Certificate Management over CMS (CMC): Transport Protocols AKAYLA, Inc.
joe@akayla.com
sn3rd
sean@sn3rd.com
SEC lamps Public Key Infrastructure Cryptographic Message Syntax Certificate Management Transport Protocols 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 .
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Table of Contents
  • .  Introduction
  • .  Requirements Terminology
  • .  Changes Since RFCs 5273 and 6402
  • .  File-Based Protocol
  • .  Mail-Based Protocol
  • .  HTTP-Based Protocol
    • .  PKI Request
    • .  PKI Response
  • .  TCP-Based Protocol
  • .  IANA Considerations
  • .  Security Considerations
  • . References
    • .  Normative References
    • .  Informative References
  • Acknowledgements
  • Contributors
  • Authors' Addresses
Introduction This document defines a number of transport methods that are used to move CMC messages (defined in ). The transport mechanisms described in this document are HTTP, file, mail, and TCP. This document obsoletes RFCs 5273 and 6402 . This document also incorporates .
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 when, and only when, they appear in all capitals, as shown here.
Changes Since RFCs 5273 and 6402 Merged 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 . Addressed . Added a reference to 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.
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: File PKI Request/Response Identification
Message Type File Extension
Simple PKI Request .p10
Full PKI Request .crq
Simple PKI Response .p7c
Full PKI Response .crp
Mail-Based Protocol MIME wrapping is defined for those environments that support MIME. The basic mime wrapping in this section is taken from . 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. Simple enrollment requests are encoded using the "application/pkcs10" content type . 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 follows:
Simple PKI Request Message Example 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 follows:
Simple PKI Response Message Example 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 follows:
Full PKI Request Message Example 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 follows:
Full PKI Response Message Example 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. MIME PKI Request/Response Identification
Item MIME Type File Extension SMIME Type
Simple PKI Request application/pkcs10 .p10 N/A
Full PKI Request application/pkcs7-mime .p7m CMC-Request
Simple PKI Response application/pkcs7-mime .p7c certs-only
Full PKI Response application/pkcs7-mime .p7m CMC-Response
HTTP-Based Protocol This section describes the conventions for use of HTTP as a data transfer protocol. Consult for additional information. The use of HTTPS 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 .
  • 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 . Although servers are not required to support TLS/QUIC, a secure channel might be available regardless depending on the HTTP version implemented , , , , or later. If TLS is used by the HTTP version, then the implementation MUST follow the recommendations in . 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 ) nor Cookies . Thus, servers cannot rely on these features to be available.
  • Clients and servers are expected to follow other rules and restrictions in . 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.
PKI Request A PKI Request using the POST method is constructed as follows. The Content-Type field MUST have the appropriate value from . 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.
PKI Response The content of an HTTP-based PKI Response is the binary value of the BER (Basic Encoding Rules) encoding of either a Simple or Full PKI Response. The Content-Type field MUST have the appropriate value from . A Content-Type field for a response: Content-Type: application/pkcs7-mime; smime-type=CMC-Response; name=response.p7m
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 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.
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 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.
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 . 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 or 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 () or AuthEnvelopedData content type 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 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 . 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.
References Normative References Deprecating TLS 1.0 and TLS 1.1 This document formally deprecates Transport Layer Security (TLS) versions 1.0 (RFC 2246) and 1.1 (RFC 4346). Accordingly, those documents have been moved to Historic status. These versions lack support for current and recommended cryptographic algorithms and mechanisms, and various government and industry profiles of applications using TLS now mandate avoiding these old TLS versions. TLS version 1.2 became the recommended version for IETF protocols in 2008 (subsequently being obsoleted by TLS version 1.3 in 2018), providing sufficient time to transition away from older versions. Removing support for older versions from implementations reduces the attack surface, reduces opportunity for misconfiguration, and streamlines library and product maintenance. This document also deprecates Datagram TLS (DTLS) version 1.0 (RFC 4347) but not DTLS version 1.2, and there is no DTLS version 1.1. This document updates many RFCs that normatively refer to TLS version 1.0 or TLS version 1.1, as described herein. This document also updates the best practices for TLS usage in RFC 7525; hence, it is part of BCP 195. Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are used to protect data exchanged over a wide range of application protocols and can also form the basis for secure transport protocols. Over the years, the industry has witnessed several serious attacks on TLS and DTLS, including attacks on the most commonly used cipher suites and their modes of operation. This document provides the latest recommendations for ensuring the security of deployed services that use TLS and DTLS. These recommendations are applicable to the majority of use cases. RFC 7525, an earlier version of the TLS recommendations, was published when the industry was transitioning to TLS 1.2. Years later, this transition is largely complete, and TLS 1.3 is widely available. This document updates the guidance given the new environment and obsoletes RFC 7525. In addition, this document updates RFCs 5288 and 6066 in view of recent attacks. Certificate Management over CMS (CMC) AKAYLA, Inc. sn3rd HTTP Semantics The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. Building Protocols with HTTP Applications often use HTTP as a substrate to create HTTP-based APIs. This document specifies best practices for writing specifications that use HTTP to define new application protocols. It is written primarily to guide IETF efforts to define application protocols using HTTP for deployment on the Internet but might be applicable in other situations. This document obsoletes RFC 3205. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Uniform Resource Identifier (URI): Generic Syntax A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] The application/pkcs10 Media Type This document specifies a media type used to carry PKCS #10 certification requests as defined in RFC 2986. It carries over the original specification from RFC 2311, which recently has been moved to Historic status, and properly links it to RFC 2986. This document is not an Internet Standards Track specification; it is published for informational purposes. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 4.0. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 5751. Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER) ITU-T Informative References Certificate Management over CMS (CMC): Transport Protocols 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. [STANDARDS-TRACK] Certificate Management over CMS (CMC) Updates This document contains a set of updates to the base syntax for CMC, a Certificate Management protocol using the Cryptographic Message Syntax (CMS). This document updates RFC 5272, RFC 5273, and RFC 5274. The new items in this document are: new controls for future work in doing server side key generation, definition of a Subject Information Access value to identify CMC servers, and the registration of a port number for TCP/IP for the CMC service to run on. [STANDARDS-TRACK] HTTP State Management Mechanism This document defines the HTTP Cookie and Set-Cookie header fields. These header fields can be used by HTTP servers to store state (called cookies) at HTTP user agents, letting the servers maintain a stateful session over the mostly stateless HTTP protocol. Although cookies have many historical infelicities that degrade their security and privacy, the Cookie and Set-Cookie header fields are widely used on the Internet. This document obsoletes RFC 2965. [STANDARDS-TRACK] RFC Errata, Erratum ID 3593, RFC 5273 Hypertext Transfer Protocol -- HTTP/1.0 The Hypertext Transfer Protocol (HTTP) is an application-level protocol with the lightness and speed necessary for distributed, collaborative, hypermedia information systems. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. HTTP/1.1 The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document specifies the HTTP/1.1 message syntax, message parsing, connection management, and related security concerns. This document obsoletes portions of RFC 7230. HTTP/2 This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced latency by introducing field compression and allowing multiple concurrent exchanges on the same connection. This document obsoletes RFCs 7540 and 8740. HTTP/3 The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC and describes how HTTP/2 extensions can be ported to HTTP/3. Security Architecture for the Internet Protocol This document describes an updated version of the "Security Architecture for IP", which is designed to provide security services for traffic at the IP layer. This document obsoletes RFC 2401 (November 1998). [STANDARDS-TRACK] SMTP Service Extension for Secure SMTP over Transport Layer Security This document describes an extension to the SMTP (Simple Mail Transfer Protocol) service that allows an SMTP server and client to use TLS (Transport Layer Security) to provide private, authenticated communication over the Internet. This gives SMTP agents the ability to protect some or all of their communications from eavesdroppers and attackers. [STANDARDS-TRACK] SMTP Require TLS Option The SMTP STARTTLS option, used in negotiating transport-level encryption of SMTP connections, is not as useful from a security standpoint as it might be because of its opportunistic nature; message delivery is, by default, prioritized over security. This document describes an SMTP service extension, REQUIRETLS, and a message header field, TLS-Required. If the REQUIRETLS option or TLS-Required message header field is used when sending a message, it asserts a request on the part of the message sender to override the default negotiation of TLS, either by requiring that TLS be negotiated when the message is relayed or by requesting that recipient-side policy mechanisms such as MTA-STS and DNS-Based Authentication of Named Entities (DANE) be ignored when relaying a message for which security is unimportant. The Transport Layer Security (TLS) Protocol Version 1.3 This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document obsoletes RFC 8446, which specified TLS 1.3. This document obsoletes RFC 5246 (specifying TLS 1.2) and RFCs 5077, 6961, 7627, and 8422, all of which pertain to TLS 1.2 or earlier, and updates RFCs 5705 and 6066. This document also specifies new requirements for TLS 1.2 implementations.
Acknowledgements Obviously, the authors would like to thank and for their work on RFC 5273. Thank you to , , , , , , , , , , , and 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 and in helping to author the original versions of this document. The authors would like to thank for his work in developing and writing up many of the concepts presented in this document. The authors would also like to thank and for their contributions.
Contributors August Cellars
TraceRoute Security, Inc.
Authors' Addresses AKAYLA, Inc.
joe@akayla.com
sn3rd
sean@sn3rd.com