to send a Piggybacked response [RFC7252] to the client or it MAY send
   a Separate response [RFC7252] in case it takes some time for CA or RA
   to process the CMP transaction.

   When transferring CMP PKIMesssage over CoAP the content-format
   "application/pkixcmp" MUST be used.

2.4.  CoAP Block-Wise Transfer Mode

   A CMP PKIMesssage consists of a header, body, protection, and
   extraCerts structures which may contain many optional and potentially
   large fields.  Thus, a CMP message can be much larger than the
   Maximum Transmission Unit (MTU) of the outgoing interface of the
   device.  The EEs and RAs or CAs, MUST use the Block-Wise transfer
   mode [RFC7959] to transfer such large messages instead of relying on
   IP fragmentation.

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   If a CoAP-to-HTTP proxy is in the path between EEs and CA or EEs and
   RA then, if the server supports, it MUST use the chunked transfer
   encoding [RFC9112] to send data over the HTTP transport.  The proxy
   MUST try to reduce the number of packets sent by using an optimal
   chunk length for the HTTP transport.

2.5.  Multicast CoAP

   CMP PKIMessages sent over CoAP MUST NOT use a Multicast destination

2.6.  Announcement PKIMessage

   A CMP server may publish announcements, that can be event triggered
   or periodic, for the other PKI entities.  Here is the list of CMP
   announcement messages prefixed by their respective ASN.1 identifier
   (section 5.1.2 [RFC4210])

         [15] CA Key Update Announcement
         [16] Certificate Announcement
         [17] Revocation Announcement
         [18] CRL Announcement

   An EE MAY use CoAP Observe option [RFC7641] to register itself to get
   any announcement messages from the RA or CA.  The EE can send a GET
   request to the server's URI suffixed by "/ann".  For example a path
   to register for announcement messages may look like this:


   If the server supports CMP Announcements messages, then it MUST send
   appropriate Success 2.xx response code, otherwise it MUST send an
   appropriate Client Error 4.xx or Server Error 5.xx response code.  If
   for some reason the server cannot add the client to its list of
   observers for the announcements, it can omit the Observe option
   [RFC7641] in the response to the client.  A client on receiving a
   2.xx success response without the Observe option [RFC7641] MAY try
   after some time to register again for announcements from the CMP
   server.  Since server can remove the EE from the list of observers
   for announcement messages, an EE SHOULD periodically re-register
   itself for announcement messages.

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   Alternatively, an EE MAY periodically poll for the current status of
   the CA via the "PKI Information Request" message, see section 6.5 of
   [RFC4210].  If supported, EEs MAY also use "Support Messages" defined
   in section 4.3 of Lightweight CMP Profile
   [I-D.ietf-lamps-lightweight-cmp-profile] to get information about the
   CA status.  These mechanisms will help constrained devices, that are
   acting as EEs, to conserve resources by eliminating the need to
   create an endpoint for receiving notifications from RA or CA.  It
   will also simplify the implementation of a CoAP-to-HTTP proxy.

3.  Proxy Support

   This section provides guidance on using a CoAP-to-HTTP proxy between
   EEs and RAs or CAs in order to avoid changes to the existing PKI

   Since CMP payload is the same over CoAP and HTTP transfer mechanisms,
   a CoAP-to-HTTP cross-protocol proxy can be implemented based on
   section 10 of [RFC7252].  The CoAP-to-HTTP proxy can either be
   located closer to the EEs or closer to the RA or CA.  The proxy MAY
   support service discovery and resource discovery as described in
   section 2.2.  The CoAP-to-HTTP proxy MUST function as a reverse
   proxy, only permitting connections to a limited set of pre-configured
   servers.  It is out of scope of this document to specify how a
   reverse proxy can route CoAP client requests to one of the configured
   servers.  Some recommended mechanisms are as follows:

   *  Use the Uri-Path option to identify a server.

   *  Use separate hostnames for each of the configured servers and then
      use the Uri-Host option for routing the CoAP requests.

   *  Use separate hostnames for each of the configured servers and then
      use Server Name Indication [RFC8446] in case of "coaps://" scheme
      for routing CoAP requests.

4.  Security Considerations

   *  If PKIProtection is used, the PKIHeader and PKIBody of the CMP
      protocol are cryptographically protected against malicious
      modifications.  As such, UDP can be used without compromising the
      security of the CMP protocol.  Security Considerations for CoAP
      are defined in [RFC7252].

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   *  The CMP protocol does not provide confidentiality of the CMP
      payloads.  If confidentiality is desired, CoAP over DTLS [RFC9147]
      SHOULD be used to provide confidentiality for the CMP payloads,
      although it cannot conceal that the CMP protocol is used within
      the DTLS layer.
   *  Section 9.1 of [RFC7252] defines how to use DTLS [RFC9147] for
      securing the CoAP.  DTLS [RFC9147] associations SHOULD be kept
      alive and re-used where possible to amortize on the additional
      overhead of DTLS on constrained devices.
   *  An EE might not witness all of the Announcement messages when
      using the CoAP Observe option [RFC7641], since the Observe option
      is a "best-effort" approach and the server might lose its state
      for subscribers to its announcement messages.  The EEs may use an
      alternate method described in section 2.6 to obtain time critical
      changes such as CRL [RFC5280] updates.
   *  Implementations SHOULD use the available datagram size and avoid
      sending small datagrams containing partial CMP PKIMessage data in
      order to reduce memory usage for packet buffering.
   *  A CoAP-to-HTTP proxy can also protect the PKI entities by handling
      UDP and CoAP messages.  The proxy can mitigate attacks like denial
      of service attacks, replay attacks and resource-exhaustion attacks
      by enforcing basic checks like validating that the ASN.1 syntax is
      compliant to CMP messages and validating the PKIMessage protection
      before sending them to PKI entities.
   *  Since the Proxy may have access to the CMP-Level metadata and
      control over the flow of CMP messages therefore proper role based
      access control should be in place.  The proxy can be deployed at
      the edge of the "End Entities" network or in front of an RA and CA
      to protect them.  The proxy however may itself be vulnerable to
      resource-exhaustion attacks as it's required to buffer the CMP
      messages received over CoAP transport before sending it to the
      HTTP endpoint.  This can be mitigated by using short timers for
      discarding the buffered messages and rate limiting clients based
      on the resource usage.

5.  IANA Considerations

   This document adds a new entry to the CoAP Content-Formats IANA
   Registry (
   parameters.xhtml#content-formats) for the code of content-type
   "application/pkixcmp", for transferring CMP transactions over CoAP,
   from the identifier range 256-9999 reserved for IETF specifications.

   Type name: application

   Subtype name: pkixcmp

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   Encoding: Content may contain arbitrary octet values.  The octet
   values are the ASN.1 DER encoding of a PKI message, as defined in the
   [RFC4210] specifications.

   Reference: This document and [RFC4210]

   This document also adds a new path segment "ann" to the CMP Well-
   Known URI Path Segments (
   cmp.xhtml#cmp-well-known-uri) IANA registry for the EEs to register
   themselves for the announcement messages.

   Path Segment: ann

   Description: The path to send a GET request with CoAP Observer Option
   to register for CMP announcement messages.

   Reference: This document.

   This document references the cmp, in the Well-Known URIs
   uris.xhtml) IANA registry.  Please add a reference of this document
   to the Well-Known URIs (
   uris/well-known-uris.xhtml) IANA registry for that entry.

   This document also refers the path segment "p" in the CMP Well-Known
   URI Path Segments (
   cmp.xhtml#cmp-well-known-uri) IANA registry.  Please add a reference
   of this document to the CMP Well-Known URI Path Segments
   for that path segment.

   [Note RFC Editor]: This document should be published together or
   after the CMP version 3 [I-D.ietf-lamps-cmp-updates] as it references
   IANA entries created by that Internet draft.

6.  Acknowledgments

   The authors would like to thank Hendrik Brockhaus, David von Oheimb,
   and Andreas Kretschmer for their guidance in writing the content of
   this document and providing valuable feedback.

7.  References

7.1.  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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC6712]  Kause, T. and M. Peylo, "Internet X.509 Public Key
              Infrastructure -- HTTP Transfer for the Certificate
              Management Protocol (CMP)", RFC 6712,
              DOI 10.17487/RFC6712, September 2012,

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,

              Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
              Management Protocol (CMP) Updates", Work in Progress,
              Internet-Draft, draft-ietf-lamps-cmp-updates-23, 29 June
              2022, <

              Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
              Certificate Management Protocol (CMP) Profile", Work in
              Progress, Internet-Draft, draft-ietf-lamps-lightweight-
              cmp-profile-21, 17 February 2023,

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   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,

   [RFC9112]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <>.

7.2.  Informative References

   [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,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,

Authors' Addresses

   Mohit Sahni (editor)
   Palo Alto Networks
   3000 Tannery Way
   Santa Clara, CA 95054
   United States of America

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   Saurabh Tripathi (editor)
   Palo Alto Networks
   3000 Tannery Way
   Santa Clara, CA 95054
   United States of America

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