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Versions: 00 01 02 03 07 08 RFC 6961

Network Working Group                                       Y. Pettersen
Internet-Draft                                        Opera Software ASA
Intended status: Standards Track                             May 9, 2012
Expires: November 10, 2012


         The TLS Multiple Certificate Status Request Extension
            draft-ietf-tls-multiple-cert-status-extension-00

Abstract

   This document defines the Transport Layer Security (TLS) Certificate
   Status Version 2 Extension to allow clients to specify and support
   multiple certificate status methods.  Also defined is a new method
   based on the Online Certificate Status Protocol (OCSP) 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].

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 November 10, 2012.

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



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   Provisions Relating to IETF Documents
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   it for publication as an RFC or to translate it into languages other
   than English.


1.  Introduction

   The Transport Layer Security (TLS) Extension [RFC6066] 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 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.  First, it does not provide functionality to request the
   status information about intermediate Certification Authority (CA)
   certificates, which means the client has to request status
   information through other methods, such as CRLs, thus adding
   additional delay.  Second, the current format of the extension and
   requirements in the TLS protocol prevents a client from offering the
   server multiple status methods.

   Many Certification Authorities are now issuing intermediate CA
   certificates that not only specify a CRL Distribution Point
   [RFC5280], but also a URL for OCSP [RFC2560] Certificate Status
   requests.  Given that client-cached CRLs are frequently out of date,
   using OCSP to access up-to-date status information about intermediate



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   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 high-traffic
   subscriber sites, and this cost is a concern for many CAs.  There are
   cases where OCSP requests for a single 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 Certification Authorities, the load on their servers
   will depend on the number of certificates they have issued, not on
   the number of visitors to those sites.

   For such a new system to be introduced seamlessly, it must be
   possible for clients to indicate support for the existing OCSP
   Certificate Status method, 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 CertificateStatusRequestItem 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








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2.1.  New extension

   The extension defined by this document is indicated by the
   "status_request_v2" in the ExtensionType enum, which uses the
   following value:

     enum {
       status_request_v2(XX) (65535)
     } ExtensionType;

   [[ EDITOR: The value used for status_request_v2 has been left as XX.
   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:

   struct {
     CertificateStatusType status_type;
     uint16 request_length; /* Length of request field in bytes */
     select (status_type) {
       case ocsp: OCSPStatusRequest;
       case ocsp_multi: OCSPStatusRequest;
     } request;
   } CertificateStatusRequestItem;

   enum { ocsp(1), ocsp_multi(YY), (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 {
     CertificateStatusRequestItem certificate_status_req_list<1..2^16-1>
   } CertificateStatusRequestList


   [[ EDITOR: The value used for ocsp_multi has been left as YY.  This
   value will be assigned when this draft progresses to RFC.]]



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   In the OCSPStatusRequestItem, 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, or are identfied by the certificates used by the server.
   "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 CertificateStatusRequestItem entries MUST be in order of
   preference.

   A server that receive a client hello containing the
   "status_request_v2" extension MAY return a suitable certificate
   status response message to the client along with the server's
   certificate message.  If OCSP is requested, it SHOULD use the
   information contained in the extension when selecting an OCSP
   responder and SHOULD include request_extensions in the OCSP request.

   The server returns a certificate status response along with its
   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 status_request_v2
   request, 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
   [RFC6066]):












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     struct {
       CertificateStatusType status_type;
       select (status_type) {
         case ocsp: OCSPResponse;
         case ocsp_multi: OCSPResponseList;
       } response;
     } CertificateStatus;

     opaque OCSPResponse<0..2^24-1>;

     struct {
       OCSPResponse ocsp_response_list<1..2^24-1>
     } OCSPResponseList


   An "OCSPResponse" element contains a complete, DER-encoded
   [CCITT.X680.2002] OCSP response (using the ASN.1 [CCITT.X680.2002]
   type OCSPResponse defined in [RFC2560]).  Only one OCSP response,
   with a length of at least one byte, may be sent for status_type
   "ocsp".

   An "ocsp_response_list" contains a list of "OCSPResponse" elements,
   as specified above, each containing the OCSP response for the
   matching corresponding certificate in the server's Certificate TLS
   handshake message.  That is, the first entry is the OCSP response for
   the first certificate in the Certificate list, the second entry is
   the response for the second certificate, and so on.  The list MAY
   contain fewer OCSP responses than there were certificates in the
   Certificate handshake message, but there MUST NOT be more responses
   than there were certificates in the list.  Individual elements of the
   list MAY have a length of 0 (zero) bytes, if the server does not have
   the OCSP response for that particular certificate stored, in which
   case, the client MUST act as if a response was not received for that
   particular certificate.  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, such as downloading the relevant CRL; OCSP SHOULD
   in this situation only be used for the end entity certificate, not
   intermediate CA certificates, for reasons stated above.

   Note that a server MAY also choose not to send a "CertificateStatus"
   message, even if it 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.  Additionally, note that that a server
   MUST NOT send the "CertificateStatus" message unless it received
   either a "status_request" or "status_request_v2" extension in the
   client hello message and sent a corresponding "status_request" or



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   "status_request_v2" extension in the server hello message.

   Clients requesting an OCSP response and receiving one or more OCSP
   responses in a "CertificateStatus" message MUST check the OCSP
   response(s) and abort the handshake, if the response is a revoked
   status or is otherwise not satisfactory with a
   bad_certificate_status_response(113) alert.  This alert is always
   fatal.

   [[Open issue: At least one reviewer has suggested that the client
   should treat an unsatisfactory (non-revoked) response as an empty
   response for that particular response and fall back to the
   alternative method described above]]


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_v2

   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.




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5.  Acknowledgements

   This document is based on [RFC6066] authored by Donald Eastlake 3rd.


6.  Normative References

   [CCITT.X680.2002]
              International International Telephone and Telegraph
              Consultative Committee, "Abstract Syntax Notation One
              (ASN.1): Specification of basic notation",
              CCITT Recommendation X.680, July 2002.

   [CCITT.X690.2002]
              International International Telephone and Telegraph
              Consultative Committee, "ASN.1 encoding rules:
              Specification of basic encoding Rules (BER), Canonical
              encoding rules (CER) and Distinguished encoding rules
              (DER)", CCITT Recommendation X.690, July 2002.

   [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.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.










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Author's Address

   Yngve N. Pettersen
   Opera Software ASA

   Email: yngve@opera.com













































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