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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.
This document defines a number of transport methods that are used to move CMC messages (defined in [CMC‑STRUCT] (Schaad, J. and M. Myers, “Certificate Management Messages over CMS,” September 2005.)). 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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
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, BER-encoded Full Enrollment Request and 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 |
|---|---|
| Full PKI Request | .crq |
| Full PKI Response | .crp |
MIME wrapping is defined for those environments that are MIME native. The basic mime wrapping in this section is taken from [SMIMEV3] (Ramsdell, B., “Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification,” July 2004.). When using a mail based protocol, MIME wrapping between the layers of CMS wrapping is optional. Note that is different from the standard S/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-enroll". 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" extensions MUST be specified as part of the content-type or content- disposition statement.
| 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 |
| Table 1: PKI Request/Response Identification |
This section describes the conventions for use of HTTP as a transport layer. In most circumstances, the use of HTTP over TLS provides any necessary content protection from ease-droppers.
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 reponses.
Clients MAY attempt to send HTTP requests using TLS 1.0 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 RFC 2616. 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.
A PKI Request using the POST method is constructed as follows:
The Content-Type header MUST have the appropriate value from Table 1 (PKI Request/Response Identification).
The body of the message is the binary value of the encoding of the PKI Request.
An HTTP-based PKI Response is composed of the appropriate HTTP headers, followed by the binary value of the BER encoding of either a Simple or Full PKI Response.
The Content-Type header MUST have the appropriate value from Table 1 (PKI Request/Response Identification).
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 connection is closed by the server after generating a response for the client. (All CMC request messages from client to server generate a response message.) If a second set of messages from the client to the server is required to complete the transaction, the client generates a new TCP-Based connection for this purpose; it cannot reuse an existing one.
Out of band setup can be used to keep a TCP-Based connection open for more than one message pair. A situation where this can occur is an RA talking to a CA over a specially setup TCP connection.
There is no specific port that is to be used when doing TCP based transport. This is to be configured out of band.
When enrollment messages and responses are sent over sockets, no wrapping is required. Messages MUST be sent in their binary, BER-encoded form.
Mechanisms for thwarting replay attacks may be required in particular implementations of this protocol depending on the operational environment. In cases where the 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 5.6 of [CMC‑STRUCT] (Schaad, J. and M. Myers, “Certificate Management Messages over CMS,” September 2005.). Developers of state-constrained PKI clients are strongly encouraged to incorporate the use of these attributes.
The authors and the Working Group are grateful for the participation of Xiaoui Lui 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.
| [CMC-STRUCT] | Schaad, J. and M. Myers, “Certificate Management Messages over CMS,” draft-ietf-pkix-2797-bis-05.txt , September 2005. |
| [RFC2119] | 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. |
| [SMIMEV2] | Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and L. Repka, “S/MIME Version 2 Message Specification,” RFC 2311, March 1998. |
| Jim Schaad | |
| Soaring Hawk Consulting | |
| PO Box 675 | |
| Gold Bar, WA 98251 | |
| Phone: | (425) 785-1031 |
| Email: | jimsch@exmsft.com |
| Michael Myers | |
| TraceRoute Security, Inc. | |
| Email: | myers@fastq.com |
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