Network Working Group Y. Pettersen Internet-Draft Opera Software ASA Intended status: Standards Track October 16, 2009 Expires: April 19, 2010 Adding Multiple TLS Certificate Status Extension requests draft-pettersen-tls-ext-multiple-ocsp-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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. 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The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 19, 2010. Copyright Notice Copyright (c) 2009 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 Pettersen Expires April 19, 2010 [Page 1] Internet-Draft Multiple Certificate Status Extension October 2009 Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document introduces a replacement of the TLS Certificate Status Extension to allow clients to specify and support multiple certificate status methods. Also being introduced is a new OCSP based method that servers can use to provide status information not just about the server's own certificate, but also the status of intermediate certificates in the chain. Requirements Language 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 RFC 2119 [RFC2119]. Pettersen Expires April 19, 2010 [Page 2] Internet-Draft Multiple Certificate Status Extension October 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Multiple Certificate Status Extension . . . . . . . . . . . . . 5 2.1. New extension . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Multiple Certificate Status Request record . . . . . . . . 5 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 4.1. Security Considerations for status_request . . . . . . . . 8 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 6. Normative References . . . . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 Pettersen Expires April 19, 2010 [Page 3] Internet-Draft Multiple Certificate Status Extension October 2009 1. Introduction The Transport Layer Security Extension [I-D.ietf-tls-rfc4366-bis] framework defines, among other extensions, the Certificate Status Extension that clients can use to request the server's copy of the current status of its certificate. The benefits of this extension include a reduced number of roundtrips and network delays for the client to verify the status of the server's certificate using other retrieval methods, and a reduced load on the certificate issuer's status response servers, thus solving a problem that can become significant when the issued certificate is presented by a frequently visited server. There are two problems with the existing Certificate Status extension as it is currently defined. First, it does not provide functionality to request the status information about intermediate CA certificates, which means the client has to request this through other retrieval methods like CRLs, which can cause significant delays when the revocation list has to be refreshed. Second, the current format of the extension and requirements in the TLS protocol prevents a client from offering the server multiple status methods. Many Certificate Authorities are now issuing intermediate CA certificates that not only specify a CRL Distribution Point[RFC5280], but a URL for OCSP [RFC2560] Certificate Status requests. Given that CRLs, particularly those cached by the client, are frequently less up-to-date than is possible for an OCSP responder, using OCSP to access up-to-date status information about intermediate CA certificates will be of great benefit to clients. The benefit to the issuing CA is less clear as providing the bandwidth for the OCSP responder can be costly, especially for CAs with many subscriber sites. The author is aware that the cost of providing this bandwidth just for site certificates has been a concern to some CAs, particularly the smaller ones, and it follows naturally that doubling or tripling this cost could be problematic for the CAs. The author is also aware of at least one case when OCSP requests for a high traffic site caused significant network problems for the issuing CA. For these reasons it will be beneficial to use the TLS server to provide the certificate status information not just for the server certificate, but also for the intermediate CA certificates. This will reduce the roundtrips needed to complete the handshake by the client to just those needed for negotiating the TLS connection. Also, for the Certificate Authorities, the load on their servers will depend on the number of certificates they have issued, noton the number of visitors to those sites. For such a new system to be introduced seamlessly it must be possible Pettersen Expires April 19, 2010 [Page 4] Internet-Draft Multiple Certificate Status Extension October 2009 for clients to indicate support for existing OCSP Certificate status, and a new multiple OCSP mode. Unfortunately, the definition of the Certificate Status extension only allows a single Certificate Status extension to be defined in a single extension record in the handshake, and the TLS Protocol only allows a single record in the extension list for any given extension. This means that it is not possible for clients to indicate support for new methods while still supporting older methods, which would cause problems for interoperability between newer clients and older servers. This will not just be an issue for the multiple status request mode proposed above, but also for any other future status methods that might be introduced. This will be true not just for the current PKIX infrastructure, but also for alternative PKI structures. The solution to this problem is to define a new extension, status_request_v2, with an extended format that allows the client to indicate support for multiple status request methods. This is implemented by using a list of CeritficateStatusRequest records in the extension record. As the server will select the single status method based on the selected cipher suite and the certificate presented, no significant changes are needed in the server's extension format. 2. Multiple Certificate Status Extension 2.1. New extension The extension added by this document is indicated by the "status_request_v2" in the ExtensionType enum, which is updated with this value: enum { status_request_v2(??) (65535) } ExtensionType; [[ EDITOR: The value used for status_request_v2 has been left as ??. This value will be assigned when this draft progresses to RFC.]] 2.2. Multiple Certificate Status Request record Clients that support a certificate status protocol like OCSP may send the status_request_v2 extension to the server in order to use the TLS handshake to transfer such data instead of downloading it through separate connections. When using this extension the "extension_data" field of the extension SHALL contain a CertificateStatusRequestList where: Pettersen Expires April 19, 2010 [Page 5] Internet-Draft Multiple Certificate Status Extension October 2009 struct { CertificateStatusType status_type; select (status_type) { case ocsp: OCSPStatusRequest; case ocsp_multi: OCSPStatusRequest; } request; } CertificateStatusRequest; enum { ocsp(1), ocsp_multi(??), (255) } CertificateStatusType; struct { ResponderID responder_id_list<0..2^16-1>; Extensions request_extensions; } OCSPStatusRequest; opaque ResponderID<1..2^16-1>; opaque Extensions<0..2^16-1>; struct { CertificateStatusRequest certificate_status_req_list<1..2^16-1> } CertificateStatusRequestList [[ EDITOR: The value used for ocsp_multi has been left as ??. This value will be assigned when this draft progresses to RFC.]] In the OCSPStatusRequest, the "ResponderIDs" provides a list of OCSP responders that the client trusts. A zero-length "responder_id_list" sequence has the special meaning that the responders are implicitly known to the server, e.g., by prior arrangement. "Extensions" is a DER encoding of OCSP request extensions. Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as defined in [RFC2560]. "Extensions" is imported from [RFC5280]. A zero-length "request_extensions" value means that there are no extensions (as opposed to a zero-length ASN.1 SEQUENCE, which is not valid for the "Extensions" type). In the case of the "id-pkix-ocsp-nonce" OCSP extension, [RFC2560] is unclear about its encoding; for clarification, the nonce MUST be a DER-encoded OCTET STRING, which is encapsulated as another OCTET STRING (note that implementations based on an existing OCSP client will need to be checked for conformance to this requirement). The list of CertificateStatusRequest entries MUST be in order of preference. Servers that receive a client hello containing the "status_request" Pettersen Expires April 19, 2010 [Page 6] Internet-Draft Multiple Certificate Status Extension October 2009 extension MAY return a suitable certificate status response to the client along with their certificate. If OCSP is requested, they SHOULD use the information contained in the extension when selecting an OCSP responder and SHOULD include request_extensions in the OCSP request. Servers return a certificate response along with their certificate by sending a "CertificateStatus" message immediately after the "Certificate" message (and before any "ServerKeyExchange" or "CertificateRequest" messages). If a server returns a "CertificateStatus" message in response to a , then the server MUST have included an extension of type "status_request_v2" with empty "extension_data" in the extended server hello. The "CertificateStatus" message is conveyed using the handshake message type "certificate_status" as follows (see also [I-D.ietf-tls-rfc4366-bis]): struct { CertificateStatusType status_type; select (status_type) { case ocsp: OCSPResponse; case ocsp_multi: OCSPResponseList; } response; } CertificateStatus; opaque OCSPResponse<1..2^24-1>; struct { OCSPResponse ocsp_response_list<1..2^24-1> } OCSPResponseList An "ocsp_response" contains a complete, DER-encoded OCSP response (using the ASN.1 type OCSPResponse defined in [RFC2560]). Only one OCSP response may be sent for status_type ocsp. An "ocsp_response_list" contains a list of "ocsp_responses", as specified above, each containing the OCSP response for the matching corresponding certificate in the server's Certificate TLS handshake message. The list MAY contain fewer OCSP responses than there was certificates in the Certificate handshake message, but there MUST NOT be more responses than certificates. If the client receives a "ocsp_response_list" that does not contain a response for one or more of the certificates in the completed certificate chain, the client SHOULD attempt to validate the certificate using an alternative retrieval method. Note that a server MAY also choose not to send a "CertificateStatus" Pettersen Expires April 19, 2010 [Page 7] Internet-Draft Multiple Certificate Status Extension October 2009 message, even if has received a "status_request_v2" extension in the client hello message and has sent a "status_request_v2" extension in the server hello message. Note in addition that a server MUST NOT send the "CertificateStatus" message unless it received a "status_request_v2" extension in the client hello message and sent a "status_request_v2" extension in the server hello message. Clients requesting an OCSP response and receiving an OCSP response in a "CertificateStatus" message MUST check the OCSP response and abort the handshake if the response is not satisfactory with a bad_certificate_status_response(113) alert. This alert is always fatal. 3. IANA Considerations Section 2.1 defines the new TLS Extension status_request_v2 enum, which should be added to the ExtensionType Values list in the IANA TLS category after IETF Concensus has decided to add the value. Section 2.2 describes a TLS CertificateStatusType Registry to be maintained by the IANA. CertificateStatusType values are to be assigned via IETF Review as defined in [RFC5226]. The initial registry corresponds to the definition of "ExtensionType" in Section 2.2. 4. Security Considerations General Security Considerations for TLS Extensions are covered in [RFC5246]. Security Considerations for the particular extension specified in this document are given below. In general, implementers should continue to monitor the state of the art and address any weaknesses identified. 4.1. Security Considerations for status_request If a client requests an OCSP response, it must take into account that an attacker's server using a compromised key could (and probably would) pretend not to support the extension. In this case, a client that requires OCSP validation of certificates SHOULD either contact the OCSP server directly or abort the handshake. Use of the OCSP nonce request extension (id-pkix-ocsp-nonce) may improve security against attacks that attempt to replay OCSP responses; see Section 4.4.1 of [RFC2560] for further details. Pettersen Expires April 19, 2010 [Page 8] Internet-Draft Multiple Certificate Status Extension October 2009 5. Acknowledgements This document is based on [I-D.ietf-tls-rfc4366-bis] authored by Donald Eastlake 3rd. 6. Normative References [I-D.ietf-tls-rfc4366-bis] 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", draft-ietf-tls-rfc4366-bis-05 (work in progress), June 2009. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. Author's Address Yngve N. Pettersen Opera Software ASA Email: yngve@opera.com Pettersen Expires April 19, 2010 [Page 9]