| Internet-Draft | HTTP Transfer for CMP | November 2024 |
| Brockhaus, et al. | Expires 22 May 2025 | [Page] |
This document describes how to layer the Certificate Management Protocol (CMP) over HTTP.¶
It includes the updates to RFC 6712 specified in RFC 9480 Section 3. These updates introduce CMP URIs using a Well-known prefix. It obsoletes RFC 6712 and together with I-D.ietf-lamps-rfc4210bis and it also obsoletes RFC 9480.¶
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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 22 May 2025.¶
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[RFC Editor: please delete:¶
During IESG telechat the CMP Updates document was approved on condition that LAMPS provides a RFC6712bis document. Version -00 of this document shall be identical to RFC 6712 and version -01 incorporates the changes specified in CMP Updates Section 3.¶
A history of changes is available in Appendix A of this document.¶
The authors of this document wish to thank Tomi Kause and Martin Peylo, the original authors of RFC 6712, for their work and invite them, next to further volunteers, to join the -bis activity as co-authors.¶
]¶
The Certificate Management Protocol (CMP) [I-D.ietf-lamps-rfc4210bis] 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 [RFC2510] 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-, E-mail-, and HTTP-based transfer, but those were not specified in detail.¶
Since the second version of the CMP specification [RFC4210] incorporated its own polling mechanism and thus 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 a reliable transport as CMP requires connection and error handling from the transfer protocol. All theses features are covered by HTTP. Additionally, delayed delivery of CMP response messages may be handled at transfer level, regardless of the message contents. Since [RFC9480] extends the polling mechanism specified in the second version of CMP [RFC4210] 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 [RFC9110] and [RFC9112]) for transferring CMP messages exclusively uses the POST method for requests, effectively tunneling CMP over HTTP. While this is generally considered bad practice (see BCP 56 [RFC9205] 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.¶
CMP Updates [RFC9480] updated Section 3.6 of [RFC6712], supporting the PKI management operations specified in the Lightweight CMP Profile [RFC9483], in the following areas:¶
This document obsoletes [RFC6712]. It includes the changes specified in Section 3 of [RFC9480] as described in Section 1.1 of this document. Additionally it adds the following changes:¶
Removed the requirement to support HTTP/1.0 [RFC1945] in accordance with Section 4.1 of [RFC9205].¶
Implementations MUST forward CMP messages when an HTTP error status code occurs, see Section 3.3.¶
Removed Section 3.8 of [RFC6712] as it contains information redundant with current HTTP specification.¶
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.¶
For direct interaction between two entities, where a reliable transport protocol like TCP [RFC9293] is available, HTTP SHOULD be utilized for conveying CMP messages.¶
This draft requires uses of the POST method (Section 3.3) and the "Content-Type" header field (Section 3.4) which are available since HTTP/1.0 [RFC1945]. This specification also specifies use of persistent connections (Section 3.2). This document refers to HTTP/1.1 as specified in [RFC9110] and [RFC9112] for further details.¶
HTTP persistent connections (Section 9.3 of [RFC9112]) allow multiple interactions to take place on the same HTTP connection. However, neither HTTP nor the protocol specified in this document are designed to correlate messages on the same connection in any meaningful way; persistent connections are only a performance optimization. In particular, intermediaries can do things like mix connections from different clients into one upstream connection, terminate persistent connections, and forward requests as non-persistent requests, etc. As such, implementations MUST NOT infer that requests on the same connection come from the same client (e.g., for correlating PKI messages with ongoing transactions); every message is to be evaluated in isolation.¶
A DER-encoded [ITU.X690.1994] PKIMessage (Section 5.1 of [I-D.ietf-lamps-rfc4210bis]) 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) status code; other Successful 2xx status codes MUST NOT be used for this purpose. HTTP responses to pushed CMP announcement messages described in Section 3.7 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 Section 5, 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.¶
The Internet Media Type "application/pkixcmp" MUST be set in the HTTP "Content-Type" header field when conveying a PKIMessage.¶
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 Section 3.7 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.¶
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 [RFC8615] 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 [RFC9483], could indicate PKI management operations using an operationLabel <operation>. The following list 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 (Section 5).¶
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 CRL, a PKI Information Request using a General Message as described in Appendix D.5 of [I-D.ietf-lamps-rfc4210bis] 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 Section 3.6.¶
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 (Section 5.1.2 of [I-D.ietf-lamps-rfc4210bis]).¶
[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 an 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 an 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.¶
Implementers should be aware that other implementations might exist that use a different approach for transferring CMP over HTTP. Further, implementations based on earlier I-Ds that led to [RFC6712] might use an unregistered "application/pkixcmp-poll" Media Type. Conforming implementations MAY handle this type like "application/pkixcmp".¶
The following aspects need to be considered by implementers and users:¶
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. After sending a CMP error message with PKIStatus other than "waiting", the server should close the connection, even if the CMP transaction is not yet fully completed.¶
Without being encapsulated in effective security protocols, such as Transport Layer Security (TLS) [RFC5246] or [RFC8446], or without using HTTP digest [RFC9530] 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).¶
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.¶
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.¶
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 [RFC9110] or using virtual private networks created, for example, by utilizing Internet Protocol Security according to [RFC7296].¶
The reference to [RFC2510] at https://www.iana.org/assignments/media-types/media-types.xhtml should be replaced with a reference to this document.¶
The reference to [RFC4210] at https://www.iana.org/assignments/core-parameters/core-parameters.xhtml should be replaced with a reference to this document.¶
The reference to [RFC9480] at https://www.iana.org/assignments/well-known-uris/well-known-uris.xhtml and https://www.iana.org/assignments/cmp/cmp.xhtmlshould should be replaced with a reference to this document.¶
No further action by the IANA is necessary for this document or any anticipated updates.¶
The authors wish to thank Tomi Kause and Martin Peylo, the original authors of [RFC6712], for their work.¶
We also thank all reviewers for their valuable feedback.¶
Note: This appendix will be deleted in the final version of the document.¶
From version 07 -> 08:¶
Addressed HTTPDIR, SECDIR, OPSDIR and ARTART review comments¶
Aligned the terminology with https://httpwg.org/admin/editors/style-guide¶
Implemented editorial changes proposed by OPSDIR reviewer¶
Removed requirement to support HTTP/1.0¶
Added normative language in Sections 3.3 and 3.7 for clarity¶
Added the requirement to provide any HTTP response message content to the application¶
Removed the paragraph on the "Content-Length" header field and Section 3.8 to reduce redundancy with current versions HTTP/1.1¶
From version 06 -> 07:¶
Updated the the page header to 'HTTP Transfer for CMP'¶
Removed one instruction to RFC Editors¶
Deprecated PKIMessages as plural of PKIMessage to prevent confusion with ASN.1 type PKIMessages¶
Fixed some nits in Section 1¶
Aligned Section 3.6 and Section 5 with RFC 9483 and draft-ietf-anima-brski-ae¶
From version 05 -> 06:¶
Updates IANA considerations addressing IANA early review (see thread "[IANA #1368693] Early review: draft-ietf-lamps-rfc4210bis-12 (IETF 120)").¶
From version 04 -> 05:¶
Added IANA considerations addressing IANA early review.¶
From version 03 -> 04:¶
Aligned with released RFC 9480 - RFC 9483.¶
From version 02 -> 03:¶
Fixing one formatting nit.¶
From version 01 -> 02:¶
Updated Section 3.4 including the requirement to add the content-length filed into the HTTP header.¶
Added a reference to TLS 1.3.¶
Addressed idnits feedback, specifically changing the following RFC references: RFC2616 -> RFC9112; RFC2818 -> RFC9110, and RFC5246 -> RFC8446¶
From version 00 -> 01:¶
Performed all updates specified in CMP Updates Section 3.¶
Version 00:¶
This version consists of the text of RFC6712 with the following changes:¶