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RFC 3546 - Transport Layer Security (TLS) Extensions

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Network Working Group                                    S. Blake-Wilson
Request for Comments: 3546                                           BCI
Updates: 2246                                                 M. Nystrom
Category: Standards Track                                   RSA Security
                                                              D. Hopwood
                                                  Independent Consultant
                                                            J. Mikkelsen
                                                               T. Wright
                                                               June 2003

               Transport Layer Security (TLS) Extensions

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This document describes extensions that may be used to add
   functionality to Transport Layer Security (TLS).  It provides both
   generic extension mechanisms for the TLS handshake client and server
   hellos, and specific extensions using these generic mechanisms.

   The extensions may be used by TLS clients and servers.  The
   extensions are backwards compatible - communication is possible
   between TLS 1.0 clients that support the extensions and TLS 1.0
   servers that do not support the extensions, and vice versa.

Conventions used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

Table of Contents

   1.  Introduction .............................................  2
   2.  General Extension Mechanisms .............................  4
       2.1. Extended Client Hello ...............................  5
       2.2. Extended Server Hello ...............................  5
       2.3. Hello Extensions ....................................  6
       2.4. Extensions to the handshake protocol ................  7
   3.  Specific Extensions ......................................  8
       3.1. Server Name Indication ..............................  8
       3.2. Maximum Fragment Length Negotiation ................. 10
       3.3. Client Certificate URLs ............................. 11
       3.4. Trusted CA Indication ............................... 14
       3.5. Truncated HMAC ...................................... 15
       3.6. Certificate Status Request........................... 16
   4. Error alerts .............................................. 18
   5. Procedure for Defining New Extensions...................... 20
   6.  Security Considerations .................................. 21
       6.1. Security of server_name ............................. 21
       6.2. Security of max_fragment_length ..................... 21
       6.3. Security of client_certificate_url .................. 22
       6.4. Security of trusted_ca_keys ......................... 23
       6.5. Security of truncated_hmac .......................... 23
       6.6. Security of status_request .......................... 24
   7.  Internationalization Considerations ...................... 24
   8.  IANA Considerations ...................................... 24
   9.  Intellectual Property Rights ............................. 26
   10. Acknowledgments .......................................... 26
   11. Normative References ..................................... 27
   12. Informative References ................................... 28
   13. Authors' Addresses ....................................... 28
   14. Full Copyright Statement ................................. 29

1. Introduction

   This document describes extensions that may be used to add
   functionality to Transport Layer Security (TLS).  It provides both
   generic extension mechanisms for the TLS handshake client and server
   hellos, and specific extensions using these generic mechanisms.

   TLS is now used in an increasing variety of operational environments
   - many of which were not envisioned when the original design criteria
   for TLS were determined.  The extensions introduced in this document
   are designed to enable TLS to operate as effectively as possible in
   new environments like wireless networks.

   Wireless environments often suffer from a number of constraints not
   commonly present in wired environments.  These constraints may
   include bandwidth limitations, computational power limitations,
   memory limitations, and battery life limitations.

   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.

   Specifically, the extensions described in this document are designed

   -  Allow TLS clients to provide to the TLS server the name of the
      server they are contacting.  This functionality is desirable 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
      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) [OCSP] 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.

   In order to support the extensions above, general extension
   mechanisms for the client hello message and the server hello message
   are introduced.

   The extensions described in this document may be used by TLS 1.0
   clients and TLS 1.0 servers.  The extensions are designed to be
   backwards compatible - meaning that TLS 1.0 clients that support the
   extensions can talk to TLS 1.0 servers that do not support the
   extensions, and vice versa.

   Backwards compatibility is primarily achieved via two considerations:

   -  Clients typically request the use of extensions via the extended
      client hello message described in Section 2.1. TLS 1.0 [TLS]
      requires servers to accept extended client hello messages, even if
      the server does not "understand" the extension.

   -  For the specific extensions described here, no mandatory server
      response is required when clients request extended functionality.

   Note however, that although backwards compatibility is supported,
   some constrained clients may be forced to reject communications with
   servers that do not support the extensions as a result of the limited
   capabilities of such clients.

   The remainder of this document is organized as follows.  Section 2
   describes general extension mechanisms for the client hello and
   server hello handshake messages.  Section 3 describes specific
   extensions to TLS 1.0.  Section 4 describes new error alerts for use
   with the TLS extensions.  The final sections of the document address
   IPR, security considerations, registration of the application/pkix-
   pkipath MIME type, acknowledgements, and references.

2. General Extension Mechanisms

   This section presents general extension mechanisms for the TLS
   handshake client hello and server hello messages.

   These general extension mechanisms are necessary in order to enable
   clients and servers to negotiate whether to use specific extensions,
   and how to use specific extensions.  The extension formats described
   are based on [MAILING LIST].

   Section 2.1 specifies the extended client hello message format,
   Section 2.2 specifies the extended server hello message format, and
   Section 2.3 describes the actual extension format used with the
   extended client and server hellos.

2.1. Extended Client Hello

   Clients MAY request extended functionality from servers by sending
   the extended client hello message format in place of the client hello
   message format.  The extended client hello message format is:

      struct {
          ProtocolVersion client_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suites<2..2^16-1>;
          CompressionMethod compression_methods<1..2^8-1>;
          Extension client_hello_extension_list<0..2^16-1>;
      } ClientHello;

   Here the new "client_hello_extension_list" field contains a list of
   extensions.  The actual "Extension" format is defined in Section 2.3.

   In the event that a client requests additional functionality using
   the extended client hello, and this functionality is not supplied by
   the server, the client MAY abort the handshake.

   Note that [TLS], Section, allows additional information to be
   added to the client hello message.  Thus the use of the extended
   client hello defined above should not "break" existing TLS 1.0

   A server that supports the extensions mechanism MUST accept only
   client hello messages in either the original or extended ClientHello
   format, and (as for all other messages) MUST check that the amount of
   data in the message precisely matches one of these formats; if not
   then it MUST send a fatal "decode_error" alert.  This overrides the
   "Forward compatibility note" in [TLS].

2.2. Extended Server Hello

   The extended server hello message format MAY be sent in place of the
   server hello message when the client has requested extended
   functionality via the extended client hello message specified in
   Section 2.1.  The extended server hello message format is:

      struct {
          ProtocolVersion server_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suite;
          CompressionMethod compression_method;
          Extension server_hello_extension_list<0..2^16-1>;
      } ServerHello;

   Here the new "server_hello_extension_list" field contains a list of
   extensions.  The actual "Extension" format is defined in Section 2.3.

   Note that the extended server hello message is only sent in response
   to an extended client hello message.  This prevents the possibility
   that the extended server hello message could "break" existing TLS 1.0

2.3. Hello Extensions

   The extension format for extended client hellos and extended server
   hellos is:

      struct {
          ExtensionType extension_type;
          opaque extension_data<0..2^16-1>;
      } Extension;


   - "extension_type" identifies the particular extension type.

   - "extension_data" contains information specific to the particular
   extension type.

   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;

   Note that for all extension types (including those defined in
   future), the extension type MUST NOT appear in the extended server
   hello unless the same extension type appeared in the corresponding
   client hello.  Thus clients MUST abort the handshake if they receive
   an extension type in the extended server hello that they did not
   request in the associated (extended) client hello.

   Nonetheless "server initiated" extensions may be provided in the
   future within this framework by requiring the client to first send an
   empty extension to indicate that it supports a particular extension.

   Also note that when multiple extensions of different types are
   present in the extended client hello or the extended server hello,
   the extensions may appear in any order.  There MUST NOT be more than
   one extension of the same type.

   Finally note that all the extensions defined in this document are
   relevant only when a session is initiated.  However, a client that
   requests resumption of a session does not in general know whether the
   server will accept this request, and therefore it SHOULD send an
   extended client hello if it would normally do so for a new session.
   If the resumption request is denied, then a new set of extensions
   will be negotiated as normal.  If, on the other hand, the older
   session is resumed, then the server MUST ignore extensions appearing
   in the client hello, and send a server hello containing no
   extensions; in this case the extension functionality negotiated
   during the original session initiation is applied to the resumed

2.4. Extensions to the handshake protocol

   This document suggests the use of two new handshake messages,
   "CertificateURL" and "CertificateStatus".  These messages are
   described in Section 3.3 and Section 3.6, 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;

3. Specific Extensions

   This section describes the specific TLS extensions specified in this

   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.

   Section 3.1 describes the extension of TLS to allow a client to
   indicate which server it is contacting.  Section 3.2 describes the
   extension to provide maximum fragment length negotiation.  Section
   3.3 describes the extension to allow client certificate URLs.
   Section 3.4 describes the extension to allow a client to indicate
   which CA root keys it possesses.  Section 3.5 describes the extension
   to allow the use of truncated HMAC.  Section 3.6 describes the
   extension to support integration of certificate status information
   messages into TLS handshakes.

3.1. 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 the server name, 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;

   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 UTF-8 encoding [UTF8], without a trailing dot.

   If the hostname labels contain only US-ASCII characters, then the
   client MUST ensure that labels are separated only by the byte 0x2E,
   representing the dot character U+002E (requirement 1 in section 3.1
   of [IDNA] notwithstanding). If the server needs to match the HostName
   against names that contain non-US-ASCII characters, it MUST perform
   the conversion operation described in section 4 of [IDNA], treating
   the HostName as a "query string" (i.e. the AllowUnassigned flag MUST
   be set). Note that IDNA allows labels to be separated by any of the
   Unicode characters U+002E, U+3002, U+FF0E, and U+FF61, therefore
   servers MUST accept any of these characters as a label separator.  If
   the server only needs to match the HostName against names containing
   exclusively ASCII characters, it MUST compare ASCII names case-

   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.

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

   If an application negotiates a server name using an application
   protocol, then upgrades to TLS, and 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.

3.2. Maximum Fragment Length Negotiation

   [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
   "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 [TLS] 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 [TLS], [KERB], and [AESSUITES]), and when null compression
   is used, the record layer output can be at most 793 bytes: 5 bytes of
   headers, 512 bytes of application data, 256 bytes of padding, and 20
   bytes of MAC.  That means that in this event a TLS record layer peer
   receiving a TLS record layer message larger than 793 bytes may
   discard the message and send a "record_overflow" alert, without
   decrypting the message.

3.3. Client Certificate URLs

   [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 to send 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 1.0 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:

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

      enum {
          false(0), true(1)
      } Boolean;

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

      struct {
          opaque url<1..2^16-1>;
          Boolean hash_present;
          select (hash_present) {
              case false: struct {};
              case true: SHA1Hash;
          } hash;
      } URLAndOptionalHash;

      opaque SHA1Hash[20];

   Here "url_and_hash_list" contains a sequence of URLs and optional

   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, or

   -  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 8.

   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 hash corresponding to each URL at the client's discretion is
   either not present or 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 [TLS] 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 a SHA-1 hash is present for
   that URL and it matches the hash of the cached copy.

   Servers that support this extension MUST support the http: URL scheme
   for certificate URLs, and MAY support other schemes.

   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" [PKIOP], and
   when a chain of X.509v3 certificates is returned, the Content-Type is
   "application/pkix-pkipath" (see Section 8).

   If a SHA-1 hash is present for an URL, then 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" alert.

   Note that 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" error alert.

3.4. 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

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

   -  "key_sha1_hash" - contains the SHA-1 hash of the CA root key.  For
      DSA and 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
      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.

3.5. Truncated HMAC

   Currently defined TLS cipher suites use the MAC construction HMAC
   with either MD5 or SHA-1 [HMAC] 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 as 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 [HMAC].  That is, CipherSpec.hash_size 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 PRF as part of handshaking or key derivation.

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

3.6. Certificate Status Request

   Constrained clients may wish to use a certificate-status protocol
   such as OCSP [OCSP] 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>;

   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 [OCSP].  "Extensions" is imported from [PKIX].  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, [OCSP] 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

   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.

      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 [OCSP]).  Note that
   only one OCSP response may be sent.

   The "CertificateStatus" message is conveyed using the handshake
   message type "certificate_status".

   Note that a server MAY also choose not to send a "CertificateStatus"
   message, even if it receives a "status_request" extension in the
   client hello message.

   Note in addition that servers MUST NOT send the "CertificateStatus"
   message unless it received a "status_request" extension in the client
   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.

4. Error Alerts

   This section defines new error alerts for use with the TLS extensions
   defined in this document.

   The following new error alerts are defined.  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.

   -  "unsupported_extension" - this alert is sent by clients that
      receive an extended server hello containing an extension that they
      did not put in the corresponding client hello (see Section 2.3).
      This message is always fatal.

   -  "unrecognized_name" - this alert is sent by servers that receive a
      server_name extension request, but do not recognize the server
      name.  This message MAY be fatal.

   -  "certificate_unobtainable" - this alert is sent by servers who are
      unable to retrieve a certificate chain from the URL supplied by
      the client (see Section 3.3).  This message MAY be fatal - for
      example if client authentication is required by the server for the
      handshake to continue and the server is unable to retrieve the
      certificate chain, it may send a fatal alert.

   -  "bad_certificate_status_response" - this alert is sent by clients
      that receive an invalid certificate status response (see Section
      3.6).  This message is always fatal.

   -  "bad_certificate_hash_value" - this alert is sent by servers when
      a certificate hash does not match a client provided
      certificate_hash.  This message is always fatal.

   These error alerts are conveyed using the following syntax:

      enum {
          /* 41 is not defined, for historical reasons */
          unsupported_extension(110),           /* new */
          certificate_unobtainable(111),        /* new */

          unrecognized_name(112),               /* new */
          bad_certificate_status_response(113), /* new */
          bad_certificate_hash_value(114),      /* new */
      } AlertDescription;

5. Procedure for Defining New Extensions

   Traditionally for Internet protocols, the Internet Assigned Numbers
   Authority (IANA) handles the allocation of new values for future
   expansion, and RFCs usually define the procedure to be used by the
   IANA.  However, there are subtle (and not so subtle) interactions
   that may occur in this protocol between new features and existing
   features which may result in a significant reduction in overall

   Therefore, requests to define new extensions (including assigning
   extension and error alert numbers) must be approved by IETF Standards

   The following considerations should be taken into account when
   designing new extensions:

   -  All of the extensions defined in this document follow the
      convention that for each extension that a client requests and that
      the server understands, the server replies with an extension of
      the same type.

   -  Some cases where a server does not agree to an extension are error
      conditions, and some simply a refusal to support a particular
      feature.  In general error alerts should be used for the former,
      and a field in the server extension response for the latter.

   -  Extensions should as far as possible be designed to prevent any
      attack that forces use (or non-use) of a particular feature by
      manipulation of handshake messages.  This principle should be
      followed regardless of whether the feature is believed to cause a
      security problem.

      Often the fact that the extension fields are included in the
      inputs to the Finished message hashes will be sufficient, but
      extreme care is needed when the extension changes the meaning of
      messages sent in the handshake phase. Designers and implementors
      should be aware of the fact that until the handshake has been
      authenticated, active attackers can modify messages and insert,
      remove, or replace extensions.

   -  It would be technically possible to use extensions to change major
      aspects of the design of TLS; for example the design of cipher
      suite negotiation.  This is not recommended; it would be more
      appropriate to define a new version of TLS - particularly since
      the TLS handshake algorithms have specific protection against
      version rollback attacks based on the version number, and the
      possibility of version rollback should be a significant
      consideration in any major design change.

6. Security Considerations

   Security considerations for the extension mechanism in general, and
   the design of new extensions, are described in the previous section.
   A security analysis of each of the extensions defined in this
   document is given below.

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

   Additional security considerations are described in the TLS 1.0 RFC

6.1. Security of 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.

   Although this document specifies an encoding for internationalized
   hostnames in the server_name extension, it does not address any
   security issues associated with the use of internationalized
   hostnames in TLS - in particular, the consequences of "spoofed" names
   that are indistinguishable from another name when displayed or
   printed.  It is recommended that server certificates not be issued
   for internationalized hostnames unless procedures are in place to
   mitigate the risk of spoofed hostnames.

6.2. Security of 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

   Note that as described in section 3.2, 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.

6.3. Security of client_certificate_url

   There are two major issues with this extension.

   The first major issue is whether or not clients should include
   certificate hashes when they send certificate URLs.

   When client authentication is used *without* the
   client_certificate_url extension, the client certificate chain is
   covered by the Finished message hashes.  The purpose of including
   hashes and checking them against the retrieved certificate chain, is
   to ensure that the same property holds when this extension is used -
   i.e., that all of the information in the certificate chain retrieved
   by the server is as the client intended.

   On the other hand, omitting certificate hashes enables functionality
   that is desirable in some circumstances - for example clients can be
   issued daily certificates that are stored at a fixed URL and need not
   be provided to the client.  Clients that choose to omit certificate
   hashes should be aware of the possibility of an attack in which the
   attacker obtains a valid certificate on the client's key that is
   different from the certificate the client intended to provide.
   Although TLS uses both MD5 and SHA-1 hashes in several other places,
   this was not believed to be necessary here.  The property required of
   SHA-1 is second pre-image resistance.

   The second major issue is that support for client_certificate_url
   involves the server acting as a client in another URL protocol.  The
   server therefore becomes subject to many of the same security
   concerns that clients of the URL 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 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 allowing the existence
   of internal hosts to be confirmed when they would otherwise be

   The detailed security concerns involved will depend on the URL
   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, the possibility for loops and
   deadlocks to be created exists and should be addressed.  In the case
   of FTP, attacks similar to FTP bounce attacks arise.

   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 being enabled by
   default.  It is also RECOMMENDED that URI protocols be enabled by the
   administrator individually, and only a minimal set of protocols be
   enabled, with unusual protocols offering limited security or whose
   security is not well-understood being avoided.

   As discussed in [URI], 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.

6.4. Security of 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

6.5. Security of 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 the handshake
   messages 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 future, if either the client or the
   server for a given session were updated to take into account the
   problem, they would be able to veto use of this extension.

6.6. Security of 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.  A client that requires
   OCSP validation of certificates SHOULD either contact the OCSP server
   directly in this case, 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 [OCSP] for further details.

7. Internationalization Considerations

   None of the extensions defined here directly use strings subject to
   localization.  Domain Name System (DNS) hostnames are encoded using
   UTF-8.  If future extensions use text strings, then
   internationalization should be considered in their design.

8. IANA Considerations

   The MIME type "application/pkix-pkipath" has been registered by the
   IANA with the following template:

   To: ietf-types@iana.org Subject: Registration of MIME media type

   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 that will be published in
      [X509-4th-TC1]; note that it is different from that in [X509-4th].

      All Certificates MUST conform to [PKIX].  (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 [PKIX] (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 [PKIX].

   Published specification: this memo, and [PKIX].

   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 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 <magnus@rsasecurity.com>

   Intended usage: COMMON

   Author/Change controller:
      Magnus Nystrom <magnus@rsasecurity.com>

9. Intellectual Property Rights

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in RFC 2028.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this document.  Please address the information to the IETF Executive

10. Acknowledgments

   The authors wish to thank the TLS Working Group and the WAP Security
   Group.  This document is based on discussion within these groups.

11. Normative References

   [HMAC]         Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                  Keyed-hashing for message authentication", RFC 2104,
                  February 1997.

   [HTTP]         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.

   [IDNA]         Faltstrom, P., Hoffman, P. and A. Costello,
                  "Internationalizing Domain Names in Applications
                  (IDNA)", RFC 3490, March 2003.

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

   [OCSP]         Myers, M., Ankney, R., Malpani, A., Galperin, S. and
                  C. Adams, "Internet X.509 Public Key Infrastructure:
                  Online Certificate Status Protocol - OCSP", RFC 2560,
                  June 1999.

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

   [PKIX]         Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
                  Public Key Infrastructure - Certificate and
                  Certificate Revocation List (CRL) Profile", RFC 3280,
                  April 2002.

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

   [URI]          Berners-Lee, T., Fielding, R. and L. Masinter,
                  "Uniform Resource Identifiers (URI): Generic Syntax",
                  RFC 2396, August 1998.

   [UTF8]         Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", RFC 2279, January 1998.

   [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.

12. Informative References

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

   [MAILING LIST] J. Mikkelsen, R. Eberhard, and J. Kistler, "General
                  ClientHello extension mechanism and virtual hosting,"
                  ietf-tls mailing list posting, August 14, 2000.

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

13. Authors' Addresses

   Simon Blake-Wilson
   EMail: sblakewilson@bcisse.com

   Magnus Nystrom
   RSA Security
   EMail: magnus@rsasecurity.com

   David Hopwood
   Independent Consultant
   EMail: david.hopwood@zetnet.co.uk

   Jan Mikkelsen
   EMail: janm@transactionware.com

   Tim Wright
   EMail: timothy.wright@vodafone.com

14.  Full Copyright Statement

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