Internet X.509 Public Key Infrastructure -- HTTP Transfer for CMP
Author(s): Tomi Kause, Martin Peylo
This document describes how to layer the Certificate Management Protocol over HTTP. It is the 'CMPtrans' document referenced in RFC 4210 and therefore updates the reference given therein....
Network Working Group T. Kause Internet-Draft Tectia Updates: 4210 (if approved) M. Peylo Intended status: Standards Track NSN Expires: January 25, 2013 July 24, 2012 Internet X.509 Public Key Infrastructure -- HTTP Transfer for CMP draft-ietf-pkix-cmp-transport-protocols-20.txt Abstract This document describes how to layer the Certificate Management Protocol over HTTP. It is the "CMPtrans" document referenced in RFC 4210 and therefore updates the reference given therein. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 25, 2013. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Kause & Peylo Expires January 25, 2013 [Page 1] Internet-Draft CMPtrans July 2012 This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 5 3. HTTP-Based Protocol . . . . . . . . . . . . . . . . . . . . . 6 3.1. HTTP Versions . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Persistent Connections . . . . . . . . . . . . . . . . . . 6 3.3. General Form . . . . . . . . . . . . . . . . . . . . . . . 6 3.4. Media Type . . . . . . . . . . . . . . . . . . . . . . . . 7 3.5. Communication Workflow . . . . . . . . . . . . . . . . . . 7 3.6. HTTP Request-URI . . . . . . . . . . . . . . . . . . . . . 7 3.7. Pushing of Announcements . . . . . . . . . . . . . . . . . 7 3.8. HTTP Considerations . . . . . . . . . . . . . . . . . . . 8 4. Implementation Considerations . . . . . . . . . . . . . . . . 10 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 8.1. Normative References . . . . . . . . . . . . . . . . . . . 14 8.2. Informative References . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 Kause & Peylo Expires January 25, 2013 [Page 2] Internet-Draft CMPtrans July 2012 1. Introduction The Certificate Management Protocol (CMP) [RFC4210] requires a well defined transfer mechanism to enable End Entities (EEs), Registration Authorities (RAs) and Certification Authorities (CAs) to pass PKIMessage sequences between them. The first version of the CMP specification [RFC2510] included a brief description of a simple transfer protocol layer on top of TCP. Its features was simple transfer level error-handling and a mechanism to poll for outstanding PKI messages. Additionally it was mentioned that PKI messages could also be conveyed using file-, E-mail- and HTTP-based transfer, but those were not specified in detail. The current version of the CMP specification [RFC4210] incorporated its own polling mechanism and thus the need for a transfer protocol providing this functionality vanished. The remaining features CMP requires from its transfer protocols are connection and error handling. Before this document was published as an RFC, the draft version underwent drastic changes during the long-lasting work process. The so-called "Direct TCP-Based Management Protocol" specified in [RFC2510] was enhanced and at some point a version existed where this protocol was again transferred over HTTP. As both approaches proved to be needless and cumbersome, implementers preferred to use plain HTTP transfer following [RFC1945] or [RFC2616]. This document now reflects that by exclusively describing HTTP as transfer protocol for CMP. The usage of HTTP for transferring CMP messages exclusively uses POST method for requests, effectively tunneling CMP over HTTP. While this is generally considered as bad practice and should not be emulated, there are good reasons to do so for transferring CMP. HTTP is used as it is generally easy to implement and able to traverse network borders utilizing ubiquitous proxies. Most importantly, HTTP is already commonly used in existing CMP implementations. Other HTTP request methods such as GET are not used as PKI management operations can only be triggered using CMP's PKI messages which need to be transferred using a POST request. With its status codes HTTP provides needed error reporting capabilities. General problems on the server side as well as those directly caused by the respective request can be reported to the client. As CMP implements a transaction ID, identifying transactions spanning over more than just a single request/response pair, the statelessness Kause & Peylo Expires January 25, 2013 [Page 3] Internet-Draft CMPtrans July 2012 of HTTP is not blocking its usage as transfer protocol for CMP messages. Kause & Peylo Expires January 25, 2013 [Page 4] Internet-Draft CMPtrans July 2012 2. Conventions Used in This Document 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]. Kause & Peylo Expires January 25, 2013 [Page 5] Internet-Draft CMPtrans July 2012 3. HTTP-Based Protocol For direct interaction between two entities, where a reliable transport protocol like TCP is available, HTTP SHOULD be utilized for conveying CMP messages. 3.1. HTTP Versions Implementations MUST support HTTP/1.0 [RFC1945], and SHOULD support HTTP/1.1 [RFC2616]. 3.2. Persistent Connections HTTP persistent connections [RFC2616] allow multiple interactions to take place on the same HTTP connection. However, neither HTTP nor the protocol specified in this document are designed to correlate messages on the same connection in any meaningful way; persistent connections are only a performance optimization. In particular, intermediaries can do things like mix connections from different clients into one "upstream" connections, terminate persistent connections and forward requests as non-persistent requests, etc. As such, implementations MUST NOT infer that requests on the same connection come from the same client (e.g., for correlating PKI messages with ongoing transactions); every message is to be evaluated in isolation. 3.3. General Form A DER-encoded [ITU.X690.1994] PKIMessage [RFC4210] is sent as the entity-body of an HTTP POST request. If this HTTP request is successful, the server returns the CMP response in the body of the HTTP response. The HTTP response status code in this case MUST be 200; other "Successful 2xx" codes MUST NOT be used for this purpose. HTTP responses to pushed CMP Announcement messages (i.e., CA Certificate Announcement, Certificate Announcement, Revocation Announcement, and CRL Announcement) utilize the status codes 201 and 202 to identify whether the received information was processed. While "Redirection 3xx" status codes MAY be supported by implementations, clients should only be enabled to automatically follow them after careful consideration of possible security implications. As described in Section 5, "301 Moved Permanently" could be misused for permanent denial of service. All applicable "Client Error 4xx" or "Server Error 5xx" status codes MAY be used to inform the client about errors. Kause & Peylo Expires January 25, 2013 [Page 6] Internet-Draft CMPtrans July 2012 3.4. Media Type The Internet Media Type "application/pkixcmp" MUST be set in the HTTP Content-Type header field when conveying a PKIMessage. 3.5. Communication Workflow In CMP most communication is initiated by the end entities where every CMP request triggers a CMP response message from the CA or RA. The CMP Announcement messages described in Section 3.7 are an exception. Their creation may be triggered by certain events or done on a regular basis by a CA. The recipient of the Announcement only replies with an HTTP status code acknowledging the receipt or indicating an error but not with a CMP response. If the receipt of an HTTP request is not confirmed by receiving an HTTP response, it MUST be assumed that the transferred CMP message was not successfully delivered to its destination. 3.6. HTTP Request-URI The Request-URI is formed as specified in [RFC3986]. A server implementation MUST handle Request-URI paths with or without a trailing slash as identical. An example of a Request-Line and a Host header field in an HTTP/1.1 header, sending a CMP request to a server, located in the "/cmp" path of the host "example.com", would be POST /cmp HTTP/1.1 Host: example.com or in the absoluteURI form POST http://example.com/cmp/ HTTP/1.1 Host: example.com 3.7. Pushing of Announcements A CMP server may create event-triggered announcements or generate them on a regular basis. It MAY utilize HTTP transfer to convey them to a suitable recipient. As no request messages are specified for those announcements they can only be pushed to the recipient. If an EE wants to poll for a potential CA Key Update Announcement or the current CRL, a PKI Information Request using a General Message as Kause & Peylo Expires January 25, 2013 [Page 7] Internet-Draft CMPtrans July 2012 described in E.5 of [RFC4210] can be used. When pushing Announcement messages, PKIMessage structures are sent as the entity-body of an HTTP POST request. Suitable recipients for CMP announcements might e.g. be repositories storing the announced information such as directory services. Those listen for incoming messages, utilizing the same HTTP Request-URI scheme as defined in Section 3.6. The following PKIMessages are announcements that may be pushed by a CA. The prefixed numbers reflect ASN.1 numbering of the respective element.  CA Key Update Announcement  Certificate Announcement  Revocation Announcement  CRL Announcement CMP Announcement messages do not require any CMP response. However, the recipient MUST acknowledge receipt with a HTTP response having an appropriate status code and an empty body. When not receiving such response it MUST be assumed that the delivery was not successful and if applicable the sending side MAY retry sending the Announcement after waiting for an appropriate time span. If the announced issue was successfully stored in a database or was already present, the answer MUST be an HTTP response with a "201 Created" status code and empty message body. In case the announced information was only accepted for further processing, the status code of the returned HTTP response MAY also be "202 Accepted". After an appropriate delay, the sender may then try to send the Announcement again and may repeat this until it receives a confirmation that it had been successfully processed. The appropriate duration of the delay and the option to increase it between consecutive attempts should be carefully considered. A receiver MUST answer with a suitable 4xx or 5xx HTTP error code when a problem occurs. 3.8. HTTP Considerations While all defined features of the HTTP protocol are available to implementations, they SHOULD keep the protocol utilization as simple as possible. E.g. there is no benefit in using chunked Transfer- Encoding as the length of an ASN.1 sequence is know when starting to send it. Kause & Peylo Expires January 25, 2013 [Page 8] Internet-Draft CMPtrans July 2012 There is no need for the clients to send an "Expect" request-header field with the "100-continue" expectation and wait for a "100 Continue" status as described in chapter 8.2.3 of [RFC2616]. The CMP payload sent by a client is relatively small, so having extra messages exchanged is more inefficient as the server will anyway only seldom reject a message without evaluating the body. Kause & Peylo Expires January 25, 2013 [Page 9] Internet-Draft CMPtrans July 2012 4. Implementation Considerations Implementors should be aware that implementations might exist that use a different approach for transferring CMP over HTTP because this document has been under development for more than a decade. Further, implementations based on earlier drafts of this document might use an unregistered "application/pkixcmp-poll" MIME type. Kause & Peylo Expires January 25, 2013 [Page 10] Internet-Draft CMPtrans July 2012 5. Security Considerations The following aspects need to be considered by implementers and users: 1. There is the risk for denial of service attacks through resource consumption by opening many connections to an HTTP server. Therefore idle connections should be terminated after an appropriate timeout, maybe also depending on the available free resources. After sending a CMP Error Message, the server should close the connection even if the CMP transaction is not yet fully completed. 2. Without being encapsulated in effective security protocols such as TLS [RFC5246] there is no integrity protection at the HTTP protocol level. Therefore information from the HTTP protocol should not be used to change state of the transaction. 3. Client users should be aware that storing the target location of a HTTP response with the "301 Moved Permanently" status code could be exploited by a man-in-the-middle attacker to block them permanently from contacting the correct server. 4. If no measures to authenticate and protect the HTTP responses to pushed Announcement messages are in place their information regarding the Announcement's processing state may not be trusted. In that case the overall design of the PKI system must not depend on the Announcements being reliably received and processed by their destination. 5. CMP provides inbuilt integrity protection and authentication. The information communicated unencrypted in CMP messages does not contain sensitive information endangering the security of the PKI when intercepted. However, it might be possible for an eavesdropper to utilize the available information to gather confidential technical or business critical information. Therefore users of the HTTP transfer for CMP might want to consider using HTTP over TLS according to [RFC2818] or virtual private networks created e.g. by utilizing Internet Protocol Security according to [RFC4301]. Compliant implementations MUST support TLS with the option to authenticate both server and client. Kause & Peylo Expires January 25, 2013 [Page 11] Internet-Draft CMPtrans July 2012 6. IANA Considerations The IANA has already registered the MIME media type "application/ pkixcmp" for identifying CMP sequences due to an request made in connection with [RFC2510]. No further action by the IANA is necessary for this document or any anticipated updates. Kause & Peylo Expires January 25, 2013 [Page 12] Internet-Draft CMPtrans July 2012 7. Acknowledgments Amit Kapoor and Ronald Tschlaer were the original authors of this document and their version focused on the so-called "TCP-Based Management Protocol", which has been removed from this document. Their contact data as originally stated by them is as follows: Amit Kapoor Certicom 25801 Industrial Blvd Hayward, CA US Email: email@example.com Ronald Tschalaer Certicom 25801 Industrial Blvd Hayward, CA US Email: firstname.lastname@example.org The authors gratefully acknowledge the contributions of various members of the IETF PKIX Working Group and the ICSA CA-talk mailing list (a list solely devoted to discussing CMP interoperability efforts). By providing ideas, giving hints and doing invaluable review work, the following alphabetically listed individuals have significantly contributed to this document: Tomas Gustavsson, Primekey Peter Gutmann, University of Auckland Wolf-Dietrich Moeller, Nokia Siemens Networks Kause & Peylo Expires January 25, 2013 [Page 13] Internet-Draft CMPtrans July 2012 8. References 8.1. Normative References [ITU.X690.1994] International Telecommunications Union, "Information Technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, 1994. [RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext Transfer Protocol -- HTTP/1.0", RFC1945, May 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC2119, March 1997. [RFC2510] Adams, C. and S. Farrell, "Internet X.509 Public Key Infrastructure Certificate Management Protocols", RFC2510, March 1999. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC2616, June 1999. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC3986, January 2005. [RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)", RFC4210, September 2005. 8.2. Informative References [RFC2818] Rescorla, E., "HTTP Over TLS", RFC2818, May 2000. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC4301, December 2005. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC5246, August 2008. Kause & Peylo Expires January 25, 2013 [Page 14] Internet-Draft CMPtrans July 2012 Authors' Addresses Tomi Kause Tectia Corporation Kumpulantie 3 Helsinki 00520 Finland Email: email@example.com Martin Peylo Nokia Siemens Networks Linnoitustie 6 Espoo 02600 Finland Email: firstname.lastname@example.org Kause & Peylo Expires January 25, 2013 [Page 15]