Internet X.509 Public Key Infrastructure -- HTTP Transfer for the Certificate Management Protocol (CMP)
RFC 6712

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)

Abstract

   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
   http://www.rfc-editor.org/info/rfc6712.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.
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   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
   handling.

   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
   client.

   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
   messages.

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 "example.com", would be

      POST /cmp HTTP/1.1
      Host: example.com

   or in the absoluteURI form

      POST http://example.com/cmp/ HTTP/1.1
      Host: example.com

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
   element.

      [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: amit@trustpoint.com

      Ronald Tschalaer
      Certicom
      25801 Industrial Blvd
      Hayward, CA
      US
      Email: ronald@trustpoint.com

   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: toka@ssh.com

   Martin Peylo
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   EMail: martin.peylo@nsn.com

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