Network Working Group                                           T. Kause
Internet-Draft                                                    Tectia
Updates: 4210 (if approved)                                     M. Peylo
Intended status: Standards Track                                     NSN
Expires: January 1, 2012                                   June 30, 2011

   Internet X.509 Public Key Infrastructure -- HTTP Transport for CMP


   This document describes how to layer the Certificate Management
   Protocol over HTTP.  It is the "CMPtrans" document referenced in RFC
   4210 and therefore updates the reference given therein.

Status of this Memo

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   This Internet-Draft will expire on January 1, 2012.

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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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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  HTTP-Based Protocol  . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  HTTP Versions  . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Persistent Connections . . . . . . . . . . . . . . . . . .  5
     3.3.  General Form . . . . . . . . . . . . . . . . . . . . . . .  5
     3.4.  Media Type . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.5.  Communication Workflow . . . . . . . . . . . . . . . . . .  6
     3.6.  HTTP Request-URI . . . . . . . . . . . . . . . . . . . . .  6
     3.7.  Pushing of Announcements . . . . . . . . . . . . . . . . .  7
     3.8.  HTTP Considerations  . . . . . . . . . . . . . . . . . . .  8
   4.  Compatibility Issues with Legacy Implementations . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  Information Security Considerations  . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 14
   Appendix B.  Registration of the application/pkixcmp Media Type  . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17

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1.  Introduction

   The Certificate Management Protocol (CMP) [RFC4210] requires well
   defined transport mechanisms to enable End Entities, RAs and CAs to
   pass PKIMessage sequences between them.  This document defines the
   transport mechanisms which were removed from the main CMP
   specification with the second release and referred to be in a
   separate document.

   The first version of the CMP specification [RFC2510] included a brief
   description of a simple transport protocol layer on top of TCP.  Its
   features was simple transport 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 transport, but those were not specified in detail.

   The current version of the CMP specification incorporated an own
   polling mechanism and thus the need for a transport protocol
   providing this functionality vanished.  The remaining features CMP
   requires from its transport protocols are connection- and error-

   During the long time it existed as draft, this RFC was undergoing
   drastic changes.  The TCP-based transport protocol was enhanced and a
   TCP-Messages-over-HTTP transport specification appeared.  As both
   proved to be needless and cumbersome, implementers preferred to use
   plain HTTP transport.  This document now reflects that by exclusively
   describing HTTP as transport protocol for CMP.

   HTTP transport is generally easy to implement, traverses network
   borders utilizing ubiquitous proxies and is already commonly found in
   existing implementations.

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2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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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.

   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 transport protocol for CMP

3.1.  HTTP Versions

   Either HTTP/1.0 as described in [RFC1945] or HTTP/1.1 as in [RFC2616]
   MAY be used.  Both server and client SHOULD be able to interact with
   counterparts utilizing either HTTP protocol version.

3.2.  Persistent Connections

   HTTP permits to reuse a connection for subsequent requests.
   Implementations may use this functionality for messages within the
   same transaction but MUST NOT rely on that, as e.g. intermediate HTTP
   proxies might terminate the connection after each request/response

3.3.  General Form

   An ASN.1 DER-encoded PKIMessage is sent as the entity-body of an HTTP
   POST request.  If this HTTP request is successful, the server returns
   the CMP reply in the body of the HTTP response.  The response status
   code in this case MUST be 200; other 2xx codes MUST NOT be used for
   this purpose.  HTTP responses with empty message body to pushed CMP
   Announcement messages utilize the status codes 201, 202 and 204 to
   identify whether the received information was processed.

   All applicable 4xx or 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
   header when conveying a PKIMessage.

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3.5.  Communication Workflow

   In CMP most communication is initiated by the end entities 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 transported CMP message
   was not successfully delivered to its destination.

3.6.  HTTP Request-URI

   The Request-URI is formed as specified in [RFC3986].

   Client requests containing a PKI message MUST be directed to an
   Request-URI depicting a directory.  A server implementation MUST
   handle Request-URIs with or without a trailing slash as identical.
   The following list contains all such CMP message types.  The prefixed
   numbers reflect the ASN.1 numbering of the respective element.

      [0] Initialization Request
      [2] Certification Request
      [4] PKCS-10 Request
      [6] pop Response
      [7] Key Update Request
      [9] Key Recovery Request
      [11] Revocation Request
      [13] Cross-Certification Request
      [15] CA Key Update Announcement
      [16] Certificate Announcement
      [17] Revocation Announcement
      [18] CRL Announcement
      [20] Nested Message
      [21] General Message
      [23] Error Message
      [24] Certificate Confirmation
      [25] Polling Request

   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"
   directory of the host, would be

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      POST /cmp HTTP/1.1

   or in the absoluteURI form

      POST HTTP/1.1

   A CMP server may be logically located either inside the root- or
   within subdirectories of an HTTP server domain.

3.7.  Pushing of Announcements

   A CMP server may create event-triggered announcements or generate
   them on a regular basis.  It MAY also utilize HTTP transport to
   convey them to a suitable recipient.  No request message for is
   specified, so 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 E.5 of [RFC4210] can be used.

   When pushing Annocements, ASN.1 encoded structures are sent as the
   entity-body of an HTTP POST request.

   Suitable recipients for CMP announcements might e.g. be repositories
   storing the announced information such as directory services.  Those
   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 a HTTP response having an
   appropriate status code and an empty body.  When not receiving such
   response it MUST be assumed that the delivery was not successful and
   if applicable the sending side may retry sending the Announcement
   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" or "204 No Content" status code and 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 server may then try
   to send the Announcement again and may repeat this until it receives
   a confirmation that it had been successfully stored.  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

   In general CMP messages are not cachable.  HTTP headers of requests
   and responses MUST include "Cache-Control: no-cache" and, in case of
   HTTP/1.0, additionally "Pragma: no-cache" to protect the client from
   getting cached responses.

   Connection management is based on the HTTP provided mechanisms
   ("Connection" and "Proxy-Connection" header fields).

   While an implementation MAY make use of all defined features of the
   HTTP protocol, it SHOULD keep the protocol utilization as simple as

   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 chapter 8.2.3 of [RFC2616].  The CMP
   payload sent by a client is relatively small, so having extra
   messages exchanged is more inefficient as the server will anyway only
   seldomly reject a message without evaluating the body.

   Content codings MAY be applied.

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4.  Compatibility Issues with Legacy Implementations

   As this document was subject of multiple changes during the long
   period of time it was created in, implementations using a different
   approach for HTTP transport may exist.  While only those
   implementations according to this specification are compliant,
   implementers should to be aware that there might be existing ones
   which behave differently.

   Legacy implementations might also use an unregistered "application/
   pkixcmp-poll" MIME type as it was specified in earlier drafts of this
   document.  Here, the entity-body of an HTTP POST request contains the
   DER-encoded PKIMessage prefixed by an additional "TCP-Messaging"
   protocol.  TCP-Messaging was described in draft versions of this
   document but was removed.

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5.  Security Considerations

   The following aspects need to be considered by server side

   1.  There is the risk for denial of service attacks through resource
       consumption by opening many connections.  Therefore idle
       connections should be terminated after an appropriate timeout,
       maybe also depending 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

   2.  There is no security at the HTTP protocol level (unless tunneled
       via TLS) and thus information from the HTTP protocol SHOULD NOT
       be used to change state of the transaction.  Change of state
       SHOULD be triggered by signed PKIMessages only.  Likewise, the
       clients SHOULD NOT support the 301 HTTP status code which could
       otherwise be used by an man-in-the-middle attacker to barr them
       permanently from contacting the correct server.

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6.  Information Security Considerations

   CMP provides inbuilt integrity protection and authentication.  Due to
   the nature of a PKI, from a security perspective the information
   communicated unencrypted does not contain sensitive information.

   However, it might be possible for an interceptor to utilize the
   available information to gather confidential technical or business
   critical information.  Therefore users of the HTTP CMP transport
   might want to use HTTP over TLS according to [RFC2818] or should
   consider to use virtual private networks created e.g. by utilizing
   Internet Protocol Security according to [RFC4301].

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7.  IANA Considerations

   The IANA has already registered TCP and UDP port 829 for "PKIX-3
   CA/RA" and the MIME media type "application/pkixcmp" for identifying
   CMP sequences.

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

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8.  References

8.1.  Normative References

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

   [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.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5646]  Phillips, A. and M. Davis, "Tags for Identifying
              Languages", BCP 47, RFC 5646, September 2009.

8.2.  Informative References

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

   [RFC2482]  Whistler, K. and G. Adams, "Language Tagging in Unicode
              Plain Text", RFC 2482, January 1999.

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

   [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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Appendix A.  Acknowledgments

   Until the fifth draft version of this document, released in November
   24th 2000, the sole authors were Amit Kapoor and Ronald Tschlaer from
   Certicom.  Up to this point the now removed TCP-Based transport was
   described in detail.  They are not available for this working on this
   document anymore at the time it is entering the "Authors Final Review
   state AUTH48".  As they therefore cannot approve this document as it
   would be necessary, their names were moved to this section.  Their
   contact data, as originally stated by them, is as follows:

      Amit Kapoor
      25801 Industrial Blvd
      Hayward, CA

      Ronald Tschalaer
      25801 Industrial Blvd
      Hayward, CA

   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

   By providing ideas, giving hints and doing invaluable review work,
   the following individuals, listed alphabetically, have significantly
   contributed to this document:

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

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Appendix B.  Registration of the application/pkixcmp Media Type

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   Subject: Registration of MIME media type application/pkixcmp

   MIME media type name: application

   MIME subtype name: pkixcmp

   Required parameters: -

   Optional parameters: -

   Encoding considerations:

   Content may contain arbitrary octet values (the ASN.1 DER encoding
   of a PKIMessage sequence, as defined in the IETF PKIX Working Group
   specifications).  base64 encoding is required for MIME e-mail; no
   encoding is necessary for HTTP.

   Security considerations:

   This MIME type may be used to transport Public-Key Infrastructure
   (PKI) messages between PKI entities.  These messages are defined by
   the IETF PKIX Working Group and are used to establish and maintain
   an Internet X.509 PKI.  There is no requirement for specific
   security mechanisms to be applied at this level if the PKI messages
   themselves are protected as defined in the PKIX specifications.

   Interoperability considerations: -

   Published specification: this document

   Applications which use this media type: Applications using
   certificate management, operational, or ancillary protocols (as
   defined by the IETF PKIX Working Group) to send PKI messages via
   E-Mail or HTTP.

   Additional information:

     Magic number (s): -
     File extension (s): ".PKI"
     Macintosh File Type Code (s): -

   Person and email address to contact for further information:
   Martin Peylo,

   Intended usage: COMMON

   Author/Change controller: Martin Peylo

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Authors' Addresses

   Tomi Kause
   Tectia Corporation
   Fredrikinkatu 42
   Helsinki  00100


   Martin Peylo
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600


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