Internet Engineering Task Force (IETF)                      Y. Pettersen
Request for Comments: 6961                                     June 2013
Category: Standards Track
ISSN: 2070-1721


                  The Transport Layer Security (TLS)
            Multiple Certificate Status Request Extension

Abstract

  This document defines the Transport Layer Security (TLS) Certificate
  Status Version 2 Extension to allow clients to specify and support
  several certificate status methods.  (The use of the Certificate
  Status extension is commonly referred to as "OCSP stapling".)  Also
  defined is a new method based on the Online Certificate Status
  Protocol (OCSP) that servers can use to provide status information
  about not only the server's own certificate but also the status of
  intermediate certificates in the chain.

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 in Section 2 of RFC 5741.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  http://www.rfc-editor.org/info/rfc6961.


















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Copyright Notice

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  document authors.  All rights reserved.

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  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.

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
  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 Certificate Revocation
  Lists (CRLs), introducing further delays.  Second, the current format
  of the extension and requirements in the TLS protocol prevent 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 [RFC6960]
  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
  sites.  Additionally, using this extension significantly reduces
  privacy concerns around the clients informing the certificate issuer
  about which sites they are visiting.

  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




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  method based on the selected cipher suite and the certificate
  presented, no significant changes are needed in the server's
  extension format.

1.1.  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].

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
  "status_request_v2" in the ExtensionType enum (originally defined by
  [RFC6066]), which uses the following value:

    enum {
      status_request_v2(17), (65535)
    } ExtensionType;

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 in Section 7.4.1.4 of [RFC5246]) of
  the extension SHALL contain a CertificateStatusRequestListV2 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>;
    } CertificateStatusRequestListV2;

  In the OCSPStatusRequest (originally defined by [RFC6066]), the
  "ResponderID" 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 identified by the certificates used by
  the server.  "Extensions" is a DER encoding [X.690] of the OCSP
  request extensions, and if the server supports the forwarding of OCSP
  request extensions, this value MUST be forwarded without
  modification.

  Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as
  defined in [RFC6960].  "Extensions" is imported from [RFC5280].  A
  zero-length "request_extensions" value means that there are no
  extensions (as opposed to a DER-encoded zero-length ASN.1 SEQUENCE,
  which is not valid for the "Extensions" type).

  Servers that support a client's selection of responders using the
  ResponderID field could implement this selection by matching the
  responder ID values from the client's list with the ResponderIDs of
  known OCSP responders, either by using a binary compare of the values
  or a hash calculation and compare method.

  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).  This
  has been clarified in [RFC6960].

  The items in the list of CertificateStatusRequestItemV2 entries are
  ordered according to 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



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  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 "Certificate" message
  (Section 7.4.2 of [RFC5246]) (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" (defined in [RFC6066]) as follows
  (updated from the definition in [RFC6066]):

    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 [X.680] type
  OCSPResponse defined in [RFC6960]).  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



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  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 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 OCSP response received from the server does not result in a
  definite "good" or "revoked" status, it is inconclusive.  A TLS
  client in such a case MAY check the validity of the server
  certificate through other means, e.g., by directly querying the
  certificate issuer.  If such processing still results in an
  inconclusive response, then the application using the TLS connection
  will have to decide whether to close the connection or not.  Note
  that this problem cannot be decided by the generic TLS client code
  without information from the application.  If the application doesn't
  provide any such information, then the client MUST abort the
  connection, since the server certificate has not been sufficiently
  validated.

  An example of where the application might wish to continue is with
  EAP-TLS (Extensible Authentication Protocol - TLS), where the
  application can use another mechanism to check the status of a
  certificate once it obtains network access.  In this case, the
  application could have the client continue with the handshake, but it
  MUST NOT 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 (17)
  enum, which has been added to the "ExtensionType Values" list in the
  IANA "Transport Layer Security (TLS) Extensions" registry.

  Section 2.2 describes a TLS CertificateStatusType registry that is
  now maintained by IANA.  The "TLS Certificate Status Types" registry
  has been created 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 "CertificateStatusType" in
  Section 2.2.

  Value   Description   Reference
  -----------------------------------------
  0       Reserved      [RFC6961]
  1       ocsp          [RFC6066] [RFC6961]
  2       ocsp_multi    [RFC6961]
  3-255   Unassigned

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 [RFC6960] for further details.

  This extension allows the client to send arbitrary data to the
  server.  The server implementers need to handle such data carefully
  to avoid introducing security vulnerabilities.

  The security considerations of [RFC6960] apply to OCSP requests and
  responses.




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5.  Acknowledgements

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

6.  References

6.1.  Normative References

  [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

  [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.

  [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
             Galperin, S., and C. Adams, "X.509 Internet Public Key
             Infrastructure Online Certificate Status Protocol - OCSP",
             RFC 6960, June 2013.

  [X.680]    ITU-T Recommendation X.680 (2008) | ISO/IEC 8824-1:2008,
             "Abstract Syntax Notation One (ASN.1): Specification of
             basic notation", November 2008.

  [X.690]    ITU-T Recommendation X.690 (2008) | ISO/IEC 8825-1:2008,
             "ASN.1 encoding rules: Specification of Basic Encoding
             Rules (BER), Canonical Encoding Rules (CER) and
             Distinguished Encoding Rules (DER)", November 2008.

6.2.  Informative References

  [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.






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Author's Address

  Yngve N. Pettersen

  EMail: [email protected]














































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