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Internet X.509 Public Key Infrastructure -- HTTP Transfer for the Certificate Management Protocol (CMP)

Document Type Active Internet-Draft (lamps WG)
Authors Hendrik Brockhaus , David von Oheimb , Mike Ounsworth , John Gray
Last updated 2023-02-10
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LAMPS Working Group                                         H. Brockhaus
Internet-Draft                                             D. von Oheimb
Obsoletes: 6712 (if approved)                                    Siemens
Intended status: Standards Track                            M. Ounsworth
Expires: 14 August 2023                                          J. Gray
                                                        10 February 2023

   Internet X.509 Public Key Infrastructure -- HTTP Transfer for the
                 Certificate Management Protocol (CMP)


   This document describes how to layer the Certificate Management
   Protocol (CMP) over HTTP.

   It includes the updates on RFC 6712 specified in CMP Updates
   [RFCAAAA] Section 3 and obsoleted both documents.  These updates
   introduce CMP URIs using a Well-known prefix.

Status of This Memo

   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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 14 August 2023.

Copyright Notice

   Copyright (c) 2023 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 (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   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

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Changes Since RFC 6712  . . . . . . . . . . . . . . . . .   4
     1.2.  Changes Made by This Document . . . . . . . . . . . . . .   4
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   3.  HTTP-Based Protocol . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  HTTP Versions . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Persistent Connections  . . . . . . . . . . . . . . . . .   5
     3.3.  General Form  . . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  Header Fields . . . . . . . . . . . . . . . . . . . . . .   6
     3.5.  Communication Workflow  . . . . . . . . . . . . . . . . .   6
     3.6.  HTTP Request-URI  . . . . . . . . . . . . . . . . . . . .   6
     3.7.  Pushing of Announcements  . . . . . . . . . . . . . . . .   7
     3.8.  HTTP Considerations . . . . . . . . . . . . . . . . . . .   8
   4.  Implementation Considerations . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  History of Changes . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   [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-


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   [RFC Editor:

   Please perform the following substitution.

   *  RFCXXXX ---> the assigned numerical RFC value for this draft

   *  RFCAAAA ---> the assigned numerical RFC value for

      Add this RFC number to the list of obsoleted RFCs.

   *  RFCBBBB ---> the assigned numerical RFC value for

   *  RFCCCCC ---> the assigned numerical RFC value for


   The Certificate Management Protocol (CMP) [RFCCCCC] requires a well-
   defined transfer mechanism to enable End Entities (EEs), Registration
   Authorities (RAs), and Certification Authorities (CAs) to pass
   PKIMessage sequences 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.

   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

   In addition to reliable transport, CMP requires connection and error
   handling from the transfer protocol, which is all covered by HTTP.
   Additionally, delayed delivery of CMP response messages may be
   handled at transfer level regardless of the message contents.  Since
   [RFCAAAA] 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.

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   The usage of HTTP for transferring CMP messages exclusively uses the
   POST method for requests, effectively tunneling CMP over HTTP.  While
   this is generally considered bad practice 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

   As CMP implements a transaction ID, 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

1.1.  Changes Since RFC 6712

   CMP Updates [RFCAAAA] updated RFC 6712 [RFC6712], supporting the PKI
   management operations specified in the Lightweight CMP Profile
   [RFCBBBB], in the following areas:

   *  Introduce the HTTP URI path prefix '/.well-known/cmp'.

   *  Add options for extending the URI structure with further segments
      and to this end define a new protocol registry group.

1.2.  Changes Made by This Document

   This document obsoletes RFC 6712 [RFC6712].  It includes the changes
   specified by CMP Updates [RFCAAAA] Section 3 as described in
   Section 1.1.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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3.  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.

3.1.  HTTP Versions

   Implementations MUST support HTTP/1.0 [RFC1945] and SHOULD support
   HTTP/1.1 [RFC9112].

3.2.  Persistent Connections

   HTTP persistent connections [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.

3.3.  General Form

   A DER-encoded [ITU.X690.1994] PKIMessage [RFCCCCC] is sent as the
   entity-body of an HTTP POST request.  If this HTTP request is
   successful, the server returns the CMP response in the body of the
   HTTP response.  The HTTP response status code in this case MUST be
   200; other "Successful 2xx" codes MUST NOT be used for this purpose.
   HTTP responses to pushed CMP Announcement messages (i.e., CA
   Certificate Announcement, Certificate Announcement, Revocation
   Announcement, and Certificate Revocation List (CRL) Announcement)
   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"
   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.

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3.4.  Header Fields

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

   The Content-Length header field SHOULD be provided, giving the length
   of the ASN.1-encoded PKIMessages.

3.5.  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 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.

3.6.  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 RFC 8615 [RFC8615] and the registered name 'cmp' to ease
   interworking in a multi-vendor environment.

   The CMP client needs to be configured with sufficient information to
   form the CMP server URI.  This is at least the authority portion of
   the URI, e.g., '', or the full operation path
   segment of the PKI management entity.  Additionally, OPTIONAL 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 [RFCBBBB], could indicate PKI management operations using an
   operationLabel <operation>.  A valid full CMP URI can look like this:<operation><name>

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3.7.  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 CRL, a PKI Information Request using a General Message as
   described in Appendix E.5 of [RFCCCCC] can be used.

   When pushing Announcement messages, PKIMessage structures are sent as
   the entity-body 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 PKIMessages are announcements that may be pushed by a
   CA.  The prefixed numbers reflect ASN.1 numbering of the respective

      [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 body.  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 message body.

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   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 HTTP error code
   when a problem occurs.

3.8.  HTTP Considerations

   While all defined features of the HTTP protocol are available to
   implementations, they SHOULD keep the protocol utilization as simple
   as possible.  For example, there is no benefit in using chunked
   Transfer-Encoding, as the length of an ASN.1 sequence is known when
   starting to send it.

   There is no need for the clients to send an "Expect" request-header
   field with the "100-continue" expectation and wait for a "100
   Continue" status as described in Section 8.2.3 of [RFC9112].  The CMP
   payload sent by a client is relatively small, so having extra
   messages exchanged is inefficient, as the server will only seldom
   reject a message without evaluating the body.

4.  Implementation Considerations

   Implementors should be aware that implementations might exist that
   use a different approach for transferring CMP over HTTP, because
   RFC 6712 [RFC6712] has been under development for more than a decade.
   Further, implementations based on earlier drafts of RFC 6712
   [RFC6712] might use an unregistered "application/pkixcmp-poll" MIME

5.  Security Considerations

   The following aspects need to be considered by implementers and

   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.  After sending a CMP Error Message, the server should
       close the connection, even if the CMP transaction is not yet
       fully completed.

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   2.  Without being encapsulated in effective security protocols, such
       as Transport Layer Security (TLS) [RFC5246] or [RFC8446], there
       is no integrity protection at the HTTP protocol level.
       Therefore, information from the HTTP protocol should not be used
       to change state of the transaction.

   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 man-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 technical or business critical information.
       Therefore, users of the HTTP transfer for CMP might want to
       consider using HTTP over TLS according to [RFC9110] or virtual
       private networks created, for example, by utilizing Internet
       Protocol Security according to [RFC4301].  Compliant
       implementations MUST support TLS with the option to authenticate
       both server and client.

6.  IANA Considerations

   The IANA has already registered what is specified in CMP Updates

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

7.  Acknowledgments

   The authors of this document wish to thank Tomi Kause and Martin
   Peylo, the original authors of [RFC6712], for their work.

   We also thank all reviewers of this document for their valuable

8.  References

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8.1.  Normative References

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945,
              DOI 10.17487/RFC1945, May 1996,

   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,

   [RFC9112]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <>.

              Brockhaus, H., von Oheimb, D., Ounsworth, M., and J. Gray,
              "Internet X.509 Public Key Infrastructure -- Certificate
              Management Protocol (CMP)", Work in Progress, Internet-
              Draft, draft-ietf-lamps-rfc4210bis-03, 24 October 2022,

              International Telecommunications Union, "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, 1994.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

8.2.  Informative References

              Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
              Management Protocol (CMP) Updates", Work in Progress,
              Internet-Draft, draft-ietf-lamps-cmp-updates-23, 29 June
              2022, <

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              Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
              Certificate Management Protocol (CMP) Profile", Work in
              Progress, Internet-Draft, draft-ietf-lamps-lightweight-
              cmp-profile-20, 12 January 2023,

   [RFC2510]  Adams, C. and S. Farrell, "Internet X.509 Public Key
              Infrastructure Certificate Management Protocols",
              RFC 2510, DOI 10.17487/RFC2510, March 1999,

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC6712]  Kause, T. and M. Peylo, "Internet X.509 Public Key
              Infrastructure -- HTTP Transfer for the Certificate
              Management Protocol (CMP)", RFC 6712,
              DOI 10.17487/RFC6712, September 2012,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

Appendix A.  History of Changes

   Note: This appendix will be deleted in the final version of the

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   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 ->

   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

   *  Introduced the authors of this document and thanked the authors of
      RFC6712 for their work.

   *  Added a paragraph to the introduction explaining the background of
      this document.

   *  Added the change history to this appendix.

Authors' Addresses

   Hendrik Brockhaus
   Werner-von-Siemens-Strasse 1
   80333 Munich

   David von Oheimb
   Werner-von-Siemens-Strasse 1
   80333 Munich

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   Mike Ounsworth
   1187 Park Place
   Minneapolis, MN 55379
   United States of America

   John Gray
   1187 Park Place
   Minneapolis, MN 55379
   United States of America

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