Siemens

Werner-von-Siemens-Strasse 1 Munich 80333 Germany hendrik.brockhaus@siemens.com https://www.siemens.com

Siemens

Werner-von-Siemens-Strasse 1 Munich 80333 Germany david.von.oheimb@siemens.com https://www.siemens.com

Entrust

1187 Park Place Minneapolis MN 55379 United States of America mike.ounsworth@entrust.com https://www.entrust.com

Entrust

1187 Park Place Minneapolis MN 55379 United States of America john.gray@entrust.com https://www.entrust.com

SEC lamps CMP HTTP Certificate management PKI This document describes how to layer the Certificate Management Protocol (CMP) over HTTP. It includes the updates to RFC 6712 specified in Section 3 of RFC 9480; these updates introduce CMP URIs using a well-known prefix. It obsoletes RFC 6712; and, together with RFC 9810, it also obsoletes RFC 9480.

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 .

Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents () in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.

Table of Contents

• .  Introduction
  • .  Changes Made by RFC 9480
  • .  Changes Made by This Document
• .  Conventions Used in This Document
• .  HTTP-Based Protocol
  • .  General Form
  • .  Media Type
  • .  Communication Workflow
  • .  HTTP Request-URI
  • .  Pushing of Announcements
• .  Implementation Considerations
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Authors' Addresses

Introduction The Certificate Management Protocol (CMP) requires a well-defined transfer mechanism to enable End Entities (EEs), Registration Authorities (RAs), and Certification Authorities (CAs) to pass PKIMessage structures between them. The first version of the CMP specification included a brief description of a simple transfer protocol layer on top of TCP. Its features were simple transfer-level error handling and a mechanism to poll for outstanding PKI messages. Additionally, it was mentioned that PKI messages could also be conveyed using file-, email-, and HTTP-based transfer, but those were not specified in detail. Since the second version of the CMP specification incorporated its own polling mechanism, the need for a transfer protocol providing this functionality vanished. The remaining features CMP requires from its transfer protocols are connection and error handling. CMP can benefit from utilizing reliable transport, as it requires connection and error handling from the transfer protocol. All these features are covered by HTTP. Additionally, delayed delivery of CMP response messages may be handled at transfer level, regardless of the message contents. Since extends the polling mechanism specified in the second version of CMP to cover all types of PKI management transactions, delays detected at application level may also be handled within CMP, using pollReq and pollRep messages. The usage of HTTP (e.g., HTTP/1.1 as specified in and ) for transferring CMP messages exclusively uses the POST method for requests, effectively tunneling CMP over HTTP. While this is generally considered bad practice (see RFC 9205 for best current practice on building protocols with HTTP) and should not be emulated, there are good reasons to do so for transferring CMP. HTTP is used as it is generally easy to implement and it is able to traverse network borders utilizing ubiquitous proxies. Most importantly, HTTP is already commonly used in existing CMP implementations. Other HTTP request methods, such as GET, are not used because PKI management operations can only be triggered using CMP's PKI messages, which need to be transferred using a POST request. With its status codes, HTTP provides needed error reporting capabilities. General problems on the server side, as well as those directly caused by the respective request, can be reported to the client. As CMP implements a transaction identification (transactionID), identifying transactions spanning over more than just a single request/response pair, the statelessness of HTTP is not blocking its usage as the transfer protocol for CMP messages.

Changes Made by RFC 9480 CMP Updates updated , supporting the PKI management operations specified in the Lightweight CMP Profile, in the following areas:

• Introduced the HTTP URI path prefix '/.well-known/cmp'.
• Added options for extending the URI structure with further segments and defined a new protocol registry group to that aim.

Changes Made by This Document This document obsoletes . It includes the changes specified in , as described in of this document. Additionally, it adds the following changes:

• Removed the requirement to support HTTP/1.0 in accordance with Section of RFC 9205 .
• Implementations MUST forward CMP messages when an HTTP error status code occurs; see .
• Removed as it contains information redundant with current HTTP specification.

Conventions Used in This Document 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.

HTTP-Based Protocol For direct interaction between two entities, where a reliable transport protocol like TCP is available, HTTP SHOULD be utilized for conveying CMP messages. This specification requires using the POST method () and the "Content-Type" header field (), which are available since HTTP/1.0 . Note: In some situations, CMP requires multiple request/response pairs to perform a PKI management operation. Their affiliation with a PKI management operation is indicated by a transaction identifier in the CMP message header (see transactionID described in ). For details on how to transfer multiple requests, see Section of RFC 9205 .

General Form A DER-encoded PKIMessage () MUST be sent as the content of an HTTP POST request. If this HTTP request is successful, the server returns the CMP response in the content of the HTTP response. The HTTP response status code in this case MUST be 200 (OK); other Successful 2xx status codes MUST NOT be used for this purpose. HTTP responses to pushed CMP announcement messages described in utilize the status codes 201 and 202 to identify whether the received information was processed. While Redirection 3xx status codes MAY be supported by implementations, clients should only be enabled to automatically follow them after careful consideration of possible security implications. As described in , the 301 (Moved Permanently) status code could be misused for permanent denial of service. All applicable Client Error 4xx or Server Error 5xx status codes MAY be used to inform the client about errors. Whenever a client receives an HTTP response with a status code in the 2xx, 4xx, or 5xx ranges, it MUST support handling response message content containing a CMP response PKIMessage.

Media Type The Internet media type "application/pkixcmp" MUST be set in the HTTP "Content-Type" header field when conveying a PKIMessage.

Communication Workflow In CMP, most communication is initiated by the EEs where every CMP request triggers a CMP response message from the CA or RA. The CMP announcement messages described in are an exception. Their creation may be triggered by certain events or done on a regular basis by a CA. The recipient of the announcement only replies with an HTTP status code acknowledging the receipt or indicating an error, but not with a CMP response. If the receipt of an HTTP request is not confirmed by receiving an HTTP response, it MUST be assumed that the transferred CMP message was not successfully delivered to its destination.

HTTP Request-URI Each CMP server on a PKI management entity supporting HTTP or HTTPS transfer MUST support the use of the path prefix '/.well-known/' as defined in and the registered name 'cmp' to ease interworking in a multi-vendor environment. CMP clients have to be configured with sufficient information to form the CMP server URI. This is at least the authority portion of the URI, e.g., 'www.example.com:80', or the full operation path segment of the PKI management entity. Additionally, path segments MAY be added after the registered application name as part of the full operation path to provide further 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 , could indicate PKI management operations using an operationLabel <operation>. The following list shows examples of valid full CMP URIs:

• http://www.example.com/.well-known/cmp
• http://www.example.com/.well-known/cmp/<operation>
• http://www.example.com/.well-known/cmp/p/<name>
• http://www.example.com/.well-known/cmp/p/<name>/<operation>

Note that https can also be used instead of http; see item 5 in the Security Considerations.

Pushing of Announcements A CMP server may create event-triggered announcements or generate them on a regular basis. It MAY utilize HTTP transfer to convey them to a suitable recipient. In this use case, the CMP server acts as an HTTP client, and the recipient needs to utilize an HTTP server. As no request messages are specified for those announcements, they can only be pushed to the recipient. If an EE wants to poll for a potential CA Key Update Announcement or the current Certificate Revocation List (CRL), a PKI Information Request using a general message as described in can be used. When pushing announcement messages, PKIMessage structures MUST be sent as the content of an HTTP POST request. Suitable recipients for CMP announcements might, for example, be repositories storing the announced information, such as directory services. Those services listen for incoming messages, utilizing the same HTTP Request-URI scheme as defined in . The following types of PKIMessage are announcements that may be pushed by a CA. The prefixed numbers reflect ASN.1 tags of the PKIBody structure (). [15] CA Key Update Announcement [16] Certificate Announcement [17] Revocation Announcement [18] CRL Announcement CMP announcement messages do not require any CMP response. However, the recipient MUST acknowledge receipt with an HTTP response having an appropriate status code and empty content. When not receiving such a response, it MUST be assumed that the delivery was not successful. If applicable, the sending side MAY try sending the announcement again after waiting for an appropriate time span. If the announced issue was successfully stored in a database or was already present, the answer MUST be an HTTP response with a 201 (Created) status code and empty content. In case the announced information was only accepted for further processing, the status code of the returned HTTP response MAY also be 202 (Accepted). After an appropriate delay, the sender may then try to send the announcement again and may repeat this until it receives a confirmation that it has been successfully processed. The appropriate duration of the delay and the option to increase it between consecutive attempts should be carefully considered. A receiver MUST answer with a suitable 4xx or 5xx error code when a problem occurs.

Implementation Considerations Implementers should be aware that other implementations might exist that use a different approach for transferring CMP over HTTP. Further, implementations based on earlier documents that led to might use an unregistered "application/pkixcmp-poll" media type. Conforming implementations MAY handle this type like "application/pkixcmp".

Security Considerations All security considerations in HTTP apply. The following items need to be considered by implementers and users:

1. There is the risk for denial-of-service attacks through resource consumption by opening many connections to an HTTP server. Therefore, idle connections should be terminated after an appropriate timeout; this may also depend on the available free resources.
2. Without being encapsulated in effective security protocols, such as Transport Layer Security (TLS) , or without using HTTP digest , there is no integrity protection at the HTTP level. Therefore, information from the HTTP should not be used to change state of the transaction, regardless of whether any mechanism was used to ensure the authenticity or integrity of HTTP messages (e.g., TLS or HTTP digests).
3. Client users should be aware that storing the target location of an HTTP response with the 301 (Moved Permanently) status code could be exploited by a meddler-in-the-middle attacker trying to block them permanently from contacting the correct server.
4. If no measures to authenticate and protect the HTTP responses to pushed announcement messages are in place, their information regarding the announcement's processing state may not be trusted. In that case, the overall design of the PKI system must not depend on the announcements being reliably received and processed by their destination.
5. CMP provides inbuilt integrity protection and authentication. The information communicated unencrypted in CMP messages does not contain sensitive information endangering the security of the PKI when intercepted. However, it might be possible for an eavesdropper to utilize the available information to gather confidential personal, technical, or business-critical information. The protection of the confidentiality of CMP messages together with an initial authentication of the RA/CA before the first CMP message is transmitted ensures the privacy of the EE requesting certificates. Therefore, users of the HTTP transfer for CMP messages should consider using HTTP over TLS according to or using virtual private networks created, for example, by utilizing Internet Protocol Security according to .

IANA Considerations IANA has made the following updates:

• the reference for "application/pkixcmp" in the "Media Types" registry refers to this document, instead of .
• the reference for "application/pkixcmp" in the "CoAP Content-Formats" registry refers to this document, instead of .
• the reference for "cmp" in the "Well-Known URIs" registry refers to this document instead of .
• the reference for "p" in the "CMP Well-Known URI Path Segments" registry refers to this document instead of .

No further action by IANA is necessary for this document or any anticipated updates.

References Normative References ITU-T 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. 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. 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. This memo defines a path prefix for "well-known locations", "/.well-known/", in selected Uniform Resource Identifier (URI) schemes. In doing so, it obsoletes RFC 5785 and updates the URI schemes defined in RFC 7230 to reserve that space. It also updates RFC 7595 to track URI schemes that support well-known URIs in their registry. 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. 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. Siemens Siemens Entrust Entrust Informative References 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. This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocols. [STANDARDS-TRACK] This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocol (CMP). Protocol messages are defined for X.509v3 certificate creation and management. CMP provides on-line interactions between PKI components, including an exchange between a Certification Authority (CA) and a client system. [STANDARDS-TRACK] This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] This document describes how to layer the Certificate Management Protocol (CMP) over HTTP. It is the "CMPtrans" document referenced in RFC 4210; therefore, this document updates the reference given therein. [STANDARDS-TRACK] This document describes version 2 of the Internet Key Exchange (IKE) protocol. IKE is a component of IPsec used for performing mutual authentication and establishing and maintaining Security Associations (SAs). This document obsoletes RFC 5996, and includes all of the errata for it. It advances IKEv2 to be an Internet Standard. 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 updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This document specifies the Transmission Control Protocol (TCP). TCP is an important transport-layer protocol in the Internet protocol stack, and it has continuously evolved over decades of use and growth of the Internet. Over this time, a number of changes have been made to TCP as it was specified in RFC 793, though these have only been documented in a piecemeal fashion. This document collects and brings those changes together with the protocol specification from RFC 793. This document obsoletes RFC 793, as well as RFCs 879, 2873, 6093, 6429, 6528, and 6691 that updated parts of RFC 793. It updates RFCs 1011 and 1122, and it should be considered as a replacement for the portions of those documents dealing with TCP requirements. It also updates RFC 5961 by adding a small clarification in reset handling while in the SYN-RECEIVED state. The TCP header control bits from RFC 793 have also been updated based on RFC 3168. This document contains a set of updates to the syntax of Certificate Management Protocol (CMP) version 2 and its HTTP transfer mechanism. This document updates RFCs 4210, 5912, and 6712. The aspects of CMP updated in this document are using EnvelopedData instead of EncryptedValue, clarifying the handling of p10cr messages, improving the crypto agility, as well as adding new general message types, extended key usages to identify certificates for use with CMP, and well-known URI path segments. CMP version 3 is introduced to enable signaling support of EnvelopedData instead of EncryptedValue and signal the use of an explicit hash AlgorithmIdentifier in certConf messages, as far as needed. This document aims at simple, interoperable, and automated PKI management operations covering typical use cases of industrial and Internet of Things (IoT) scenarios. This is achieved by profiling the Certificate Management Protocol (CMP), the related Certificate Request Message Format (CRMF), and transfer based on HTTP or Constrained Application Protocol (CoAP) in a succinct but sufficiently detailed and self-contained way. To make secure certificate management for simple scenarios and constrained devices as lightweight as possible, only the most crucial types of operations and options are specified as mandatory. More specialized or complex use cases are supported with optional features. This document defines HTTP fields that support integrity digests. The Content-Digest field can be used for the integrity of HTTP message content. The Repr-Digest field can be used for the integrity of HTTP representations. Want-Content-Digest and Want-Repr-Digest can be used to indicate a sender's interest and preferences for receiving the respective Integrity fields. This document obsoletes RFC 3230 and the Digest and Want-Digest HTTP fields.

Acknowledgements The authors wish to thank and , the original authors of , for their work. We also thank all reviewers for their valuable feedback.

Authors' Addresses Siemens

Werner-von-Siemens-Strasse 1 Munich 80333 Germany hendrik.brockhaus@siemens.com https://www.siemens.com

Siemens

Werner-von-Siemens-Strasse 1 Munich 80333 Germany david.von.oheimb@siemens.com https://www.siemens.com

Entrust

1187 Park Place Minneapolis MN 55379 United States of America mike.ounsworth@entrust.com https://www.entrust.com

Entrust

1187 Park Place Minneapolis MN 55379 United States of America john.gray@entrust.com https://www.entrust.com