Network Working Group                                          J. Schaad
Request for Comments: 5273                       Soaring Hawk Consulting
Category: Standards Track                                       M. Myers
                                               TraceRoute Security, Inc.
                                                               June 2008

       Certificate Management over CMS (CMC): Transport Protocols

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.


   This document defines a number of transport mechanisms that are used
   to move CMC (Certificate Management over CMS (Cryptographic Message
   Syntax)) messages.  The transport mechanisms described in this
   document are HTTP, file, mail, and TCP.

1.  Overview

   This document defines a number of transport methods that are used to
   move CMC messages (defined in [CMC-STRUCT]).  The transport
   mechanisms described in this document are HTTP, file, mail, and TCP.

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

2.  File-Based Protocol

   Enrollment messages and responses may be transferred between clients
   and servers using file-system-based mechanisms, such as when
   enrollment is performed for an off-line client.  When files are used
   to transport binary, Full PKI Request or Full PKI Response messages,
   there MUST be only one instance of a request or response message in a
   single file.  The following file type extensions SHOULD be used:

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RFC 5273                CMC: Transport Protocols               June 2008

                 | Message Type        | File Extension |
                 | Simple PKI Request  | .p10           |
                 | Full PKI Request    | .crq           |
                 | Simple PKI Response | .p7c           |
                 | Full PKI Response   | .crp           |

                 File PKI Request/Response Identification

3.  Mail-Based Protocol

   MIME wrapping is defined for those environments that are MIME native.
   The basic mime wrapping in this section is taken from [SMIMEV3].
   When using a mail-based protocol, MIME wrapping between the layers of
   CMS wrapping is optional.  Note that this is different from the
   standard S/MIME (Secure MIME) message.

   Simple enrollment requests are encoded using the "application/pkcs10"
   content type.  A file name MUST be included either in a content-type
   or a content-disposition statement.  The extension for the file MUST
   be ".p10".

   Simple enrollment response messages MUST be encoded as content type
   "application/pkcs7-mime".  An smime-type parameter MUST be on the
   content-type statement with a value of "certs-only".  A file name
   with the ".p7c" extension MUST be specified as part of the content-
   type or content-disposition statement.

   Full enrollment request messages MUST be encoded as content type
   "application/pkcs7-mime".  The smime-type parameter MUST be included
   with a value of "CMC-Request".  A file name with the ".p7m" extension
   MUST be specified as part of the content-type or content-disposition

   Full enrollment response messages MUST be encoded as content type
   "application/pkcs7-mime".  The smime-type parameter MUST be included
   with a value of "CMC-response".  A file name with the ".p7m"
   extension MUST be specified as part of the content-type or content-
   disposition statement.

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RFC 5273                CMC: Transport Protocols               June 2008

   | Item         | MIME Type              | File       | SMIME Type   |
   |              |                        | Extension  |              |
   | Simple PKI   | application/pkcs10     | .p10       | N/A          |
   | Request      |                        |            |              |
   | Full PKI     | application/pkcs7-mime | .p7m       | CMC-request  |
   | Request      |                        |            |              |
   | Simple PKI   | application/pkcs7-mime | .p7c       | certs-only   |
   | Response     |                        |            |              |
   | Full PKI     | application/pkcs7-mime | .p7m       | CMC-response |
   | Response     |                        |            |              |

             Table 1: MIME PKI Request/Response Identification

4.  HTTP/HTTPS-Based Protocol

   This section describes the conventions for use of HTTP [HTTP] as a
   transport layer.  In most circumstances, the use of HTTP over TLS
   [TLS] provides any necessary content protection from eavesdroppers.

   In order for CMC clients and servers using HTTP to interoperate, the
   following rules apply.

      Clients MUST use the POST method to submit their requests.

      Servers MUST use the 200 response code for successful responses.

      Clients MAY attempt to send HTTP requests using TLS 1.0 [TLS] or
      later, although servers are not required to support TLS.

      Servers MUST NOT assume client support for any type of HTTP
      authentication such as cookies, Basic authentication, or Digest

      Clients and servers are expected to follow the other rules and
      restrictions in [HTTP].  Note that some of those rules are for
      HTTP methods other than POST; clearly, only the rules that apply
      to POST are relevant for this specification.

4.1.  PKI Request

   A PKI Request using the POST method is constructed as follows:

   The Content-Type header MUST have the appropriate value from Table 1.

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RFC 5273                CMC: Transport Protocols               June 2008

   The body of the message is the binary value of the encoding of the
   PKI Request.

4.2.  PKI Response

   An HTTP-based PKI Response is composed of the appropriate HTTP
   headers, followed by the binary value of the BER (Basic Encoding
   Rules) encoding of either a Simple or Full PKI Response.

   The Content-Type header MUST have the appropriate value from Table 1.

5.  TCP-Based Protocol

   When CMC messages are sent over a TCP-based connection, no wrapping
   is required of the message.  Messages are sent in their binary
   encoded form.

   The client closes a connection after receiving a response, or it
   issues another request to the server using the same connection.
   Reusing one connection for multiple successive requests, instead of
   opening multiple connections that are only used for a single request,
   is RECOMMENDED for performance and resource conservation reasons.  A
   server MAY close a connection after it has been idle for some period
   of time; this timeout would typically be several minutes long.

   There is no specific port that is to be used when doing TCP-based
   transport.  Only the Private Ports 49152-65535 may be used in this
   manner (without registration).  The ports in the range of 1-49151
   SHOULD NOT be used.  The port to be used is configured out of band.

6.   Security Considerations

   Mechanisms for thwarting replay attacks may be required in particular
   implementations of this protocol depending on the operational
   environment.  In cases where the Certification Authority (CA)
   maintains significant state information, replay attacks may be
   detectable without the inclusion of the optional nonce mechanisms.
   Implementers of this protocol need to carefully consider
   environmental conditions before choosing whether or not to implement
   the senderNonce and recipientNonce attributes described in Section
   6.6 of [CMC-STRUCT].  Developers of state-constrained PKI clients are
   strongly encouraged to incorporate the use of these attributes.

   Initiation of a secure communications channel between an end-entity
   and a CA or Registration Authority (RA) -- and, similarly, between an
   RA and another RA or CA -- necessarily requires an out-of-band trust
   initiation mechanism.  For example, a secure channel may be
   constructed between the end-entity and the CA via IPsec [IPsec] or

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RFC 5273                CMC: Transport Protocols               June 2008

   TLS [TLS].  Many such schemes exist, and the choice of any particular
   scheme for trust initiation is outside the scope of this document.
   Implementers of this protocol are strongly encouraged to consider
   generally accepted principles of secure key management when
   integrating this capability within an overall security architecture.

   In some instances, no prior out-of-band trust will have been
   initiated prior to use of this protocol.  This can occur when the
   protocol itself is being used to download onto the system the set of
   trust anchors to be used for these protocols.  In these instances,
   the Enveloped Data content type (Section in [CMC-STRUCT])
   must be used to provide the same shrouding that TLS would have

7.  Acknowledgments

   The authors and the PKIX Working Group are grateful for the
   participation of Xiaoyi Liu and Jeff Weinstein in helping to author
   the original versions of this document.

   The authors would like to thank Brian LaMacchia for his work in
   developing and writing up many of the concepts presented in this
   document.  The authors would also like to thank Alex Deacon and Barb
   Fox for their contributions.

8.  References

8.1.  Normative References

   [CMC-STRUCT]  Schaad, J. and M. Myers, "Certificate Management over
                 CMS (CMC)", RFC 5272, June 2008.

   [HTTP]        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.

   [IPsec]       Kent, S. and K. Seo, "Security Architecture for the
                 Internet Protocol", RFC 4301, December 2005.

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

   [SMIMEV3]     Ramsdell, B., "Secure/Multipurpose Internet Mail
                 Extensions (S/MIME) Version 3.1 Message Specification",
                 RFC 3851, July 2004.

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RFC 5273                CMC: Transport Protocols               June 2008

8.2.  Informative References

   [TLS]         Dierks, T. and E. Rescorla, "The Transport Layer
                 Security (TLS) Protocol Version 1.1", RFC 4346,
                 April 2006.

Authors' Addresses

   Jim Schaad
   Soaring Hawk Consulting
   PO Box 675
   Gold Bar, WA  98251

   Phone: (425) 785-1031

   Michael Myers
   TraceRoute Security, Inc.


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RFC 5273                CMC: Transport Protocols               June 2008

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