Transport Layer Security (TLS) Extensions: Extension Definitions
RFC 6066

Internet Engineering Task Force (IETF)                   D. Eastlake 3rd
Request for Comments: 6066                                        Huawei
Obsoletes: 4366                                             January 2011
Category: Standards Track
ISSN: 2070-1721

    Transport Layer Security (TLS) Extensions: Extension Definitions

Abstract

   This document provides specifications for existing TLS extensions.
   It is a companion document for RFC 5246, "The Transport Layer
   Security (TLS) Protocol Version 1.2".  The extensions specified are
   server_name, max_fragment_length, client_certificate_url,
   trusted_ca_keys, truncated_hmac, and status_request.

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/rfc6066.

Copyright Notice

   Copyright (c) 2011 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
   Provisions Relating to IETF Documents
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   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.

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RFC 6066                TLS Extension Definitions           January 2011

   This document may contain material from IETF Documents or IETF
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   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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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................3
      1.1. Specific Extensions Covered ................................3
      1.2. Conventions Used in This Document ..........................5
   2. Extensions to the Handshake Protocol ............................5
   3. Server Name Indication ..........................................6
   4. Maximum Fragment Length Negotiation .............................8
   5. Client Certificate URLs .........................................9
   6. Trusted CA Indication ..........................................12
   7. Truncated HMAC .................................................13
   8. Certificate Status Request .....................................14
   9. Error Alerts ...................................................16
   10. IANA Considerations ...........................................17
      10.1. pkipath MIME Type Registration ...........................17
      10.2. Reference for TLS Alerts, TLS HandshakeTypes, and
            ExtensionTypes ...........................................19
   11. Security Considerations .......................................19
      11.1. Security Considerations for server_name ..................19
      11.2. Security Considerations for max_fragment_length ..........20
      11.3. Security Considerations for client_certificate_url .......20
      11.4. Security Considerations for trusted_ca_keys ..............21
      11.5. Security Considerations for truncated_hmac ...............21
      11.6. Security Considerations for status_request ...............22
   12. Normative References ..........................................22
   13. Informative References ........................................23
   Appendix A. Changes from RFC 4366 .................................24
   Appendix B. Acknowledgements ......................................25

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1.  Introduction

   The Transport Layer Security (TLS) Protocol Version 1.2 is specified
   in [RFC5246].  That specification includes the framework for
   extensions to TLS, considerations in designing such extensions (see
   Section 7.4.1.4 of [RFC5246]), and IANA Considerations for the
   allocation of new extension code points; however, it does not specify
   any particular extensions other than Signature Algorithms (see
   Section 7.4.1.4.1 of [RFC5246]).

   This document provides the specifications for existing TLS
   extensions.  It is, for the most part, the adaptation and editing of
   material from RFC 4366, which covered TLS extensions for TLS 1.0 (RFC
   2246) and TLS 1.1 (RFC 4346).

1.1.  Specific Extensions Covered

   The extensions described here focus on extending the functionality
   provided by the TLS protocol message formats.  Other issues, such as
   the addition of new cipher suites, are deferred.

   The extension types defined in this document are:

      enum {
          server_name(0), max_fragment_length(1),
          client_certificate_url(2), trusted_ca_keys(3),
          truncated_hmac(4), status_request(5), (65535)
      } ExtensionType;

   Specifically, the extensions described in this document:

   -  Allow TLS clients to provide to the TLS server the name of the
      server they are contacting.  This functionality is desirable in
      order to facilitate secure connections to servers that host
      multiple 'virtual' servers at a single underlying network address.

   -  Allow TLS clients and servers to negotiate the maximum fragment
      length to be sent.  This functionality is desirable as a result of
      memory constraints among some clients, and bandwidth constraints
      among some access networks.

   -  Allow TLS clients and servers to negotiate the use of client
      certificate URLs.  This functionality is desirable in order to
      conserve memory on constrained clients.

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   -  Allow TLS clients to indicate to TLS servers which certification
      authority (CA) root keys they possess.  This functionality is
      desirable in order to prevent multiple handshake failures
      involving TLS clients that are only able to store a small number
      of CA root keys due to memory limitations.

   -  Allow TLS clients and servers to negotiate the use of truncated
      Message Authentication Codes (MACs).  This functionality is
      desirable in order to conserve bandwidth in constrained access
      networks.

   -  Allow TLS clients and servers to negotiate that the server sends
      the client certificate status information (e.g., an Online
      Certificate Status Protocol (OCSP) [RFC2560] response) during a
      TLS handshake.  This functionality is desirable in order to avoid
      sending a Certificate Revocation List (CRL) over a constrained
      access network and therefore saving bandwidth.

   TLS clients and servers may use the extensions described in this
   document.  The extensions are designed to be backwards compatible,
   meaning that TLS clients that support the extensions can talk to TLS
   servers that do not support the extensions, and vice versa.

   Note that any messages associated with these extensions that are sent
   during the TLS handshake MUST be included in the hash calculations
   involved in "Finished" messages.

   Note also that all the extensions defined in this document are
   relevant only when a session is initiated.  A client that requests
   session resumption does not in general know whether the server will
   accept this request, and therefore it SHOULD send the same extensions
   as it would send if it were not attempting resumption.  When a client
   includes one or more of the defined extension types in an extended
   client hello while requesting session resumption:

   -  The server name indication extension MAY be used by the server
      when deciding whether or not to resume a session as described in
      Section 3.

   -  If the resumption request is denied, the use of the extensions is
      negotiated as normal.

   -  If, on the other hand, the older session is resumed, then the
      server MUST ignore the extensions and send a server hello
      containing none of the extension types.  In this case, the
      functionality of these extensions negotiated during the original
      session initiation is applied to the resumed session.

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1.2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

2.  Extensions to the Handshake Protocol

   This document specifies the use of two new handshake messages,
   "CertificateURL" and "CertificateStatus".  These messages are
   described in Sections 5 and 8, respectively.  The new handshake
   message structure therefore becomes:

   enum {
       hello_request(0), client_hello(1), server_hello(2),
       certificate(11), server_key_exchange (12),
       certificate_request(13), server_hello_done(14),
       certificate_verify(15), client_key_exchange(16),
       finished(20), certificate_url(21), certificate_status(22),
       (255)
   } HandshakeType;

   struct {
       HandshakeType msg_type;    /* handshake type */
       uint24 length;             /* bytes in message */
       select (HandshakeType) {
           case hello_request:       HelloRequest;
           case client_hello:        ClientHello;
           case server_hello:        ServerHello;
           case certificate:         Certificate;
           case server_key_exchange: ServerKeyExchange;
           case certificate_request: CertificateRequest;
           case server_hello_done:   ServerHelloDone;
           case certificate_verify:  CertificateVerify;
           case client_key_exchange: ClientKeyExchange;
           case finished:            Finished;
           case certificate_url:     CertificateURL;
           case certificate_status:  CertificateStatus;
       } body;
   } Handshake;

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3.  Server Name Indication

   TLS does not provide a mechanism for a client to tell a server the
   name of the server it is contacting.  It may be desirable for clients
   to provide this information to facilitate secure connections to
   servers that host multiple 'virtual' servers at a single underlying
   network address.

   In order to provide any of the server names, clients MAY include an
   extension of type "server_name" in the (extended) client hello.  The
   "extension_data" field of this extension SHALL contain
   "ServerNameList" where:

      struct {
          NameType name_type;
          select (name_type) {
              case host_name: HostName;
          } name;
      } ServerName;

      enum {
          host_name(0), (255)
      } NameType;

      opaque HostName<1..2^16-1>;

      struct {
          ServerName server_name_list<1..2^16-1>
      } ServerNameList;

   The ServerNameList MUST NOT contain more than one name of the same
   name_type.  If the server understood the ClientHello extension but
   does not recognize the server name, the server SHOULD take one of two
   actions: either abort the handshake by sending a fatal-level
   unrecognized_name(112) alert or continue the handshake.  It is NOT
   RECOMMENDED to send a warning-level unrecognized_name(112) alert,
   because the client's behavior in response to warning-level alerts is
   unpredictable.  If there is a mismatch between the server name used
   by the client application and the server name of the credential
   chosen by the server, this mismatch will become apparent when the
   client application performs the server endpoint identification, at
   which point the client application will have to decide whether to
   proceed with the communication.  TLS implementations are encouraged
   to make information available to application callers about warning-
   level alerts that were received or sent during a TLS handshake.  Such
   information can be useful for diagnostic purposes.

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      Note: Earlier versions of this specification permitted multiple
      names of the same name_type.  In practice, current client
      implementations only send one name, and the client cannot
      necessarily find out which name the server selected.  Multiple
      names of the same name_type are therefore now prohibited.

   Currently, the only server names supported are DNS hostnames;
   however, this does not imply any dependency of TLS on DNS, and other
   name types may be added in the future (by an RFC that updates this
   document).  The data structure associated with the host_name NameType
   is a variable-length vector that begins with a 16-bit length.  For
   backward compatibility, all future data structures associated with
   new NameTypes MUST begin with a 16-bit length field.  TLS MAY treat
   provided server names as opaque data and pass the names and types to
   the application.

   "HostName" contains the fully qualified DNS hostname of the server,
   as understood by the client.  The hostname is represented as a byte
   string using ASCII encoding without a trailing dot.  This allows the
   support of internationalized domain names through the use of A-labels
   defined in [RFC5890].  DNS hostnames are case-insensitive.  The
   algorithm to compare hostnames is described in [RFC5890], Section
   2.3.2.4.

   Literal IPv4 and IPv6 addresses are not permitted in "HostName".

   It is RECOMMENDED that clients include an extension of type
   "server_name" in the client hello whenever they locate a server by a
   supported name type.

   A server that receives a client hello containing the "server_name"
   extension MAY use the information contained in the extension to guide
   its selection of an appropriate certificate to return to the client,
   and/or other aspects of security policy.  In this event, the server
   SHALL include an extension of type "server_name" in the (extended)
   server hello.  The "extension_data" field of this extension SHALL be
   empty.

   When the server is deciding whether or not to accept a request to
   resume a session, the contents of a server_name extension MAY be used
   in the lookup of the session in the session cache.  The client SHOULD
   include the same server_name extension in the session resumption
   request as it did in the full handshake that established the session.
   A server that implements this extension MUST NOT accept the request
   to resume the session if the server_name extension contains a
   different name.  Instead, it proceeds with a full handshake to
   establish a new session.  When resuming a session, the server MUST
   NOT include a server_name extension in the server hello.

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   If an application negotiates a server name using an application
   protocol and then upgrades to TLS, and if a server_name extension is
   sent, then the extension SHOULD contain the same name that was
   negotiated in the application protocol.  If the server_name is
   established in the TLS session handshake, the client SHOULD NOT
   attempt to request a different server name at the application layer.

4.  Maximum Fragment Length Negotiation

   Without this extension, TLS specifies a fixed maximum plaintext
   fragment length of 2^14 bytes.  It may be desirable for constrained
   clients to negotiate a smaller maximum fragment length due to memory
   limitations or bandwidth limitations.

   In order to negotiate smaller maximum fragment lengths, clients MAY
   include an extension of type "max_fragment_length" in the (extended)
   client hello.  The "extension_data" field of this extension SHALL
   contain:

      enum{
          2^9(1), 2^10(2), 2^11(3), 2^12(4), (255)
      } MaxFragmentLength;

   whose value is the desired maximum fragment length.  The allowed
   values for this field are: 2^9, 2^10, 2^11, and 2^12.

   Servers that receive an extended client hello containing a
   "max_fragment_length" extension MAY accept the requested maximum
   fragment length by including an extension of type
   "max_fragment_length" in the (extended) server hello.  The
   "extension_data" field of this extension SHALL contain a
   "MaxFragmentLength" whose value is the same as the requested maximum
   fragment length.

   If a server receives a maximum fragment length negotiation request
   for a value other than the allowed values, it MUST abort the
   handshake with an "illegal_parameter" alert.  Similarly, if a client
   receives a maximum fragment length negotiation response that differs
   from the length it requested, it MUST also abort the handshake with
   an "illegal_parameter" alert.

   Once a maximum fragment length other than 2^14 has been successfully
   negotiated, the client and server MUST immediately begin fragmenting
   messages (including handshake messages) to ensure that no fragment
   larger than the negotiated length is sent.  Note that TLS already
   requires clients and servers to support fragmentation of handshake
   messages.

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   The negotiated length applies for the duration of the session
   including session resumptions.

   The negotiated length limits the input that the record layer may
   process without fragmentation (that is, the maximum value of
   TLSPlaintext.length; see [RFC5246], Section 6.2.1).  Note that the
   output of the record layer may be larger.  For example, if the
   negotiated length is 2^9=512, then, when using currently defined
   cipher suites (those defined in [RFC5246] and [RFC2712]) and null
   compression, the record-layer output can be at most 805 bytes: 5
   bytes of headers, 512 bytes of application data, 256 bytes of
   padding, and 32 bytes of MAC.  This means that in this event a TLS
   record-layer peer receiving a TLS record-layer message larger than
   805 bytes MUST discard the message and send a "record_overflow"
   alert, without decrypting the message.  When this extension is used
   with Datagram Transport Layer Security (DTLS), implementations SHOULD
   NOT generate record_overflow alerts unless the packet passes message
   authentication.

5.  Client Certificate URLs

   Without this extension, TLS specifies that when client authentication
   is performed, client certificates are sent by clients to servers
   during the TLS handshake.  It may be desirable for constrained
   clients to send certificate URLs in place of certificates, so that
   they do not need to store their certificates and can therefore save
   memory.

   In order to negotiate sending certificate URLs to a server, clients
   MAY include an extension of type "client_certificate_url" in the
   (extended) client hello.  The "extension_data" field of this
   extension SHALL be empty.

   (Note that it is necessary to negotiate the use of client certificate
   URLs in order to avoid "breaking" existing TLS servers.)

   Servers that receive an extended client hello containing a
   "client_certificate_url" extension MAY indicate that they are willing
   to accept certificate URLs by including an extension of type
   "client_certificate_url" in the (extended) server hello.  The
   "extension_data" field of this extension SHALL be empty.

   After negotiation of the use of client certificate URLs has been
   successfully completed (by exchanging hellos including
   "client_certificate_url" extensions), clients MAY send a
   "CertificateURL" message in place of a "Certificate" message as
   follows (see also Section 2):

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      enum {
          individual_certs(0), pkipath(1), (255)
      } CertChainType;

      struct {
          CertChainType type;
          URLAndHash url_and_hash_list<1..2^16-1>;
      } CertificateURL;

      struct {
          opaque url<1..2^16-1>;
          unint8 padding;
          opaque SHA1Hash[20];
      } URLAndHash;

   Here, "url_and_hash_list" contains a sequence of URLs and hashes.
   Each "url" MUST be an absolute URI reference according to [RFC3986]
   that can be immediately used to fetch the certificate(s).

   When X.509 certificates are used, there are two possibilities:

   -  If CertificateURL.type is "individual_certs", each URL refers to a
      single DER-encoded X.509v3 certificate, with the URL for the
      client's certificate first.

   -  If CertificateURL.type is "pkipath", the list contains a single
      URL referring to a DER-encoded certificate chain, using the type
      PkiPath described in Section 10.1.

   When any other certificate format is used, the specification that
   describes use of that format in TLS should define the encoding format
   of certificates or certificate chains, and any constraint on their
   ordering.

   The "padding" byte MUST be 0x01.  It is present to make the structure
   backwards compatible.

   The hash corresponding to each URL is the SHA-1 hash of the
   certificate or certificate chain (in the case of X.509 certificates,
   the DER-encoded certificate or the DER-encoded PkiPath).

   Note that when a list of URLs for X.509 certificates is used, the
   ordering of URLs is the same as that used in the TLS Certificate
   message (see [RFC5246], Section 7.4.2), but opposite to the order in
   which certificates are encoded in PkiPath.  In either case, the self-
   signed root certificate MAY be omitted from the chain, under the
   assumption that the server must already possess it in order to
   validate it.

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   Servers receiving "CertificateURL" SHALL attempt to retrieve the
   client's certificate chain from the URLs and then process the
   certificate chain as usual.  A cached copy of the content of any URL
   in the chain MAY be used, provided that the SHA-1 hash matches the
   hash of the cached copy.

   Servers that support this extension MUST support the 'http' URI
   scheme for certificate URLs and MAY support other schemes.  Use of
   other schemes than 'http', 'https', or 'ftp' may create unexpected
   problems.

   If the protocol used is HTTP, then the HTTP server can be configured
   to use the Cache-Control and Expires directives described in
   [RFC2616] to specify whether and for how long certificates or
   certificate chains should be cached.

   The TLS server MUST NOT follow HTTP redirects when retrieving the
   certificates or certificate chain.  The URLs used in this extension
   MUST NOT be chosen to depend on such redirects.

   If the protocol used to retrieve certificates or certificate chains
   returns a MIME-formatted response (as HTTP does), then the following
   MIME Content-Types SHALL be used: when a single X.509v3 certificate
   is returned, the Content-Type is "application/pkix-cert" [RFC2585],
   and when a chain of X.509v3 certificates is returned, the Content-
   Type is "application/pkix-pkipath" (Section 10.1).

   The server MUST check that the SHA-1 hash of the contents of the
   object retrieved from that URL (after decoding any MIME Content-
   Transfer-Encoding) matches the given hash.  If any retrieved object
   does not have the correct SHA-1 hash, the server MUST abort the
   handshake with a bad_certificate_hash_value(114) alert.  This alert
   is always fatal.

   Clients may choose to send either "Certificate" or "CertificateURL"
   after successfully negotiating the option to send certificate URLs.
   The option to send a certificate is included to provide flexibility
   to clients possessing multiple certificates.

   If a server is unable to obtain certificates in a given
   CertificateURL, it MUST send a fatal certificate_unobtainable(111)
   alert if it requires the certificates to complete the handshake.  If
   the server does not require the certificates, then the server
   continues the handshake.  The server MAY send a warning-level alert
   in this case.  Clients receiving such an alert SHOULD log the alert
   and continue with the handshake if possible.

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6.  Trusted CA Indication

   Constrained clients that, due to memory limitations, possess only a
   small number of CA root keys may wish to indicate to servers which
   root keys they possess, in order to avoid repeated handshake
   failures.

   In order to indicate which CA root keys they possess, clients MAY
   include an extension of type "trusted_ca_keys" in the (extended)
   client hello.  The "extension_data" field of this extension SHALL
   contain "TrustedAuthorities" where:

      struct {
          TrustedAuthority trusted_authorities_list<0..2^16-1>;
      } TrustedAuthorities;

      struct {
          IdentifierType identifier_type;
          select (identifier_type) {
              case pre_agreed: struct {};
              case key_sha1_hash: SHA1Hash;
              case x509_name: DistinguishedName;
              case cert_sha1_hash: SHA1Hash;
          } identifier;
      } TrustedAuthority;

      enum {
          pre_agreed(0), key_sha1_hash(1), x509_name(2),
          cert_sha1_hash(3), (255)
      } IdentifierType;

      opaque DistinguishedName<1..2^16-1>;

   Here, "TrustedAuthorities" provides a list of CA root key identifiers
   that the client possesses.  Each CA root key is identified via
   either:

   -  "pre_agreed": no CA root key identity supplied.

   -  "key_sha1_hash": contains the SHA-1 hash of the CA root key.  For
      Digital Signature Algorithm (DSA) and Elliptic Curve Digital
      Signature Algorithm (ECDSA) keys, this is the hash of the
      "subjectPublicKey" value.  For RSA keys, the hash is of the big-
      endian byte string representation of the modulus without any
      initial zero-valued bytes.  (This copies the key hash formats
      deployed in other environments.)

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   -  "x509_name": contains the DER-encoded X.509 DistinguishedName of
      the CA.

   -  "cert_sha1_hash": contains the SHA-1 hash of a DER-encoded
      Certificate containing the CA root key.

   Note that clients may include none, some, or all of the CA root keys
   they possess in this extension.

   Note also that it is possible that a key hash or a Distinguished Name
   alone may not uniquely identify a certificate issuer (for example, if
   a particular CA has multiple key pairs).  However, here we assume
   this is the case following the use of Distinguished Names to identify
   certificate issuers in TLS.

   The option to include no CA root keys is included to allow the client
   to indicate possession of some pre-defined set of CA root keys.

   Servers that receive a client hello containing the "trusted_ca_keys"
   extension MAY use the information contained in the extension to guide
   their selection of an appropriate certificate chain to return to the
   client.  In this event, the server SHALL include an extension of type
   "trusted_ca_keys" in the (extended) server hello.  The
   "extension_data" field of this extension SHALL be empty.

7.  Truncated HMAC

   Currently defined TLS cipher suites use the MAC construction HMAC
   [RFC2104] to authenticate record-layer communications.  In TLS, the
   entire output of the hash function is used as the MAC tag.  However,
   it may be desirable in constrained environments to save bandwidth by
   truncating the output of the hash function to 80 bits when forming
   MAC tags.

   In order to negotiate the use of 80-bit truncated HMAC, clients MAY
   include an extension of type "truncated_hmac" in the extended client
   hello.  The "extension_data" field of this extension SHALL be empty.

   Servers that receive an extended hello containing a "truncated_hmac"
   extension MAY agree to use a truncated HMAC by including an extension
   of type "truncated_hmac", with empty "extension_data", in the
   extended server hello.

   Note that if new cipher suites are added that do not use HMAC, and
   the session negotiates one of these cipher suites, this extension
   will have no effect.  It is strongly recommended that any new cipher
   suites using other MACs consider the MAC size an integral part of the

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   cipher suite definition, taking into account both security and
   bandwidth considerations.

   If HMAC truncation has been successfully negotiated during a TLS
   handshake, and the negotiated cipher suite uses HMAC, both the client
   and the server pass this fact to the TLS record layer along with the
   other negotiated security parameters.  Subsequently during the
   session, clients and servers MUST use truncated HMACs, calculated as
   specified in [RFC2104].  That is, SecurityParameters.mac_length is 10
   bytes, and only the first 10 bytes of the HMAC output are transmitted
   and checked.  Note that this extension does not affect the
   calculation of the pseudo-random function (PRF) as part of
   handshaking or key derivation.

   The negotiated HMAC truncation size applies for the duration of the
   session including session resumptions.

8.  Certificate Status Request

   Constrained clients may wish to use a certificate-status protocol
   such as OCSP [RFC2560] to check the validity of server certificates,
   in order to avoid transmission of CRLs and therefore save bandwidth
   on constrained networks.  This extension allows for such information
   to be sent in the TLS handshake, saving roundtrips and resources.

   In order to indicate their desire to receive certificate status
   information, clients MAY include an extension of type
   "status_request" in the (extended) client hello.  The
   "extension_data" field of this extension SHALL contain
   "CertificateStatusRequest" where:

      struct {
          CertificateStatusType status_type;
          select (status_type) {
              case ocsp: OCSPStatusRequest;
          } request;
      } CertificateStatusRequest;

      enum { ocsp(1), (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>;

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   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).

   Servers that receive a client hello containing the "status_request"
   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, then the server MUST have included an
   extension of type "status_request" 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 Section 2):

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

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

   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.

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   Note that a server MAY also choose not to send a "CertificateStatus"
   message, even if has received a "status_request" extension in the
   client hello message and has sent a "status_request" extension in the
   server hello message.

   Note in addition that a server MUST NOT send the "CertificateStatus"
   message unless it received a "status_request" extension in the client
   hello message and sent a "status_request" 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
   bad_certificate_status_response(113) alert.  This alert is always
   fatal.

9.  Error Alerts

   Four new error alerts are defined for use with the TLS extensions
   defined in this document.  To avoid "breaking" existing clients and
   servers, these alerts MUST NOT be sent unless the sending party has
   received an extended hello message from the party they are
   communicating with.  These error alerts are conveyed using the
   following syntax.  The new alerts are the last four, as indicated by
   the comments on the same line as the error alert number.

      enum {
          close_notify(0),
          unexpected_message(10),
          bad_record_mac(20),
          decryption_failed(21),
          record_overflow(22),
          decompression_failure(30),
          handshake_failure(40),
          /* 41 is not defined, for historical reasons */
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter(47),
          unknown_ca(48),
          access_denied(49),
          decode_error(50),
          decrypt_error(51),
          export_restriction(60),
          protocol_version(70),
          insufficient_security(71),

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          internal_error(80),
          user_canceled(90),
          no_renegotiation(100),
          unsupported_extension(110),
          certificate_unobtainable(111),        /* new */
          unrecognized_name(112),               /* new */
          bad_certificate_status_response(113), /* new */
          bad_certificate_hash_value(114),      /* new */
          (255)
      } AlertDescription;

   "certificate_unobtainable" is described in Section 5.
   "unrecognized_name" is described in Section 3.
   "bad_certificate_status_response" is described in Section 8.
   "bad_certificate_hash_value" is described in Section 5.

10.  IANA Considerations

   IANA Considerations for TLS extensions and the creation of a registry
   are covered in Section 12 of [RFC5246] except for the registration of
   MIME type application/pkix-pkipath, which appears below.

   The IANA TLS extensions and MIME type application/pkix-pkipath
   registry entries that reference RFC 4366 have been updated to
   reference this document.

10.1.  pkipath MIME Type Registration

   MIME media type name: application
   MIME subtype name: pkix-pkipath
   Required parameters: none

   Optional parameters: version (default value is "1")

   Encoding considerations:
      Binary; this MIME type is a DER encoding of the ASN.1 type
      PkiPath, defined as follows:
        PkiPath ::= SEQUENCE OF Certificate
        PkiPath is used to represent a certification path.  Within the
        sequence, the order of certificates is such that the subject of
        the first certificate is the issuer of the second certificate,
        etc.
      This is identical to the definition published in [X509-4th-TC1];
      note that it is different from that in [X509-4th].

      All Certificates MUST conform to [RFC5280].  (This should be
      interpreted as a requirement to encode only PKIX-conformant
      certificates using this type.  It does not necessarily require

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      that all certificates that are not strictly PKIX-conformant must
      be rejected by relying parties, although the security consequences
      of accepting any such certificates should be considered
      carefully.)

      DER (as opposed to BER) encoding MUST be used.  If this type is
      sent over a 7-bit transport, base64 encoding SHOULD be used.

   Security considerations:
      The security considerations of [X509-4th] and [RFC5280] (or any
      updates to them) apply, as well as those of any protocol that uses
      this type (e.g., TLS).

      Note that this type only specifies a certificate chain that can be
      assessed for validity according to the relying party's existing
      configuration of trusted CAs; it is not intended to be used to
      specify any change to that configuration.

   Interoperability considerations:
      No specific interoperability problems are known with this type,
      but for recommendations relating to X.509 certificates in general,
      see [RFC5280].

   Published specification: This document and [RFC5280].

   Applications that use this media type:
      TLS.  It may also be used by other protocols or for general
      interchange of PKIX certificate chains.

   Additional information:
      Magic number(s): DER-encoded ASN.1 can be easily recognized.
        Further parsing is required to distinguish it from other ASN.1
        types.
      File extension(s): .pkipath
      Macintosh File Type Code(s): not specified

   Person & email address to contact for further information:
      Magnus Nystrom <mnystrom@microsoft.com>

   Intended usage: COMMON

   Change controller: IESG <iesg@ietf.org>

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10.2.  Reference for TLS Alerts, TLS HandshakeTypes, and ExtensionTypes

   The following values in the TLS Alert Registry have been updated to
   reference this document:

      111 certificate_unobtainable
      112 unrecognized_name
      113 bad_certificate_status_response
      114 bad_certificate_hash_value

   The following values in the TLS HandshakeType Registry have been
   updated to reference this document:

      21 certificate_url
      22 certificate_status

   The following ExtensionType values have been updated to reference
   this document:

      0 server_name
      1 max_fragment_length
      2 client_certificate_url
      3 trusted_ca_keys
      4 truncated_hmac
      5 status_request

11.  Security Considerations

   General security considerations for TLS extensions are covered in
   [RFC5246].  Security Considerations for particular extensions
   specified in this document are given below.

   In general, implementers should continue to monitor the state of the
   art and address any weaknesses identified.

11.1.  Security Considerations for server_name

   If a single server hosts several domains, then clearly it is
   necessary for the owners of each domain to ensure that this satisfies
   their security needs.  Apart from this, server_name does not appear
   to introduce significant security issues.

   Since it is possible for a client to present a different server_name
   in the application protocol, application server implementations that
   rely upon these names being the same MUST check to make sure the
   client did not present a different name in the application protocol.

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   Implementations MUST ensure that a buffer overflow does not occur,
   whatever the values of the length fields in server_name.

11.2.  Security Considerations for max_fragment_length

   The maximum fragment length takes effect immediately, including for
   handshake messages.  However, that does not introduce any security
   complications that are not already present in TLS, since TLS requires
   implementations to be able to handle fragmented handshake messages.

   Note that, as described in Section 4, once a non-null cipher suite
   has been activated, the effective maximum fragment length depends on
   the cipher suite and compression method, as well as on the negotiated
   max_fragment_length.  This must be taken into account when sizing
   buffers and checking for buffer overflow.

11.3.  Security Considerations for client_certificate_url

   Support for client_certificate_url involves the server's acting as a
   client in another URI-scheme-dependent protocol.  The server
   therefore becomes subject to many of the same security concerns that
   clients of the URI scheme are subject to, with the added concern that
   the client can attempt to prompt the server to connect to some
   (possibly weird-looking) URL.

   In general, this issue means that an attacker might use the server to
   indirectly attack another host that is vulnerable to some security
   flaw.  It also introduces the possibility of denial-of-service
   attacks in which an attacker makes many connections to the server,
   each of which results in the server's attempting a connection to the
   target of the attack.

   Note that the server may be behind a firewall or otherwise able to
   access hosts that would not be directly accessible from the public
   Internet.  This could exacerbate the potential security and denial-
   of-service problems described above, as well as allow the existence
   of internal hosts to be confirmed when they would otherwise be
   hidden.

   The detailed security concerns involved will depend on the URI
   schemes supported by the server.  In the case of HTTP, the concerns
   are similar to those that apply to a publicly accessible HTTP proxy
   server.  In the case of HTTPS, loops and deadlocks may be created,
   and this should be addressed.  In the case of FTP, attacks arise that
   are similar to FTP bounce attacks.

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   As a result of this issue, it is RECOMMENDED that the
   client_certificate_url extension should have to be specifically
   enabled by a server administrator, rather than be enabled by default.
   It is also RECOMMENDED that URI schemes be enabled by the
   administrator individually, and only a minimal set of schemes be
   enabled.  Unusual protocols that offer limited security or whose
   security is not well understood SHOULD be avoided.

   As discussed in [RFC3986], URLs that specify ports other than the
   default may cause problems, as may very long URLs (which are more
   likely to be useful in exploiting buffer overflow bugs).

   This extension continues to use SHA-1 (as in RFC 4366) and does not
   provide algorithm agility.  The property required of SHA-1 in this
   case is second pre-image resistance, not collision resistance.
   Furthermore, even if second pre-image attacks against SHA-1 are found
   in the future, an attack against client_certificate_url would require
   a second pre-image that is accepted as a valid certificate by the
   server and contains the same public key.

   Also note that HTTP caching proxies are common on the Internet, and
   some proxies do not check for the latest version of an object
   correctly.  If a request using HTTP (or another caching protocol)
   goes through a misconfigured or otherwise broken proxy, the proxy may
   return an out-of-date response.

11.4.  Security Considerations for trusted_ca_keys

   Potentially, the CA root keys a client possesses could be regarded as
   confidential information.  As a result, the CA root key indication
   extension should be used with care.

   The use of the SHA-1 certificate hash alternative ensures that each
   certificate is specified unambiguously.  This context does not
   require a cryptographic hash function, so the use of SHA-1 is
   considered acceptable, and no algorithm agility is provided.

11.5.  Security Considerations for truncated_hmac

   It is possible that truncated MACs are weaker than "un-truncated"
   MACs.  However, no significant weaknesses are currently known or
   expected to exist for HMAC with MD5 or SHA-1, truncated to 80 bits.

   Note that the output length of a MAC need not be as long as the
   length of a symmetric cipher key, since forging of MAC values cannot
   be done off-line: in TLS, a single failed MAC guess will cause the
   immediate termination of the TLS session.

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   Since the MAC algorithm only takes effect after all handshake
   messages that affect extension parameters have been authenticated by
   the hashes in the Finished messages, it is not possible for an active
   attacker to force negotiation of the truncated HMAC extension where
   it would not otherwise be used (to the extent that the handshake
   authentication is secure).  Therefore, in the event that any security
   problems were found with truncated HMAC in the future, if either the
   client or the server for a given session were updated to take the
   problem into account, it would be able to veto use of this extension.

11.6.  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.

12.  Normative References

   [RFC2104]      Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                  Keyed-Hashing for Message Authentication", RFC 2104,
                  February 1997.

   [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.

   [RFC2585]      Housley, R. and P. Hoffman, "Internet X.509 Public Key
                  Infrastructure Operational Protocols: FTP and HTTP",
                  RFC 2585, May 1999.

   [RFC2616]      Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                  Masinter, L., Leach, P., and T. Berners-Lee,
                  "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                  June 1999.

   [RFC3986]      Berners-Lee, T., Fielding, R., and L. Masinter,
                  "Uniform Resource Identifier (URI): Generic Syntax",
                  STD 66, RFC 3986, January 2005.

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   [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.

   [RFC5890]      Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Definitions and Document
                  Framework", RFC 5890, August 2010.

13.  Informative References

   [RFC2712]      Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
                  Suites to Transport Layer Security (TLS)", RFC 2712,
                  October 1999.

   [X509-4th]     ITU-T Recommendation X.509 (2000) | ISO/IEC
                  9594-8:2001, "Information Systems - Open Systems
                  Interconnection - The Directory: Public key and
                  attribute certificate frameworks".

   [X509-4th-TC1] ITU-T Recommendation X.509(2000) Corrigendum 1(2001) |
                  ISO/IEC 9594-8:2001/Cor.1:2002, Technical Corrigendum
                  1 to ISO/IEC 9594:8:2001.

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Appendix A.  Changes from RFC 4366

   The significant changes between RFC 4366 and this document are
   described below.

   RFC 4366 described both general extension mechanisms (for the TLS
   handshake and client and server hellos) as well as specific
   extensions.  RFC 4366 was associated with RFC 4346, TLS 1.1.  The
   client and server hello extension mechanisms have been moved into RFC
   5246, TLS 1.2, so this document, which is associated with RFC 5246,
   includes only the handshake extension mechanisms and the specific
   extensions from RFC 4366.  RFC 5246 also specifies the unknown
   extension error and new extension specification considerations, so
   that material has been removed from this document.

   The Server Name extension now specifies only ASCII representation,
   eliminating UTF-8.  It is provided that the ServerNameList can
   contain more than only one name of any particular name_type.  If a
   server name is provided but not recognized, the server should either
   continue the handshake without an error or send a fatal error.
   Sending a warning-level message is not recommended because client
   behavior will be unpredictable.  Provision was added for the user
   using the server_name extension in deciding whether or not to resume
   a session.  Furthermore, this extension should be the same in a
   session resumption request as it was in the full handshake that
   established the session.  Such a resumption request must not be
   accepted if the server_name extension is different, but instead a
   full handshake must be done to possibly establish a new session.

   The Client Certificate URLs extension has been changed to make the
   presence of a hash mandatory.

   For the case of DTLS, the requirement to report an overflow of the
   negotiated maximum fragment length is made conditional on passing
   authentication.

   TLS servers are now prohibited from following HTTP redirects when
   retrieving certificates.

   The material was also re-organized in minor ways.  For example,
   information as to which errors are fatal is moved from the "Error
   Alerts" section to the individual extension specifications.

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Appendix B.  Acknowledgements

   This document is based on material from RFC 4366 for which the
   authors were S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen,
   and T. Wright.  Other contributors include Joseph Salowey, Alexey
   Melnikov, Peter Saint-Andre, and Adrian Farrel.

Author's Address

   Donald Eastlake 3rd
   Huawei
   155 Beaver Street
   Milford, MA 01757 USA

   Phone: +1-508-333-2270
   EMail: d3e3e3@gmail.com

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