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Internet Engineering Task Force (IETF)                      Y. PettersenRequest for Comments: 6961                                     June 2013Category: Standards TrackISSN: 2070-1721The Transport Layer Security (TLS)Multiple Certificate Status Request ExtensionAbstract   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 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/rfc6961.Pettersen                    Standards Track                    [Page 1]

RFC 6961          Multiple Certificate Status Extension        June 2013Copyright Notice   Copyright (c) 2013 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.   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.Pettersen                    Standards Track                    [Page 2]

RFC 6961          Multiple Certificate Status Extension        June 2013   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 statusPettersen                    Standards Track                    [Page 3]

RFC 6961          Multiple Certificate Status Extension        June 2013   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 inRFC 2119 [RFC2119].1.2.  Presentation Language   This document defines protocol structures using the same conventions   and presentation language as defined inSection 4 of [RFC5246].2.  Multiple Certificate Status Extension2.1.  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 inSection 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;Pettersen                    Standards Track                    [Page 4]

RFC 6961          Multiple Certificate Status Extension        June 2013     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'sPettersen                    Standards Track                    [Page 5]

RFC 6961          Multiple Certificate Status Extension        June 2013   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 whichPettersen                    Standards Track                    [Page 6]

RFC 6961          Multiple Certificate Status Extension        June 2013   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.Pettersen                    Standards Track                    [Page 7]

RFC 6961          Multiple Certificate Status Extension        June 20133.  IANA ConsiderationsSection 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" inSection 2.2.   Value   Description   Reference   -----------------------------------------   0       Reserved      [RFC6961]   1       ocsp          [RFC6066] [RFC6961]   2       ocsp_multi    [RFC6961]   3-255   Unassigned4.  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; seeSection 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.Pettersen                    Standards Track                    [Page 8]

RFC 6961          Multiple Certificate Status Extension        June 20135.  Acknowledgements   This document is based on [RFC6066], authored by Donald Eastlake 3rd.6.  References6.1.  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.Pettersen                    Standards Track                    [Page 9]

RFC 6961          Multiple Certificate Status Extension        June 2013Author's Address   Yngve N. Pettersen   EMail: yngve@spec-work.netPettersen                    Standards Track                   [Page 10]