Certificate Management over CMS (CMC): Transport Protocols
draft-ietf-pkix-cmc-trans-08.txt

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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] (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 [MUST] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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:

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] (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 (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-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.

4.  HTTP/HTTPS based protocol

This section describes the conventions for use of HTTP [HTTP] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) as a transport layer. In most circumstances, the use of HTTP over TLS [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) 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 [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) 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] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.). 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 (MIME PKI Request/Response Identification).

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 (MIME PKI Request/Response Identification).

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 connection is closed by the client after recieving a final response. If a second round of messages is needed, the client can either re-use the same connection or use a new one.

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 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.

Initiation of a secure communications channel between an end-entity and a CA or 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] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.) or TLS [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.). 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] (Schaad, J. and M. Myers, “Certificate Management Messages over CMS,” September 2005.)) must be used to provide the same shrouding that TLS would have provided.

7.  IANA Considerations

There are no IANA considerations in this document.

8.  Acknowledgments

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.

9.  References

9.1. Normative References

9.2. Informative References

Authors' Addresses

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