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<pre>Network Working Group J. Schaad
Request for Comments: 5273 Soaring Hawk Consulting
Category: Standards Track M. Myers
TraceRoute Security, Inc.
June 2008
<span class="h1">Certificate Management over CMS (CMC): Transport Protocols</span>
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.
Abstract
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.
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Overview</span>
This document defines a number of transport methods that are used to
move CMC messages (defined in [<a href="#ref-CMC-STRUCT" title=""Certificate Management over CMS (CMC)"">CMC-STRUCT</a>]). The transport
mechanisms described in this document are HTTP, file, mail, and TCP.
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 [<a href="#ref-MUST" title=""Key words for use in RFCs to Indicate Requirement Levels"">MUST</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. File-Based Protocol</span>
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:
<span class="grey">Schaad & Myers Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc5273">RFC 5273</a> CMC: Transport Protocols June 2008</span>
+---------------------+----------------+
| 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
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Mail-Based Protocol</span>
MIME wrapping is defined for those environments that are MIME native.
The basic mime wrapping in this section is taken from [<a href="#ref-SMIMEV3" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification"">SMIMEV3</a>].
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
statement.
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.
<span class="grey">Schaad & Myers Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc5273">RFC 5273</a> CMC: Transport Protocols June 2008</span>
+--------------+------------------------+------------+--------------+
| 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
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. HTTP/HTTPS-Based Protocol</span>
This section describes the conventions for use of HTTP [<a href="#ref-HTTP" title=""Hypertext Transfer Protocol -- HTTP/1.1"">HTTP</a>] as a
transport layer. In most circumstances, the use of HTTP over TLS
[<a href="#ref-TLS" title=""The Transport Layer Security (TLS) Protocol Version 1.1"">TLS</a>] 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 [<a href="#ref-TLS" title=""The Transport Layer Security (TLS) Protocol Version 1.1"">TLS</a>] 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
authentication.
Clients and servers are expected to follow the other rules and
restrictions in [<a href="#ref-HTTP" title=""Hypertext Transfer Protocol -- HTTP/1.1"">HTTP</a>]. 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.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. PKI Request</span>
A PKI Request using the POST method is constructed as follows:
The Content-Type header MUST have the appropriate value from Table 1.
<span class="grey">Schaad & Myers Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc5273">RFC 5273</a> CMC: Transport Protocols June 2008</span>
The body of the message is the binary value of the encoding of the
PKI Request.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. PKI Response</span>
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.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. TCP-Based Protocol</span>
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.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Security Considerations</span>
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 <a href="#section-6.6">Section</a>
<a href="#section-6.6">6.6</a> of [<a href="#ref-CMC-STRUCT" title=""Certificate Management over CMS (CMC)"">CMC-STRUCT</a>]. 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 [<a href="#ref-IPsec" title=""Security Architecture for the Internet Protocol"">IPsec</a>] or
<span class="grey">Schaad & Myers Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc5273">RFC 5273</a> CMC: Transport Protocols June 2008</span>
TLS [<a href="#ref-TLS" title=""The Transport Layer Security (TLS) Protocol Version 1.1"">TLS</a>]. 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 3.2.1.3.3 in [<a href="#ref-CMC-STRUCT" title=""Certificate Management over CMS (CMC)"">CMC-STRUCT</a>])
must be used to provide the same shrouding that TLS would have
provided.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Acknowledgments</span>
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.
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. References</span>
<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>. Normative References</span>
[<a id="ref-CMC-STRUCT">CMC-STRUCT</a>] Schaad, J. and M. Myers, "Certificate Management over
CMS (CMC)", <a href="./rfc5272">RFC 5272</a>, June 2008.
[<a id="ref-HTTP">HTTP</a>] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", <a href="./rfc2616">RFC 2616</a>, June 1999.
[<a id="ref-IPsec">IPsec</a>] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", <a href="./rfc4301">RFC 4301</a>, December 2005.
[<a id="ref-MUST">MUST</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="./rfc2119">RFC 2119</a>, <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, March 1997.
[<a id="ref-SMIMEV3">SMIMEV3</a>] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
<a href="./rfc3851">RFC 3851</a>, July 2004.
<span class="grey">Schaad & Myers Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc5273">RFC 5273</a> CMC: Transport Protocols June 2008</span>
<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>. Informative References</span>
[<a id="ref-TLS">TLS</a>] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.1", <a href="./rfc4346">RFC 4346</a>,
April 2006.
Authors' Addresses
Jim Schaad
Soaring Hawk Consulting
PO Box 675
Gold Bar, WA 98251
Phone: (425) 785-1031
EMail: jimsch@nwlink.com
Michael Myers
TraceRoute Security, Inc.
EMail: mmyers@fastq.com
<span class="grey">Schaad & Myers Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc5273">RFC 5273</a> CMC: Transport Protocols June 2008</span>
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
<a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Schaad & Myers Standards Track [Page 7]
</pre>
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