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Versions: 00 01 02 03 04 05 06 07 08 rfc6961             Standards Track
Network Working Group                                       Y. Pettersen
Internet-Draft                                            March 29, 2013
Intended status: Standards Track
Expires: September 30, 2013

         The TLS Multiple Certificate Status Request Extension


   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

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
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   This Internet-Draft will expire on September 30, 2013.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   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
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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
   (also referred to as "OCSP stapling") 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

   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 Certificate Revocation
   Lists (CRLs), introducing further delays.  Second, the current format
   of the extension and requirements in the TLS protocol prevents a
   client from offering the server multiple status methods.

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   Many CAs are now issuing intermediate CA certificates that not only
   specify the publication point for their CRLs in a CRL Distribution
   Point [RFC5280], but also specify a URL for their OCSP [RFC2560]
   server in Authority Information Access [RFC5280].  Given that client-
   cached CRLs are frequently out of date, clients would benefit from
   using OCSP to access up-to-date status information about intermediate
   CA certificates.  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.

   Clients will benefit from the TLS server providing certificate status
   information regardless of type, not just for the server certificate,
   but also for the intermediate CA certificates.  Combining the status
   checks into one extension 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

   For such a new system to be introduced seamlessly, clients need to be
   able 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
   [RFC5246] 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 [RFC5280], 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 using a list of CertificateStatusRequestItemV2 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.

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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Presentation language

   This document defines protocol structures using the same conventions
   and presentation language as defined in Section 4 of [RFC5246].

2.  Multiple Certificate Status Extension

2.1.  New extension

   The extension defined by this document is indicated by the
   "status_request_v2" in the ExtensionType enum (originally defined by
   [RFC6066]), 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

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 (defined by Section in[RFC5246]) 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;
     } CertificateStatusRequestItemV2;

     enum { ocsp(1), ocsp_multi(2), (255) } CertificateStatusType;

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     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 {
       CertificateStatusRequestItemV2 certificate_status_req_list<1..2^16-1>;
     } CertificateStatusRequestList;

   In the OCSPStatusRequest (originally defined by [RFC6066]), 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 [CCITT.X690.2002] of the
   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 items in the list of CertificateStatusRequestItemV2 entries are
   in order of the client's preference (favorite choice first).

   A server that receives 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 (originally
   defined by [RFC6066]) immediately after the [RFC5246] (Section 7.4.2)
   "Certificate" message (and before any "ServerKeyExchange" or

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   "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

     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 (originally defined by [RFC6066]) contains
   a complete, DER-encoded 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".

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   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
   "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 other unacceptable responses (as determined by client
   policy), with a bad_certificate_status_response(113) alert.  This
   alert is always fatal.

   If the response is inconclusive, then the client MAY decide to allow
   the connection if it believes it will have the opportunity to check
   the validity of the certificate through another means, e.g., by
   directly querying the issuer's CRL or OCSP responders.  The client
   MUST abort the connection if it needs to engage in activities that
   require trust in the server, and the server certificate has not been
   sufficiently validated.  An example of where the client might wish to
   continue is with EAP-TLS, where the client can use another mechanism
   to check the status of a certificate once it obtains network access.
   In this case, the client could continue with the handshake, but it
   would be inappropriate for the client to disclose a username and
   password until it has fully validated the server certificate.

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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
   Transport Layer Security (TLS) Extensions registry.

   Section 2.2 describes a TLS CertificateStatusType Registry to be
   maintained by the IANA.  The new registry is called TLS Certificate
   Status Types and should be defined under the Transport Layer Security
   (TLS) Extensions registry.  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.

   Value   Description   Reference
   1       ocsp          [This RFC]
   2       ocsp_multi    [This RFC]

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.

   The security considerations of [RFC2560] apply to OCSP requests and

5.  Acknowledgements

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

6.  Normative References

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

Author's Address

   Yngve N. Pettersen

   Email: yngve@spec-work.net

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