Certificate Management over CMS (CMC): Transport Protocols
draft-ietf-lamps-rfc5273bis-11

LAMPS Working Group                                        J. Mandel, Ed
Internet-Draft                                              AKAYLA, Inc.
Obsoletes: 5273, 6402 (if approved)                        S. Turner, Ed
Intended status: Standards Track                                   sn3rd
Expires: 30 August 2026                                 26 February 2026

       Certificate Management over CMS (CMC): Transport Protocols
                     draft-ietf-lamps-rfc5273bis-11

Abstract

   This document defines a number of transport mechanisms that are used
   to move CMC (Certificate Management over CMS (Cryptographic Message
   Syntax)) messages.  The transport mechanisms described in this
   document are HTTP, file, mail, and TCP.

   This document obsoletes RFC 5273 and RFC 6402.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-lamps-rfc5273bis/.

   Discussion of this document takes place on the WG LAMPS mailing list
   (mailto:spasm@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/spasm/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/spasm/.

   Source for this draft and an issue tracker can be found at
   https://github.com/seanturner/cmcbis.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 30 August 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Terminology  . . . . . . . . . . . . . . . . . .   3
   3.  Changes Since 5273 and 6402 . . . . . . . . . . . . . . . . .   3
   4.  File-Based Protocol . . . . . . . . . . . . . . . . . . . . .   4
   5.  Mail-Based Protocol . . . . . . . . . . . . . . . . . . . . .   4
   6.  HTTP-Based Protocol . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  PKI Request . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  PKI Response  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  TCP-Based Protocol  . . . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   This document defines a number of transport methods that are used to
   move CMC messages (defined in [CMC-STRUCT]).  The transport
   mechanisms described in this document are HTTP, file, mail, and TCP.

   This document obsoletes RFC 5273 [CMC-TRANSv1] and RFC 6402
   [CMC-Updates].  This document also incorporates [erratum3593].

2.  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.

3.  Changes Since 5273 and 6402

   Merged [CMC-Updates] text.

   IANA assigned TCP port 5318 for the use of CMC.

   Clarified the file extensions for Full PKI Requests and Responses.

   Added examples of encoding types for mail-based Requests and
   Responses.

   Replaced TLS 1.0 with TLS 1.2 or later, and added that implemetations
   are required to follow the recommendations in [BCP195].

   Addressed [erratum3593].

   Added reference to RFC9205 for HTTP guidance.

   Restrict early data (0-RTT) if using TLS 1.3 or QUIC.

   Restrict the use of TCP-Pipelining.

   Clarified the limitations of SMTP-over-TLS and the use of
   authenticated TLS for message delivery.

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4.  File-Based Protocol

   Enrollment messages and responses may be transferred between clients
   and servers using file-system-based mechanisms, such as when
   enrollment is performed for an off-line client.  When files are used
   to transport Full PKI Request or Full PKI Response messages, there
   MUST be only one instance of a request or response message in a
   single file and the file MUST be binary encoded.  The abbreviations
   crq and crp stand for Full PKI Request/Response, respectively; for
   clarity we define file extensions for them.  The following file type
   extensions SHOULD be used:

                 +=====================+================+
                 | Message Type        | File Extension |
                 +=====================+================+
                 | Simple PKI Request  | .p10           |
                 +---------------------+----------------+
                 | Full PKI Request    | .crq           |
                 +---------------------+----------------+
                 | Simple PKI Response | .p7c           |
                 +---------------------+----------------+
                 | Full PKI Response   | .crp           |
                 +---------------------+----------------+

                    Table 1: File PKI Request/Response
                              Identification

5.  Mail-Based Protocol

   MIME wrapping is defined for those environments that are MIME native.
   The basic mime wrapping in this section is taken from [SMIMEV4].
   When using a mail-based protocol, MIME wrapping between the layers of
   CMS wrapping is optional.  Note that this is different from the
   standard S/MIME (Secure MIME) message.

   What follows is a set of Simple PKI Request and Response messages and
   a set of Full PKI Request and Response messages.  The headers
   discussed below appear in the top-level content of the message and
   the messages' contents are the entire messages' bodies.

      |  The examples that follow are purposely truncated for brevity.

   Simple enrollment requests are encoded using the "application/pkcs10"
   content type [RFC5967].  A file name MUST be included either in a
   Content-Type or a Content-Disposition header in the name or filename
   parameter, respectively.  The extension for the file MUST be ".p10”.
   An example similar to that from [RFC5967] follows:

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   From: cmc-client@example.com
   Message-Id: <E06C3FA6-FF15-4851-AC7F-DB9F3B1C2C7A@example.com>
   To: cmc-server@example.com
   Subject: Simple Enrollment Request
   Date: Tue, 3 Feb 2026 16:08:28 -0500
   MIME-Version: 1.0
   Content-Type: application/pkcs10; name=smime.p10
   Content-Transfer-Encoding: base64
   Content-Disposition: inline; filename=smime.p10

   < message contents >

   Simple PKI Response messages MUST be encoded as content type
   "application/pkcs7-mime".  A smime-type parameter MUST be on the
   Content-Type header with a value of "certs-only".  A file name with
   the ".p7c" extension MUST be specified as part of the Content-Type or
   Content-Disposition header in the name or filename parameter,
   respectively.  An example similar to that from [SMIMEV4] follows:

   From: cmc-server@example.com
   Message-Id: <E06C3FA6-FF15-4851-AC7F-DB9F3B1C2C7B@example.com>
   To: cmc-client@example.com
   Subject: Re: Simple Enrollment Request
   Date: Tue, 3 Feb 2026 16:09:28 -0500
   MIME-Version: 1.0
   References: <E06C3FA6-FF15-4851-AC7F-DB9F3B1C2C7A@example.com>
   In-Reply-To: <E06C3FA6-FF15-4851-AC7F-DB9F3B1C2C7A@example.com>
   Content-Type: application/pkcs7-mime; smime-type=certs-only;
     name=smime.p7c
   Content-Transfer-Encoding: base64
   Content-Disposition: inline; filename=smime.p7c

   < message contents >

   Full PKI Request messages MUST be encoded as content type
   "application/pkcs7-mime".  The smime-type parameter MUST be included
   with a value of "CMC-Request".  A file name with the ".p7m" extension
   MUST be specified as part of the Content-Type or Content-Disposition
   header in the name or filename parameter, respectively.  An example
   similar to that from [SMIMEV4] follows:

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   From: cmc-client@example.com
   Message-Id: <E06C3FA6-FF15-4851-AC7F-DB9F3B1C2C7C@example.com>
   To: cmc-server@example.com
   Subject: Full Enrollment Request
   Date: Tue, 3 Feb 2026 16:10:28 -0500
   MIME-Version: 1.0
   Content-Type: application/pkcs7-mime; smime-type=CMC-Request;
     name=smime.p7c
   Content-Transfer-Encoding: base64
   Content-Disposition: inline; filename=smime.p7m

   < message contents >

   Full PKI Response messages MUST be encoded as content type
   "application/pkcs7-mime".  The smime-type parameter MUST be included
   with a value of "CMC-Response".  A file name with the ".p7m"
   extension MUST be specified as part of the Content-Type or Content-
   Disposition statement.  An example similar to that from [SMIMEV4]
   follows:

   From: cmc-server@example.com
   Message-Id: <E06C3FA6-FF15-4851-AC7F-DB9F3B1C2C7D@example.com>
   To: cmc-client@example.com
   Subject: Re: Full Enrollment Request
   Date: Tue, 3 Feb 2026 16:11:28 -0500
   MIME-Version: 1.0
   References: <E06C3FA6-FF15-4851-AC7F-DB9F3B1C2C7C@example.com>
   In-Reply-To: <E06C3FA6-FF15-4851-AC7F-DB9F3B1C2C7C@example.com>
   Content-Type: application/pkcs7-mime; smime-type=CMC-Response;
     name=smime.p7m
   Content-Transfer-Encoding: base64
   Content-Disposition: inline; filename=smime.p7m

   < message contents >

   For file names present in the name or filename parameters, non-ASCII
   text is prohibited.

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   +====================+==============+================+==============+
   | Item               | MIME Type    | File Extension | SMIME Type   |
   +====================+==============+================+==============+
   | Simple PKI         | application/ | .p10           | N/A          |
   | Request            | pkcs10       |                |              |
   +--------------------+--------------+----------------+--------------+
   | Full PKI           | application/ | .p7m           | CMC-Request  |
   | Request            | pkcs7-mime   |                |              |
   +--------------------+--------------+----------------+--------------+
   | Simple PKI         | application/ | .p7c           | certs-only   |
   | Response           | pkcs7-mime   |                |              |
   +--------------------+--------------+----------------+--------------+
   | Full PKI           | application/ | .p7m           | CMC-Response |
   | Response           | pkcs7-mime   |                |              |
   +--------------------+--------------+----------------+--------------+

             Table 2: MIME PKI Request/Response Identification

6.  HTTP-Based Protocol

   This section describes the conventions for use of HTTP [HTTP] as a
   data transfer protocol.  Consult [HTTP-IMP] for additional
   information.  The use of HTTPS [HTTP] provides any necessary content
   protection from eavesdroppers.

   In order for CMC clients and servers using HTTP to interoperate, the
   following rules apply.

      Clients are configured with sufficient information to form the
      server URI [RFC3986].

      Client requests are submitted by use of the POST method.

      Servers MUST use the 2XX response codes for successful responses.

      Clients MAY attempt to send certification requests using HTTPS
      [HTTP], although servers are not required to support TLS/QUIC but
      a secure channel might be available regardless depending on the
      HTTP version implemented [HTTP_1.0], [HTTP_1.1], [HTTP_2],
      [HTTP_3], or later.  If TLS is used by the HTTP version, then the
      implementation MUST follow the recommendations in [BCP195].  CMC
      implementations that support TLS 1.3 or QUIC MUST NOT use early
      data (i.e., 0-RTT) because POST is not idempotent.

      Clients are not required to support any type of HTTP
      authentication (Section 11 of [HTTP]) nor Cookies [COOKIES].
      Thus, servers can not rely on these features to be available.

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      Clients and servers are expected to follow other rules and
      restrictions in [HTTP].  Note that some of those rules are for
      HTTP methods other than POST; clearly, only the rules that apply
      to POST are relevant for this specification.

6.1.  PKI Request

   A PKI Request using the POST method is constructed as follows:

   The Content-Type field MUST have the appropriate value from Table 2.

   A Content-Type field for a request:

      Content-Type: application/pkcs7-mime; smime-type=CMC-Request;
      name=request.p7m

   The content of the message is the binary value of the encoding of the
   PKI Request.

6.2.  PKI Response

   The content of an HTTP-based PKI Response is the binary value of the
   BER (Basic Encoding Rules) encoding [X690] of either a Simple or Full
   PKI Response.

   The Content-Type field MUST have the appropriate value from Table 2.

   A Content-Type field for a response:

      Content-Type: application/pkcs7-mime; smime-type=CMC-Response;
      name=response.p7m

7.  TCP-Based Protocol

   When CMC messages are sent over a TCP-based connection, no wrapping
   is required of the message.  Messages are sent in their binary
   encoded form.

   The client closes a connection after receiving a response, or it
   issues another request to the server using the same connection.
   Reusing one connection for multiple successive requests, instead of
   opening multiple connections that are only used for a single request,
   is RECOMMENDED for performance and resource conservation reasons.
   The client MUST wait for the full response before making another
   request on the same connection.  A server MAY close a connection
   after it has been idle for some period of time; this timeout would
   typically be several minutes long.

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   CMC requires a registered port number to send and receive CMC
   messages over TCP.  The Service Name is "pkix-cmc".  The TCP port
   number is 5318.

   Prior to [CMC-Updates], CMC did not have a registered port number and
   used an externally configured port from the Private Port range.
   Client implementations MAY continue to use a port chosen from the
   Private Port range.  A TCP Server SHOULD use port 5318 assigned to
   the CMC service.  It is expected that HTTP will continue to be the
   primary transport method used by CMC installations.

8.  IANA Considerations

   IANA has assigned a TCP port number in the Registered Port Number
   range for the use of CMC.

     Service name: pkix-cmc
     Port Number: 5318
     Transport protocol: TCP
     Description: PKIX Certificate Management using CMS (CMC)
     Reference: [RFC-to-be]
     Assignee: iesg@ietf.org
     Contact: chair@ietf.org

   IANA is requested to update the existing references to [CMC-TRANSv1]
   in the Media Type Sub-Parameter Registries for CMC-Request and CMC-
   Response to [ RFC-to-be ].

9.  Security Considerations

   Mechanisms for thwarting replay attacks may be required in particular
   implementations of this protocol depending on the operational
   environment.  In cases where the Certification Authority (CA)
   maintains significant state information, replay attacks may be
   detectable without the inclusion of the (optional) CMC nonce
   mechanisms.  Implementers and Designers of this protocol need to
   carefully consider environmental conditions before choosing whether
   or not to implement or use the senderNonce and recipientNonce
   attributes described in Section 6.6 of [CMC-STRUCT].  Developers of
   state-constrained PKI clients are strongly encouraged to incorporate
   the use of these attributes.

   Initiation of a secure communications channel between an end-entity
   and a CA or Registration Authority (RA) -- and, similarly, between an
   RA and another RA or CA -- necessarily requires an out-of-band trust
   initiation mechanism.  For example, a secure channel may be
   constructed between the end-entity and the CA via IPsec [IPsec] or
   TLS [TLS].  Many such schemes exist, and the choice of any particular

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   scheme for trust initiation is outside the scope of this document.
   Implementers of this protocol are strongly encouraged to consider
   generally accepted principles of secure key management when
   integrating this capability within an overall security architecture.

   In some instances, no prior out-of-band trust will have been
   initiated prior to use of this protocol.  This can occur when the
   protocol itself is being used to download onto the system the set of
   trust anchors to be used for these protocols.  In these instances,
   the Enveloped Data content type (Section 3.2.1.3.3 of [CMC-STRUCT])
   or Authenticated Enveloped Data content type Section 3.2.1.3.5 of
   [CMC-STRUCT] provides the same shrouding that TLS would have
   provided.

   For the mail-based protocol, the Enveloped Data or Authenticated
   Enveloped Data content types can also be used to apply
   confidentiality protection (content shrouding) to the conveyed
   messages.  Note that even if the application uses SMTP-over-TLS
   [RFC3207] with its preferred Message Submission Agent (MSA) for
   initial submission of the message for delivery, SMTP in subsequent
   relay hops may not be either authenticated or encrypted.  For some
   combinations of initial MSA and destination domains it may be
   possible to request use of authenticated TLS at every relay "hop" of
   message delivery via the mechanism specified in [RFC8689].  This MAY
   be used, when supported, and expected to work, but risks non-delivery
   if some of the SMTP servers along the relay chain do not support the
   REQUIRETLS ESMTP extension.

   For the file-based protocol, an additional method of applying
   confidentiality protection (content shrouding) to the conveyed
   messages is usually available in the form of filesystem permissions.
   The local system may allow for read access to be limited to just a
   single user or group that corresponds to the entity authorized to
   read the request or response, respectively, and diligent use of these
   filesystem permissions can be a useful mechanism in multi-user
   environments.

10.  References

10.1.  Normative References

   [BCP195]   Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/rfc/rfc9325>.

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   [CMC-STRUCT]
              Mandel, J. and S. Turner, "Certificate Management over CMS
              (CMC)", Work in Progress, Internet-Draft, draft-ietf-
              lamps-rfc5272bis-10, 6 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
              rfc5272bis-10>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [HTTP-IMP] Nottingham, M., "Building Protocols with HTTP", BCP 56,
              RFC 9205, DOI 10.17487/RFC9205, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9205>.

   [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/rfc/rfc2119>.

   [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/rfc/rfc3986>.

   [RFC5967]  Turner, S., "The application/pkcs10 Media Type", RFC 5967,
              DOI 10.17487/RFC5967, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5967>.

   [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/rfc/rfc8174>.

   [SMIMEV4]  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/rfc/rfc8551>.

   [X690]     ITU-T, "Information Technology -- Abstract Syntax Notation
              One (ASN.1): ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", ITU-T
              Recommendation X.690, ISO/IEC 8825-1:2021, February 2021,
              <https://www.itu.int/rec/T-REC-X.690>.

10.2.  Informative References

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   [CMC-TRANSv1]
              Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC): Transport Protocols", RFC 5273,
              DOI 10.17487/RFC5273, June 2008,
              <https://www.rfc-editor.org/rfc/rfc5273>.

   [CMC-Updates]
              Schaad, J., "Certificate Management over CMS (CMC)
              Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011,
              <https://www.rfc-editor.org/rfc/rfc6402>.

   [COOKIES]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6265>.

   [erratum3593]
              "RFC 5273 erratum 3593", April 2013,
              <https://www.rfc-editor.org/errata/eid3593>.

   [HTTP_1.0] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945,
              DOI 10.17487/RFC1945, May 1996,
              <https://www.rfc-editor.org/rfc/rfc1945>.

   [HTTP_1.1] 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/rfc/rfc9112>.

   [HTTP_2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9113>.

   [HTTP_3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.

   [IPsec]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/rfc/rfc4301>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <https://www.rfc-editor.org/rfc/rfc3207>.

   [RFC8689]  Fenton, J., "SMTP Require TLS Option", RFC 8689,
              DOI 10.17487/RFC8689, November 2019,
              <https://www.rfc-editor.org/rfc/rfc8689>.

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   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/rfc/rfc5246>.

Acknowledgements

   Obviously, the authors of this version of the document would like to
   thank Jim Schaad and Michael Myers for their work on the previous
   version of this document.

   Thank you to Julian Reschke, Benjamin Kaduk, Vidhi Goel, Thomas
   Fossati, Gorry Fairhurst, Éric Vyncke, Gunter Van de Velde, Mahesh
   Jethanandani, Mike Bishop, Mohamed Boucadair, Viktor Dukhovni, and
   Eliot Lear for reviewing the document and providing comments.

   The acknowledgement from the previous version of this document
   follows:

   The authors and the PKIX Working Group are grateful for the
   participation of Xiaoyi Liu and Jeff Weinstein in helping to author
   the original versions of this document.

   The authors would like to thank Brian LaMacchia for his work in
   developing and writing up many of the concepts presented in this
   document.  The authors would also like to thank Alex Deacon and Barb
   Fox for their contributions.

Contributors

   Jim Schaad
   August Cellars

   Michael Myers
   TraceRoute Security, Inc.

Authors' Addresses

   Joseph Mandel
   AKAYLA, Inc.
   Email: joe@akayla.com

   Sean Turner (editor)
   sn3rd
   Email: sean@sn3rd.com

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