TLS Working Group                                    Donald Eastlake 3rd
INTERNET-DRAFT                                          Stellar Switches
Obsoletes: RFC 4366
Intended status: Proposed Standard
Expires: December 21, 2009                                 June 22, 2009

    Transport Layer Security (TLS) Extensions: Extension Definitions

Status of This Document

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79. 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.

   Distribution of this document is unlimited.  Comments should be sent
   to the TLS working group mailing list <>.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at

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   This document provides specifications for existing TLS extensions. It
   is a companion document for the TLS 1.2 specification [RFC5246]. The
   extensions specified are server_name, max_fragment_length,
   client_certificate_url, trusted_ca_keys, truncated_hmac, and

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   This draft is based on material from RFC 4366 for which the authors
   were S. Blake-Wilson, M. Nystron, D. Hopwood, J. Mikkelsen, and T.

Table of Contents

      Status of This Document....................................1
      Table of Contents..........................................2

      1. Introduction............................................3
      1.1 Specific Extensions Covered............................3
      1.2 Conventions Used in This Document......................4

      2. Extensions to the Handshake Protocol....................5
      3. Server Name Indication..................................6
      4. Maximum Fragment Length Negotiation.....................8
      5. Client Certificate URLs................................10
      6. Trusted CA Indication..................................13
      7. Truncated HMAC.........................................15
      8. Certificate Status Request.............................16
      9. Error Alerts...........................................18
      10. IANA Considerations...................................19

      11. Security Considerations...............................19
      11.1 Security Considerations for server_name..............19
      11.2 Security Considerations for max_fragment_length......19
      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...........21

      12. Normative References..................................22
      13. Informative References................................22

      Annex A: pkipath MIME Type Registration...................24
      Annex B: Changes from RFC 4366............................26
      Author's Address..........................................27
      Copyright and IPR Provisions..............................28

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1. Introduction

   The TLS (Transport Layer Security) Protocol Version 1.2 is specified
   in [RFC5246]. That specification includes the framework for
   extensions to TLS, considerations in designing such extensions (see
   Section 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 of [RFC5246]).

   This document provides the specifications for existing TLS
   extensions. It is, for the most part, the adaptation and editing of
   material from [RFC4366], which covered TLS extensions for TLS 1.0
   [RFC2246] and TLS 1.1 [RFC4346].

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.

   -  Allow TLS clients to indicate to TLS servers which CA root keys
      they possess. This functionality is desirable in order to prevent
      multiple handshake failures involving TLS clients that are only

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      able to store a small number of CA root keys due to memory

   -  Allow TLS clients and servers to negotiate the use of truncated
      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 save 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.

1.2 Conventions Used in This Document

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

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2. Extensions to the Handshake Protocol

   This document specifies the use of two new handshake messages,
   "CertificateURL" and "CertificateStatus".  These messages are
   described in Section 5 and Section 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),
      } 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;

   If the server understood the client hello extension but does not
   recognize any of the server names, it SHOULD send an
   unrecognized_name(112) alert (which MAY be fatal).

   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). 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.

   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.

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   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

   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.

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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

          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

   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 for currently defined cipher
   suites (those defined in [RFC5246], [RFC2712], and [RFC3268]), and

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   when null compression is used, 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 may discard the message and send a "record_overflow"
   alert, without decrypting the message.

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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 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):

      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>;
          opaque padding<1>;
          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

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   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 Annex A.

   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

   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.

   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

   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 is not required to follow HTTP redirects when
   retrieving the certificates or certificate chain. The URLs used in
   this extension SHOULD therefore be chosen not to depend on such

   If the protocol used to retrieve certificates or certificate chains
   returns a MIME-formatted response (as HTTP does), then the following

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   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" Annex A.

   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 encounters an unreasonable delay in obtaining
   certificates in a given CertificateURL, it SHOULD time out and signal
   a certificate_unobtainable(111) error alert. This alert MAY be fatal;
   for example, if client authentication is required by the server for
   the handshake to continue.

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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

   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 0-valued bytes. (This copies the key hash formats deployed
      in other environments.)

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

   -  "cert_sha1_hash": contains the SHA-1 hash of a DER-encoded

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      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.

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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
   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

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

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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"

      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>;

   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"

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   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

   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.

   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

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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 {
          /* 41 is not defined, for historical reasons */
          certificate_unobtainable(111),        /* new */
          unrecognized_name(112),               /* new */
          bad_certificate_status_response(113), /* new */
          bad_certificate_hash_value(114),      /* new */
      } 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.

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10. IANA Considerations

   IANA Considerations for TLS Extensions and the creation of a Registry
   therefore are covered in Section 12 of [RFC5246] except for the
   registration of MIME type application/pkix-pkipath.  This MIME type
   has already been registered but is reproduced in Annex A for

   The IANA TLS extensions registry entries that reference [RFC4366]
   should be updated to reference this document on its publication as an

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.

   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.

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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.

   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).

   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.

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11.4 Security Considerations for trusted_ca_keys

   It is possible that which 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. As for the previous
   extension, it was not believed necessary to use both MD5 and SHA-1

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.

   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
   problem 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.

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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

   [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

   [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

13. Informative References

   [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
   RFC 2246, January 1999.

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

   [RFC3268] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites
   for Transport Layer Security (TLS)", RFC 3268, June 2002.

   [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
   (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
   and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366,

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   April 2006.

   [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

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Annex A: pkipath MIME Type Registration

   The MIME type application/pkix-pkipath has been registered.  A copy
   of its template is included here for convenience:

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

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

   Encoding considerations:
      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,
      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
      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

      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: [RFC4366], and [RFC5280].

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

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

   Person & email address to contact for further information:
      Magnus Nystrom <>

   Intended usage: COMMON

   Change controller: IESG <>

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Annex B: 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.

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

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

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Author's Address

   Donald Eastlake 3rd
   Stellar Switches, Inc.
   155 Beaver Street
   Milford, MA 01757 USA

   Tel:   +1-508-634-2066

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Copyright and IPR Provisions

   Copyright (c) 2009 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 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.

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   the avoidance of doubt, each Contributor to the IETF Standards
   Process licenses each Contribution that he or she makes as part of
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