Certificate Management over CMS (CMC): Transport Protocols
RFC 10003
| Document | Type | RFC - Proposed Standard (July 2026) | |
|---|---|---|---|
| Authors | J. Mandel , S. Turner | ||
| Last updated | 2026-07-17 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| IESG | Responsible AD | Deb Cooley | |
| Send notices to | (None) |
RFC 10003
Internet Engineering Task Force (IETF) J. Mandel, Ed.
Request for Comments: 10003 AKAYLA, Inc.
Obsoletes: 5273, 6402 S. Turner, Ed.
Category: Standards Track sn3rd
ISSN: 2070-1721 July 2026
Certificate Management over CMS (CMC): Transport Protocols
Abstract
This document defines a number of transport mechanisms that are used
to move Certificate Management over CMS (CMC) messages. The
transport mechanisms described in this document are HTTP, file, mail,
and TCP.
This document obsoletes RFCs 5273 and 6402.
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
https://www.rfc-editor.org/info/rfc10003.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Without obtaining an adequate license from the person(s) controlling
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than English.
Table of Contents
1. Introduction
2. Requirements Terminology
3. Changes Since RFCs 5273 and 6402
4. File-Based Protocol
5. Mail-Based Protocol
6. HTTP-Based Protocol
6.1. PKI Request
6.2. PKI Response
7. TCP-Based Protocol
8. IANA Considerations
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
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 RFCs 5273 [CMC-TRANSv1] and 6402
[CMC-Updates]. This document also incorporates [Err3593].
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 RFCs 5273 and 6402
Merged [CMC-Updates] text.
IANA assigned TCP port 5318 for the use of CMC.
Clarified the file extensions for Full Public Key Infrastructure
(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 implementations
are required to follow the recommendations in [BCP195].
Addressed [Err3593].
Added a reference to [HTTP-IMP] for HTTP guidance.
Restricted early data (0-RTT) if using TLS 1.3 or QUIC.
Restricted the use of TCP-Pipelining.
Clarified the limitations of SMTP-over-TLS and the use of
authenticated TLS for message delivery.
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 offline 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 support MIME.
The basic mime wrapping in this section is taken from [SMIMEV4].
When using a mail-based protocol, MIME wrapping between the layers of
Cryptographic Message Syntax (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 messages, and
the messages' contents are the entire messages' bodies.
| WARNING: 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:
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 >
Figure 1: Simple PKI Request Message Example
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 >
Figure 2: Simple PKI Response Message Example
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:
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 >
Figure 3: Full PKI Request Message Example
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 >
Figure 4: Full PKI Response Message Example
For file names present in the name or filename parameters, non-ASCII
text is prohibited.
+====================+==============+================+==============+
| 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, 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 (see Section 11 of [HTTP]) nor Cookies [COOKIES].
Thus, servers cannot rely on these features to be available.
* 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.
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 the port assigned to the
CMC service: 5318. 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 "Dynamic and/or Private
Ports Range" of the "Service Name and Transport Protocol Port Number
Registry" for the use of CMC.
Service Name: pkix-cmc
Port Number: 5318
Transport Protocol: tcp
Description: PKIX Certificate Management using CMS (CMC)
Assignee: iesg@ietf.org
Contact: chair@ietf.org
Reference: RFC 10003
IANA has updated the references to [CMC-TRANSv1] in the "Parameter
Values for the smime-type Parameter" registry in the "Media Type Sub-
Parameter Registries" registry group for CMC-Request and CMC-Response
to instead point to this document.
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
(EE) 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 EE and the CA via IPsec [IPsec] or TLS
[TLS]. Many such schemes exist, and the choice of any particular
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 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
EnvelopedData content type (Section 3.2.1.3.3 of [CMC-STRUCT]) or
AuthEnvelopedData 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 EnvelopedData or AuthEnvelopedData
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] Best Current Practice 195,
<https://www.rfc-editor.org/info/bcp195>.
At the time of writing, this BCP comprises the following:
Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.
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/info/rfc9325>.
[CMC-STRUCT]
Mandel, J., Ed. and S. Turner, Ed., "Certificate
Management over CMS (CMC)", RFC 10002,
DOI 10.17487/RFC10002, July 2026,
<https://www.rfc-editor.org/info/rfc10002>.
[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/info/rfc9110>.
[HTTP-IMP] Nottingham, M., "Building Protocols with HTTP", BCP 56,
RFC 9205, DOI 10.17487/RFC9205, June 2022,
<https://www.rfc-editor.org/info/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/info/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/info/rfc3986>.
[RFC5967] Turner, S., "The application/pkcs10 Media Type", RFC 5967,
DOI 10.17487/RFC5967, August 2010,
<https://www.rfc-editor.org/info/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/info/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/info/rfc8551>.
[X690] ITU-T, "Information technology - 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
[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/info/rfc5273>.
[CMC-Updates]
Schaad, J., "Certificate Management over CMS (CMC)
Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011,
<https://www.rfc-editor.org/info/rfc6402>.
[COOKIES] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[Err3593] RFC Errata, Erratum ID 3593, RFC 5273,
<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/info/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/info/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/info/rfc9113>.
[HTTP_3] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, <https://www.rfc-editor.org/info/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/info/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/info/rfc3207>.
[RFC8689] Fenton, J., "SMTP Require TLS Option", RFC 8689,
DOI 10.17487/RFC8689, November 2019,
<https://www.rfc-editor.org/info/rfc8689>.
[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 9846, DOI 10.17487/RFC9846, July 2026,
<https://www.rfc-editor.org/info/rfc9846>.
Acknowledgements
Obviously, the authors would like to thank Jim Schaad and Michael
Myers for their work on RFC 5273.
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 Acknowledgements section from RFC 5273 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 (editor)
AKAYLA, Inc.
Email: joe@akayla.com
Sean Turner (editor)
sn3rd
Email: sean@sn3rd.com