Certificate Management over CMS (CMC): Transport Protocols
Four ways to move CMC certificate request and response messages: files, mail, HTTP and plain TCP.
Schaad, J. and M. Myers, "Certificate Management over CMS (CMC): Transport Protocols", RFC 5273, DOI 10.17487/RFC5273, June 2008, <https://www.rfc-editor.org/info/rfc5273>.
@misc{rfc5273,
series = {Request for Comments},
number = 5273,
howpublished = {RFC 5273},
publisher = {RFC Editor},
doi = {10.17487/RFC5273},
url = {https://www.rfc-editor.org/info/rfc5273},
author = {J. Schaad and M. Myers},
title = {{Certificate Management over CMS (CMC): Transport Protocols}},
pagetotal = 7,
year = 2008,
month = jun,
} RFC text
Contents
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.
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.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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:
+---------------------+----------------+
| 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
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.
+--------------+------------------------+------------+--------------+
| 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
authentication.
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.
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
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 3.2.1.3.3 in [CMC-STRUCT])
must be used to provide the same shrouding that TLS would have
provided.
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.
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
EMail: jimsch@nwlink.com
Michael Myers
TraceRoute Security, Inc.
EMail: mmyers@fastq.com
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AI ELI5
On this page
§1Overview
This document defines transport methods for moving CMC messages (Certificate Management over CMS), which are defined in RFC 5272. The mechanisms are HTTP, file, mail and TCP. It uses the usual RFC 2119 key words such as MUST and SHOULD.
CMC messages come as Simple PKI Requests and Responses, and Full PKI Requests and Responses. The transports differ mostly in how they label and wrap those four message types.
A certificate request is a letter. This document lists the ways to deliver the letter, and how to write on the envelope so the receiver knows what is inside.
§2File and Mail Transports
With the file-based transport, requests and responses move by file-system means, for example when enrollment is done for an off-line client. There MUST be only one request or response message in a single file. File extensions SHOULD be .p10 (Simple PKI Request), .crq (Full PKI Request), .p7c (Simple PKI Response) and .crp (Full PKI Response).
The mail-based transport uses MIME wrapping, taken from S/MIME. MIME wrapping between the layers of CMS wrapping is optional, which differs from a standard S/MIME message. A simple request uses content type application/pkcs10 with a .p10 file name. The other three message types use application/pkcs7-mime with an smime-type parameter and a file name that MUST be included in the content-type or content-disposition statement.
On a disk you put one message per file and end the file name with a code that says what kind it is. In email you stick a label on the attachment that says the same thing.
| Item | MIME type | File extension | SMIME type |
|---|---|---|---|
| Simple PKI Request | application/pkcs10 | .p10 | N/A |
| Full PKI Request | application/pkcs7-mime | .p7m | CMC-request |
| Simple PKI Response | application/pkcs7-mime | .p7c | certs-only |
| Full PKI Response | application/pkcs7-mime | .p7m | CMC-response |
§3HTTP and TCP Transports
For HTTP, clients MUST use the POST method to submit requests and servers MUST use the 200 response code for successful responses. The Content-Type header MUST carry the right value from the MIME table, and the body is the binary encoding of the PKI Request or PKI Response (a BER encoding for responses). Clients MAY try TLS 1.0 or later, although servers are not required to support TLS. Servers MUST NOT assume client support for any HTTP authentication such as cookies, Basic or Digest.
For TCP, no wrapping is required: messages are sent in their binary encoded form. The client closes the connection after the response or sends another request on it, and reusing one connection for successive requests is RECOMMENDED. A server MAY close an idle connection, typically after several minutes. No specific port is assigned. Only the Private Ports 49152-65535 may be used without registration, ports 1-49151 SHOULD NOT be used, and the port is configured out of band.
On the web, you post the request like a form and get the answer back. With plain TCP you just send the bytes down a pipe, and both sides agree on the door number beforehand.
§4Security Considerations
Replay attacks may need to be thwarted depending on the operating environment. If the CA keeps significant state, replays may be detectable without the optional nonces. Implementers need to weigh whether to use the senderNonce and recipientNonce attributes, and developers of state-constrained PKI clients are strongly encouraged to use them.
Setting up a secure channel between an end-entity and a CA or RA requires an out-of-band trust initiation mechanism, for example IPsec or TLS, and the choice is outside the scope of this document. When no prior trust exists, such as when the protocol downloads the trust anchors, the Enveloped Data content type must be used to give the shrouding TLS would have provided.
Someone could copy an old message and send it again, so some systems add one-time numbers. And you need some way to trust who you are talking to before you start.