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RFC 6712

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

Pages: 10
Proposed Standard
Updates:  42109480

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Internet Engineering Task Force (IETF)                          T. Kause
Request for Comments: 6712                                           SSH
Updates: 4210                                                   M. Peylo
Category: Standards Track                                            NSN
ISSN: 2070-1721                                           September 2012

       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 is the "CMPtrans" document referenced in RFC 4210; therefore, this document updates the reference given therein. 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 5741. 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) 2012 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

1. Introduction ....................................................2 2. Conventions Used in This Document ...............................3 3. HTTP-Based Protocol .............................................3 3.1. HTTP Versions ..............................................4 3.2. Persistent Connections .....................................4 3.3. General Form ...............................................4 3.4. Media Type .................................................4 3.5. Communication Workflow .....................................5 3.6. HTTP Request-URI ...........................................5 3.7. Pushing of Announcements ...................................5 3.8. HTTP Considerations ........................................6 4. Implementation Considerations ...................................7 5. Security Considerations .........................................7 6. IANA Considerations .............................................8 7. Acknowledgments .................................................8 8. References ......................................................9 8.1. Normative References .......................................9 8.2. Informative References .....................................9

1. Introduction

The Certificate Management Protocol (CMP) [RFC4210] 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.
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   The current 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

   Before this document was published as an RFC, the draft version
   underwent drastic changes during the long-lasting work process.  The
   so-called "Direct TCP-Based Management Protocol" specified in
   [RFC2510] was enhanced, and at some point a version existed where
   this protocol was again transferred over HTTP.  As both approaches
   proved to be needless and cumbersome, implementers preferred to use
   plain HTTP transfer following [RFC1945] or [RFC2616].  This document
   now reflects that by exclusively describing HTTP as the transfer
   protocol for CMP.

   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

2. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

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.
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3.1. HTTP Versions

Implementations MUST support HTTP/1.0 [RFC1945] and SHOULD support HTTP/1.1 [RFC2616].

3.2. Persistent Connections

HTTP persistent connections [RFC2616] 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 [RFC4210] 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.

3.4. Media Type

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

The Request-URI is formed as specified in [RFC3986]. A server implementation MUST handle Request-URI paths, with or without a trailing slash, as identical. An example of a Request-Line and a Host header field in an HTTP/1.1 header, sending a CMP request to a server, located in the "/cmp" path of the host "", would be POST /cmp HTTP/1.1 Host: or in the absoluteURI form POST HTTP/1.1 Host:

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 [RFC4210] can be used.
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   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.

   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.
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   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 [RFC2616].  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 this document has been under development for more than a decade. Further, implementations based on earlier drafts of this document might use an unregistered "application/pkixcmp-poll" MIME type.

5. Security Considerations

The following aspects 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. After sending a CMP Error Message, the server should close the connection, even if the CMP transaction is not yet fully completed. 2. Without being encapsulated in effective security protocols, such as Transport Layer Security (TLS) [RFC5246], 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.
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   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 [RFC2818] 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 the MIME media type "application/ pkixcmp" for identifying CMP sequences due to an request made in connection with [RFC2510]. No further action by the IANA is necessary for this document or any anticipated updates.

7. Acknowledgments

Amit Kapoor and Ronald Tschlaer were the original authors of this document, and their version focused on the so-called "TCP-Based Management Protocol", which has been removed from this document. Their contact data, as originally stated by them, is as follows: Amit Kapoor Certicom 25801 Industrial Blvd Hayward, CA US Email: Ronald Tschalaer Certicom 25801 Industrial Blvd Hayward, CA US Email: The authors gratefully acknowledge the contributions of various members of the IETF PKIX working group and the ICSA CA-talk mailing list (a list solely devoted to discussing CMP interoperability efforts).
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   By providing ideas, giving hints, and doing invaluable review work,
   the following alphabetically listed individuals have significantly
   contributed to this document:

      Tomas Gustavsson, Primekey
      Peter Gutmann, University of Auckland
      Wolf-Dietrich Moeller, Nokia Siemens Networks

8. References

8.1. Normative References

[ITU.X690.1994] 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. [RFC1945] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2510] Adams, C. and S. Farrell, "Internet X.509 Public Key Infrastructure Certificate Management Protocols", RFC 2510, March 1999. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)", RFC 4210, September 2005.

8.2. Informative References

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005.
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   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

Authors' Addresses

Tomi Kause SSH Communications Security Takomotie 8 Helsinki 00380 Finland EMail: Martin Peylo Nokia Siemens Networks Linnoitustie 6 Espoo 02600 Finland EMail: