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Internet Engineering Task Force (IETF)                          T. KauseRequest for Comments: 6712                                           SSHUpdates:4210                                                   M. PeyloCategory: Standards Track                                            NSNISSN: 2070-1721                                           September 2012Internet X.509 Public Key Infrastructure -- HTTP Transferfor the Certificate Management Protocol (CMP)Abstract   This document describes how to layer the Certificate Management   Protocol (CMP) over HTTP.  It is the "CMPtrans" document referenced   inRFC 4210; therefore, this document updates the reference given   therein.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6712.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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                Standards Track                    [Page 1]

RFC 6712                        CMPtrans                  September 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 Contents1. Introduction ....................................................22. Conventions Used in This Document ...............................33. HTTP-Based Protocol .............................................33.1. HTTP Versions ..............................................43.2. Persistent Connections .....................................43.3. General Form ...............................................43.4. Media Type .................................................43.5. Communication Workflow .....................................53.6. HTTP Request-URI ...........................................53.7. Pushing of Announcements ...................................53.8. HTTP Considerations ........................................64. Implementation Considerations ...................................75. Security Considerations .........................................76. IANA Considerations .............................................87. Acknowledgments .................................................88. References ......................................................98.1. Normative References .......................................98.2. Informative References .....................................91.  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 were 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.Kause & Peylo                Standards Track                    [Page 2]

RFC 6712                        CMPtrans                  September 2012   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 the transfer   protocol for CMP.   The usage of HTTP for transferring CMP messages exclusively uses the   POST method for requests, effectively tunneling CMP over HTTP.  While   this is generally considered 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 it is 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 because 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   of HTTP is not blocking its usage as the transfer protocol for CMP   messages.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].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.Kause & Peylo                Standards Track                    [Page 3]

RFC 6712                        CMPtrans                  September 20123.1.  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" connection, 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 Certificate Revocation List (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 inSection 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.3.4.  Media Type   The Internet Media Type "application/pkixcmp" MUST be set in the HTTP   Content-Type header field when conveying a PKIMessage.Kause & Peylo                Standards Track                    [Page 4]

RFC 6712                        CMPtrans                  September 20123.5.  Communication Workflow   In CMP, most communication is initiated by the EEs where every CMP   request triggers a CMP response message from the CA or RA.   The CMP Announcement messages described inSection 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.com3.7.  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.  In this use case, the CMP server acts as an   HTTP client, and the recipient needs to utilize an HTTP server.  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   described inAppendix E.5 of [RFC4210] can be used.Kause & Peylo                Standards Track                    [Page 5]

RFC 6712                        CMPtrans                  September 2012   When pushing Announcement messages, PKIMessage structures are sent as   the entity-body of an HTTP POST request.   Suitable recipients for CMP announcements might, for example, be   repositories storing the announced information, such as directory   services.  Those services listen for incoming messages, utilizing the   same HTTP Request-URI scheme as defined inSection 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.      [15] CA Key Update Announcement      [16] Certificate Announcement      [17] Revocation Announcement      [18] CRL Announcement   CMP Announcement messages do not require any CMP response.  However,   the recipient MUST acknowledge receipt with an HTTP response having   an appropriate status code and an empty body.  When not receiving   such a response, it MUST be assumed that the delivery was not   successful.  If applicable, the sending side MAY try sending the   Announcement again 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 an 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 has 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.  For example, there is no benefit in using chunked   Transfer-Encoding, as the length of an ASN.1 sequence is known when   starting to send it.Kause & Peylo                Standards Track                    [Page 6]

RFC 6712                        CMPtrans                  September 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 inSection 8.2.3 of [RFC2616].  The CMP   payload sent by a client is relatively small, so having extra   messages exchanged is inefficient, as the server will only seldom   reject a message without evaluating the body.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.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; this may also depend 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 Transport Layer Security (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       an HTTP response with the "301 Moved Permanently" status code       could be exploited by a man-in-the-middle attacker trying 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.Kause & Peylo                Standards Track                    [Page 7]

RFC 6712                        CMPtrans                  September 2012   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, for example, by utilizing Internet       Protocol Security according to [RFC4301].  Compliant       implementations MUST support TLS with the option to authenticate       both server and client.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.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: amit@trustpoint.com      Ronald Tschalaer      Certicom      25801 Industrial Blvd      Hayward, CA      US      Email: ronald@trustpoint.com   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).Kause & Peylo                Standards Track                    [Page 8]

RFC 6712                        CMPtrans                  September 2012   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 Networks8.  References8.1.  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. Frystyk, "Hypertext              Transfer Protocol -- HTTP/1.0",RFC 1945, May 1996.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, 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",RFC 2616, 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)",RFC 4210, September 2005.8.2.  Informative References   [RFC2818]  Rescorla, E., "HTTP Over TLS",RFC 2818, May 2000.   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the              Internet Protocol",RFC 4301, December 2005.Kause & Peylo                Standards Track                    [Page 9]

RFC 6712                        CMPtrans                  September 2012   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security              (TLS) Protocol Version 1.2",RFC 5246, August 2008.Authors' Addresses   Tomi Kause   SSH Communications Security   Takomotie 8   Helsinki  00380   Finland   EMail: toka@ssh.com   Martin Peylo   Nokia Siemens Networks   Linnoitustie 6   Espoo  02600   Finland   EMail: martin.peylo@nsn.comKause & Peylo                Standards Track                   [Page 10]