Internet Draft
PKIX Working Group                              Amit Kapoor (Certicom)
Expires in 6 months                         Ronald Tschal„r (Certicom)

                                                      November 24 2000

                        Transport Protocols for CMP

Status of this Memo

  This document is an Internet-Draft and is in full conformance with
  all provisions of Section 10 of RFC2026.

  Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

  Copyright (C) The Internet Society (2000). All Rights Reserved.


  This document describes how to layer Certificate Management
  Protocols [CMP] over various transport protocols.

  The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document (in
  uppercase, as shown) are to be interpreted as described in

1. Motivation

  Section 5 of the [RFC2510] spec specifies how to exchange CMP
  messages over various transports. However, implementors, during
  various interoperability workshops, found the protocol lacking in
  the following respects:

  1. For CMP-over-TCP (section 5.2):
      A. No clear definition on when the connection is to be
         closed and by whom.
      B. No version number specified to allow for extensions.
      C. Error messages cannot be processed by applications.

  2. For CMP-over-HTTP (section 5.4):
      A. No support for polling
      B. No specification of what to do in the case where there
         is no CMP response message (e.g. after a pkiConf)

  Realizing that this could not be achieved in a backward compatible
  way, and acknowledging the changes being made to [RFC2510], the
  decision was made to enhance the protocol now to avoid
  interoperability conflicts later and to pull the transport section
  out in a separate draft. This enhancement tries to keep as much of
  the older protocol as possible, while ensuring that implementations
  using the old protocol will not mistake a new message for a valid
  message in the [RFC2510] format.

  For CMP-over-HTTP a new content type is specified which carries a
  TCP-message instead of a plain DER-encoded PKIMessage.

2. TCP-Based Management Protocol

   While this section is called TCP-Based and the messages are called
   TCP-message's, the same protocol can be used over any reliable,
   connection oriented transport protocol (e.g. SNA, DECnet, etc.).
   This protocol is suitable for cases where an end entity (or an RA)
   initiates a transaction and can poll to pick up the results.

   The client sends a TCP-message to the server, and the server
   responds with another TCP-message. Note that a response MUST be
   sent for every request, even if the encapsulated CMP message in the
   request does not have a corresponding response.

   The protocol basically assumes a listener process on an RA or CA
   which can accept TCP-messages on a well-defined port (default port
   number is 829). Typically a client initiates connection to the
   server and submits a PKI message. The server replies with a PKI
   message or with a reference number to be used later when polling
   for the actual PKI message response.

   If a polling-reference was supplied then the client will send a
   polling request using this polling-reference after waiting for at
   least the specified time. The server may again reply with a
   polling-reference or with the actual PKI message response.

   When the final PKI response message has been picked up by the
   client then no new polling reference is supplied.

   If a transaction is initiated by a PKI entity (RA or CA) then an
   end entity must either supply a listener process or be supplied
   with a polling reference (see below) in order to allow it to pick
   up the PKI message from the PKI management component.

2.1 General Form

   A TCP-message consists of:

         length (32-bits)
         version (8-bits)
         flags (variable length)
         message-type (8-bits),
         value (defined below)

   The length field contains the number of octets of the remainder of
   the TCP-message (i.e., number of octets of <value> plus <flags-length>
   plus 2).  All bit values in this protocol are specified to be in
   network byte order.

   The version field indicates the version of the TCP-message. It MUST
   be incremented for each specification which changes the flags field
   in a way that is not fully backwards compatible with the previous
   version (e.g. when the length of the flags field is changed).

   The flags field is for transporting TCP-message specific data. The
   length of this field is version dependent and is fixed for a given

   The message-type field is used to indicate the type of TCP-message.

   The value field contains message-type dependent data.

2.2 Version Negotiation

   If a client knows the protocol version(s) supported by the
   server (e.g. from a previous TCP-message exchange or via some
   out-of-band means) then it SHOULD send a TCP-message with the highest
   version supported both by it and the server. If a client does
   not know what version(s) the server supports then it SHOULD send
   a TCP-message using the highest version it supports.

   If a server receives a TCP-message version that it supports, then it
   MUST reply with a TCP-message of the same version. If the version
   received is higher than what the server supports, it MUST send
   back a VersionNotSupported errorMsgRep (defined below) containing
   the highest version it supports.

2.3 TCP-message Version 10

   The TCP-message version will be 10 for this document. The number
   has deliberately been chosen to prevent [RFC2510] compliant
   applications from treating it as a valid message type. Applications
   receiving a version less than 10 SHOULD interpret the message as
   being an [RFC2510] style message.

   The length of the flags field for this version is 1 octet. The LSB
   is used to indicate a connection close; all other bits in the flags
   octet MUST be ignored by receivers, and MUST be set to zero by

   By default connections are kept open after the receipt of a
   response. Either party (client or server) MAY set the connection
   close bit at any time.  If the connection close bit is set on a
   request, then the server MUST set the bit in the response and
   close the connection after sending the response. If the bit is set
   on a response from the server, the client MUST NOT send any further
   requests on that connection. Applications MAY decide to close an
   idle connection (one on which no response is outstanding) after
   some time-out. Because of the problem where a client sends a
   request and the server closes the connection while the request is
   still in flight, clients SHOULD automatically retry a request for
   which no part of the response could be read due to a connection
   close or reset.

   If the connection is kept open, it MUST only be used for subsequent
   request/response transactions started by the client - the server
   MUST NOT use it to send requests to the client. Different
   transactions may be freely interwoven on the same connection. E.g.
   a CR/CP need not immediately be followed by the Confirm, but may be
   followed by any other request from a different transaction.

2.4 Detecting and Interoperating with RFC-2510 Conformant Implementations

   Servers wishing to interoperate with clients conforming to
   [RFC2510] can do so by treating any received message with a version
   less than 10 as an [RFC2510] message and responding in that format.
   Servers not wishing to support [RFC2510] messages MUST respond with
   a [RFC2510] errorMsgRep.

   Clients wishing to interoperate with [RFC2510] compliant servers
   SHOULD treat a response with a version less than 10 as an [RFC2510]
   style message. If this message is an errorMsgRep (message-type 06)
   then the client MAY automatically retry the request using the
   [RFC2510] format; if the message is not an errorMsgRep or the
   implementation does not wish to support [RFC2510] then it MUST
   abort the corresponding CMP transaction.

2.5 Message Types

   message-types 0-127 are reserved and will be issued under IANA
   auspices. message-types 128-255 are reserved for application use.

   The message-type's currently defined are:

    Message name            Message-type

    pkiReq                  '00'H

    pollRep                 '01'H

    pollReq                 '02'H

    finRep                  '03'H

    pkiRep                  '05'H

    errorMsgRep             '06'H

   If server receives an unknown message-type then it MUST reply with
   an InvalidMessageType errorMsgRep. If a client receives an unknown
   message-type then it MUST abort the CMP transaction.

   The different TCP-messages are discussed in the following sections:

2.5.1 pkiReq

   The pkiReq is to be used to carry a PKIMessage from the client to the
   server.  The <value> portion of this TCP-message will contain:

           DER-encoded PKIMessage.

   The type of PKIMessages that can be carried by this TCP-message are:

           CRL Announcement
           Certificate Confirmation
           Poll Request
           Subscription Request
           CA Key Update Announcement
           Certificate Announcement
           Certification Request
           Cross-Certification Request
           Error Message
           General Message
           Initialization Request
           Key Recovery Request
           Key Update Request
           Nested Message
           PKCS-10 Request
           POP Response
           Revocation Request

2.5.2 pkiRep

   This TCP-message is to be used to send back the response to the request.
   The <value> portion of the pkiRep will contain:

           DER encoded PKI message

   The type of PKIMessages that can be carried by this TCP-message are:

           Poll Response
           Subscription Response
           Certification Response
           Error Message
           General Response
           Initialization Response
           Key Recovery Response
           Key Update Response
           POP Challenge
           Revocation Response

2.5.3 pollReq

   The pollReq will be the used by the client to check the status of a
   pending TCP-message.  The <value> portion of the pollReq will

           polling-reference (32 bits)

   The <polling-reference> MUST be the one returned via the pollRep

2.5.4 pollRep

   The pollRep will be the response sent by the server to the client
   when there are no TCP-message response ready.  The <value> portion of
   the pollRep will contain:

           polling-reference (32 bits)
           time-to-check-back (32 bits)

   The <polling-reference> is a unique 32-bit number sent by the server.
   The <time-to-check-back> is the time in seconds indicating the minimum
   interval after which the client SHOULD check the status again.

   The duration for which the server keeps the <polling-reference>
   unique is left to the implementation.

2.5.5 finRep

    finRep is sent by the server whenever no other response applies
    (such as after receiving a CMP pkiConf), and usually indicates
    the end of the CMP transaction. The <value> portion of the finRep
    SHALL contain:

           '00'H (8 bits)

2.5.6 errorMsgRep

   This TCP-message is sent when a TCP-message level protocol error is
   detected. Please note that PKIError messages MUST NOT be sent
   using this. Examples of TCP-message level errors are:

   1.  Invalid protocol version
   2.  Invalid TCP message-type
   3.  Invalid polling reference number

   The <value> field of the TCP-message SHALL contain:

            error-type (16-bits)
            data-length (16-bits)
            data (<data-length> octets)
            UTF8 String (SHOULD include a RFC 1766 language tag)

   The <error-type> is of the form MMNN where M and N are hex digits
   (0-F) and MM represents the major category and NN the minor. The
   major categories defined by this specification are:

     '01'H   TCP-message version negotiation
     '02'H   client errors
     '03'H   server errors

   The major categories '80'H-'FF'H are reserved for application use.

   The <data-length> and <data> are additional information about the
   error to be used by programs for further processing and recovery.
   <data-length> contains the length of the <data> field in number of
   octets. Error messages not needing additional information to be
   conveyed MUST set the <data-length> to 0.

   The UTF8 text string is for user readable error messages. Note that
   it does not contain a terminating null character at the end. VersionNotSupported errorMsgRep

   The VersionNotSupported errorMsgRep is defined as follows:

      error-type:                        '0101'H
      data-length:                             1
      data:                            <version>
      UTF8-text String:   implementation defined

   where <version> is the highest version the server supports. GeneralClientError errorMsgRep

   The GeneralClientError errorMsgRep is defined as follows:

      error-type:                        '0200'H
      data-length:                             0
      data:                              <empty>
      UTF8-text String:   implementation defined InvalidMessageType errorMsgRep

   The InvalidMessageType errorMsgRep is defined as follows:

      error-type:                        '0201'H
      data-length:                             1
      data:                       <message-type>
      UTF8-text String:   implementation defined

   where <message-type> is the message-type received by the
   server. InvalidPollID errorMsgRep

   The InvalidPollID errorMsgRep is defined as follows:

      error-type:                        '0202'H
      data-length:                             4
      data:                  <polling-reference>
      UTF8-text String:   implementation defined

   where <polling-reference> is the polling-reference received by
   the server. GeneralServerError errorMsgRep

   The GeneralServerError errorMsgRep is defined as follows:

      error-type:                        '0300'H
      data-length:                             0
      data:                              <empty>
      UTF8-text String:   implementation defined

3. HTTP-Based Management Protocol

  A client creates a TCP-message, as specified in section 2.0. The
  message is then sent as the entity-body of an HTTP POST request. If
  the HTTP request is successful then the server returns a similar
  message in the body of the response. The response status code in
  this case MUST be 200; other 2xx codes MUST NOT be used. The content
  type of the request and response MUST be "application/pkixcmp-poll".
  Applications MAY wish to also recognized and use the
  "application/x-pkixcmp-poll" MIME type (specified in earlier
  versions of this document) in order to support backward
  compatibility wherever applicable. Content codings may be applied.

  Note that a server may return any 1xx, 3xx, 4xx, or 5xx code if the
  HTTP request needs further handling or is otherwise not acceptable.

  Because in general CMP messages are not cacheable, requests and
  responses should include a "Cache-Control: no-cache" (and, if either
  side uses HTTP/1.0, a "Pragma: no-cache") to prevent the client from
  getting cached responses. This is especially important for polling
  requests and responses.

  Connection management SHOULD be based on the HTTP provided mechanisms
  (Connection and Proxy-Connection header fields) and not on the
  connection flag carried in the TCP-message.

4. File based protocol

   A file containing a PKI message MUST contain only the DER encoding of
   one PKI message, i.e., there MUST be no extraneous header or trailer
   information in the file.

   Such files can be used to transport PKI messages using, e.g., FTP.

5. Mail based protocol

   This subsection specifies a means for conveying ASN.1-encoded
   messages for the protocol exchanges via Internet mail.

   A simple MIME object is specified as follows.

      Content-Type: application/pkixcmp
      Content-Transfer-Encoding: base64

      <<the ASN.1 DER-encoded PKIX-CMP message, base64-encoded>>

   This MIME object can be sent and received using common MIME
   processing engines and provides a simple Internet mail transport for
   PKIX-CMP messages.  Implementations MAY wish to also recognize and
   use the "application/x-pkixcmp" MIME type (specified in earlier
   versions of this document) in order to support backward compatibility
   wherever applicable.

6. Security Considerations

  Three aspects need to be considered by server side implementors:

  1.  There is no security at the TCP and HTTP protocol level (unless
      tunneled via SSL/TLS) and thus TCP-message should not be used
      to change state of the transaction.  Change of state should be
      done on the signed PKIMessage being carried within the

  2.  If the server is going to be sending messages with sensitive
      information (not meant for public consumption) in the clear, it
      is RECOMMENDED that the server send back the message directly
      and not use the pollRep.

  3.  The polling request/response mechanism can be used for all kinds
      of denial of service attacks.  It is RECOMMENDED that the server
      not change the polling-reference between polling requests.

7. Acknowledgments

   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

8. References

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

  [CMP]     Adams, C., Farrell, S., "Internet X.509 Public Key
            Infrastructure, Certificate Management Protocols",
            draft-ietf-pkix-rfc2510bis-01.txt,  July 2000

  [HTTP]    Fielding, R.T., et. al, "Hypertext Transfer Protocol --
            HTTP/1.1", RFC 2616, June 1999.

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

  [RFC821]  Postel, J., "Simple Mail Transfer Protocol", RFC 821,
            August 1982.

Authors' Addresses

  Amit Kapoor
  25801 Industrial Blvd
  Hayward, CA 94545


  Ronald Tschal„r
  25801 Industrial Blvd
  Hayward, CA 94545


Appendix A: Registration of MIME Type for Section 3

   Subject: Registration of MIME media type application/pkixcmp-poll

   MIME media type name: application

   MIME subtype name: pkixcmp-poll

   Required parameters: none

   Optional parameters: none

   Encoding considerations:
   Content may contain arbitrary octet values (the ASN.1 DER encoding of
   a PKI message, 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): ".cmp"
     Macintosh File Type Code (s): -

   Person and email address to contact for further information:
   Carlisle Adams,

   Intended usage: COMMON

   Author/Change controller: Carlisle Adams

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