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/rfc9482.¶

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Table of Contents

1. Introduction

The Certificate Management Protocol (CMP) [RFC4210] is used by the PKI entities for the generation and management of certificates. One of the requirements of CMP is to be independent of the transport protocol in use. CMP has mechanisms to take care of required transactions, error reporting, and protection of messages.¶

The Constrained Application Protocol (CoAP) defined in [RFC7252], [RFC7959], and [RFC8323] is a client-server protocol like HTTP. It is designed to be used by constrained devices over constrained networks. The recommended transport for CoAP is UDP; however, [RFC8323] specifies the support of CoAP over TCP, TLS, and WebSockets.¶

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

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

2. CoAP Transfer Mechanism for CMP

A CMP transaction consists of exchanging PKIMessages [RFC4210] between PKI end entities (EEs), registration authorities (RAs), and certification authorities (CAs). If the EEs are constrained devices, then they may prefer, as a CMP client, the use of CoAP instead of HTTP as the transfer mechanism. In general, the RAs and CAs are not constrained and can support both CoAP and HTTP client and server implementations. This section specifies how to use CoAP as the transfer mechanism for CMP.¶

    coap://www.example.com/.well-known/cmp
    coap://www.example.com/.well-known/cmp/<operation>
    coap://www.example.com/.well-known/cmp/p/<profileLabel>
    coap://www.example.com/.well-known/cmp/p/<profileLabel>/<operation>

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

    coap://www.example.com/.well-known/cmp/ann
    coap://www.example.com/.well-known/cmp/p/<profileLabel>/ann

3. Proxy Support

This section provides guidance on using a CoAP-to-HTTP proxy between EEs and RAs or CAs in order to avoid changes to the existing PKI implementation.¶

Since the CMP payload is the same over CoAP and HTTP transfer mechanisms, a CoAP-to-HTTP cross-protocol proxy can be implemented based on Section 10 of [RFC7252]. The CoAP-to-HTTP proxy can either be located closer to the EEs or closer to the RA or CA. The proxy MAY support service discovery and resource discovery, as described in Section 2.2. The CoAP-to-HTTP proxy MUST function as a reverse proxy, only permitting connections to a limited set of preconfigured servers. It is out of scope of this document to specify how a reverse proxy can route CoAP client requests to one of the configured servers. Some recommended mechanisms are as follows:¶

• Use the Uri-Path option to identify a server.¶
• Use separate hostnames for each of the configured servers and then use the Uri-Host option for routing the CoAP requests.¶
• Use separate hostnames for each of the configured servers and then use Server Name Indication [RFC8446] in case of the "coaps://" scheme for routing CoAP requests.¶

4. Security Considerations

• If PKIProtection is used, the PKIHeader and PKIBody of the CMP are cryptographically protected against malicious modifications. As such, UDP can be used without compromising the security of the CMP. Security considerations for CoAP are defined in [RFC7252].¶
• The CMP does not provide confidentiality of the CMP payloads. If confidentiality is desired, CoAP over DTLS [RFC9147] SHOULD be used to provide confidentiality for the CMP payloads; although, it cannot conceal that the CMP is used within the DTLS layer.¶
• Section 9.1 of [RFC7252] defines how to use DTLS [RFC9147] for securing CoAP. DTLS [RFC9147] associations SHOULD be kept alive and reused where possible to amortize on the additional overhead of DTLS on constrained devices.¶
• An EE might not witness all of the announcement messages when using the CoAP Observe Option [RFC7641], since the Observe Option is a "best-effort" approach and the server might lose its state for subscribers to its announcement messages. The EEs may use an alternate method described in Section 2.6 to obtain time critical changes, such as Certificate Revocation List (CRL) [RFC5280] updates.¶
• Implementations SHOULD use the available datagram size and avoid sending small datagrams containing partial CMP PKIMessage data in order to reduce memory usage for packet buffering.¶
• A CoAP-to-HTTP proxy can also protect the PKI entities by handling UDP and CoAP messages. The proxy can mitigate attacks, like denial-of-service attacks, replay attacks, and resource-exhaustion attacks, by enforcing basic checks, like validating that the ASN.1 syntax is compliant to CMP messages and validating the PKIMessage protection before sending them to PKI entities.¶
• Since the proxy may have access to the CMP-level metadata and control over the flow of CMP messages, proper role-based access control should be in place. The proxy can be deployed at the edge of the "end entities" network or in front of an RA and CA to protect them. However, the proxy may itself be vulnerable to resource-exhaustion attacks as it's required to buffer the CMP messages received over CoAP transport before sending it to the HTTP endpoint. This can be mitigated by using short timers for discarding the buffered messages and rate limiting clients based on the resource usage.¶

5. IANA Considerations

IANA has registered "application/pkixcmp" (ID 259) in the "CoAP Content-Formats" registry to transfer CMP transactions over CoAP.¶

Type name:

application¶

Subtype name:

pkixcmp¶

Reference:

RFC 9482 [RFC4210]¶

IANA has also registered a new path segment "ann" in the "CMP Well-Known URI Path Segments" registry for the EEs to register themselves for the announcement messages.¶

Path Segment:

ann¶

Description:

The path to send a GET request with the CoAP Observe Option to register for CMP announcement messages.¶

Reference:

RFC 9482¶

IANA has added this document as a reference for the "cmp" entry in the "Well-Known URIs" registry.¶

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

Acknowledgements

The authors would like to thank Hendrik Brockhaus, David von Oheimb, and Andreas Kretschmer for their guidance in writing the content of this document and providing valuable feedback.¶

Authors' Addresses