Internet Engineering Task Force (IETF)                     M. Sahni, Ed.
Request for Comments: 9482                              S. Tripathi, Ed.
Category: Standards Track                             Palo Alto Networks
ISSN: 2070-1721                                            November 2023

  Constrained Application Protocol (CoAP) Transfer for the Certificate
                          Management Protocol


   This document specifies the use of the 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 Internet of Things space.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Table of Contents

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

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 CMP is to be independent of the transport
   protocol in use.  CMP has mechanisms to take care of required
   transactions, error reporting, and protection of messages.

   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

   This document specifies the use of CoAP over UDP as a transport
   medium for CMP version 2 [RFC4210], CMP version 3 [RFC9480]
   (designated as CMP in this document), and the Lightweight CMP Profile
   [RFC9483].  In general, this document follows the HTTP transfer for
   CMP specifications defined in [RFC6712] and specifies the
   requirements for using CoAP as a transfer mechanism for CMP.

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

1.1.  Requirements Language

   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.  In general, the RAs and CAs 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 CMP.

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, for example, as specified in Lightweight CMP Profile
   [RFC9483], could indicate PKI management operations using an
   operationLabel <operation>.  A valid full CMP URI can look like this:


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., "example.com:5683".  If the port number is not specified
   in the authority, then the default port numbers MUST be assumed for
   the "coap:" and "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 RA or 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 Constrained RESTful Environments (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 request 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, the 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
   the RA or CA to process the CMP transaction.

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

2.4.  CoAP Block-Wise Transfer Mode

   A CMP PKIMessage consists of a header, body, protection, and
   extraCerts structure, 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

   If a CoAP-to-HTTP proxy is in the path between EEs and an RA or EEs
   and a CA and if the server supports, then 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 triggered by an
   event or periodically for the other PKI entities.  Here is the list
   of CMP announcement messages prefixed by their respective ASN.1
   identifier (see Section 5.1.2 of [RFC4210]):

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

   An EE MAY use the 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 announcement messages, then it MUST send
   an 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.  Upon receiving a Success
   2.xx response without the Observe Option [RFC7641], after some time,
   a client MAY try to register again for announcements from the CMP
   server.  Since a server can remove the EE from the list of observers
   for announcement messages, an EE SHOULD periodically reregister
   itself for announcement messages.

   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 [RFC9483] 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 the 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 the 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 preconfigured
   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 the "coaps://"
      scheme for routing CoAP requests.

4.  Security Considerations

   *  If PKIProtection is used, the PKIHeader and PKIBody of the CMP are
      cryptographically protected against malicious modifications.  As
      such, UDP can be used without compromising the security of the
      CMP.  Security considerations for CoAP are defined in [RFC7252].
   *  The CMP 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 is used within the DTLS layer.
   *  Section 9.1 of [RFC7252] defines how to use DTLS [RFC9147] for
      securing CoAP.  DTLS [RFC9147] associations SHOULD be kept alive
      and reused 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 Certificate Revocation List (CRL) [RFC5280]
   *  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, 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.  However, the proxy 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

   IANA has registered "application/pkixcmp" (ID 259) in the "CoAP
   Content-Formats" registry <https://www.iana.org/assignments/core-
   parameters> to transfer CMP transactions over CoAP.

   Type name:  application
   Subtype name:  pkixcmp
   Reference:  RFC 9482 [RFC4210]

   IANA has also registered a new path segment "ann" in the "CMP Well-
   Known URI Path Segments" registry <https://www.iana.org/assignments/
   cmp> for the EEs to register themselves for the announcement

   Path Segment:  ann
   Description:  The path to send a GET request with the CoAP Observe
      Option to register for CMP announcement messages.
   Reference:  RFC 9482

   IANA has added this document as a reference for the "cmp" entry in
   the "Well-Known URIs" registry <https://www.iana.org/assignments/

   IANA has also added this document as a reference for the "p" entry in
   the "CMP Well-Known URI Path Segments" registry

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,

   [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,

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

   [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,

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

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

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

   [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>.

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

   [RFC9112]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <https://www.rfc-editor.org/info/rfc9112>.

   [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,

   [RFC9480]  Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
              Management Protocol (CMP) Updates", RFC 9480,
              DOI 10.17487/RFC9480, November 2023,

   [RFC9483]  Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
              Certificate Management Protocol (CMP) Profile", RFC 9483,
              DOI 10.17487/RFC9483, November 2023,

6.2.  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,

   [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,

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


   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.

Authors' Addresses

   Mohit Sahni (editor)
   Palo Alto Networks
   3000 Tannery Way
   Santa Clara, CA 95054
   United States of America
   Email: msahni@paloaltonetworks.com

   Saurabh Tripathi (editor)
   Palo Alto Networks
   3000 Tannery Way
   Santa Clara, CA 95054
   United States of America
   Email: stripathi@paloaltonetworks.com