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Versions: 00 01 02 03 04 05 RFC 3546

TLS Working Group                           Simon Blake-Wilson, Certicom
INTERNET-DRAFT                              Magnus Nystrom, RSA Security
February 22, 2002                  David Hopwood, Independent Consultant
Expires August 22, 2002                   Jan Mikkelsen, Transactionware
Intended Category: Standards track                  Tim Wright, Vodafone


                             TLS Extensions

                   <draft-ietf-tls-extensions-03.txt>

                          Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or made obsolete by other documents at
   any time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as work in progress.

   The list of current Internet-Drafts may be found at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories may be found at
   http://www.ietf.org/shadow.html.

                                 Abstract

   This document describes extensions that may be used to add
   functionality to 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.

   This document is based on discussions within the TLS working group
   and within the WAP security group.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [KEYWORDS].



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   Please send comments on this document to the TLS mailing list.

                             Table of Contents

    1. Introduction .............................................. 2
    2. General Extension Mechanisms .............................. 4
    2.1. Extended Client Hello ................................... 4
    2.2. Extended Server Hello ................................... 5
    2.3. Hello Extensions ........................................ 6
    2.4. Extensions to the handshake protocol .................... 7
    3. Specific Extensions ....................................... 7
    3.1. Server Name Indication .................................. 8
    3.2. Maximum Fragment Length Negotiation ..................... 9
    3.3. Client Certificate URLs ................................ 11
    3.4. Trusted CA Indication .................................. 12
    3.5. Truncated HMAC ......................................... 14
    3.6. Certificate Status Request.............................. 15
    4. Error alerts ............................................. 16
    5. Procedure for Defining New Extensions..................... 18
    6. Security Considerations .................................. 19
    6.1. Security of server_name ................................ 19
    6.2. Security of max_fragment_length ........................ 19
    6.3. Security of client_certificate_url ..................... 20
    6.4. Security of trusted_ca_keys ............................ 20
    6.5. Security of truncated_hmac ............................. 21
    6.6. Security of status_request ............................. 21
    7. Internationalization Considerations .......................21
    8. Intellectual Property Rights ............................. 22
    9. Acknowledgments .......................................... 22
   10. References ............................................... 22
   11. Authors' Addresses ....................................... 23

1. Introduction

   This document describes extensions that may be used to add
   functionality to 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,



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   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
   to:

   - 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
     limitations.

   - 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 OCSP [OCSP]
     response) during a TLS handshake. This functionality is desirable
     in order to avoid sending a 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.



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   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, 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>;



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           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 clients request additional functionality using the
   extended client hello, and this functionality is not supplied by the
   server, clients MAY abort the handshake.

   Note that [TLS], Section 7.4.1.2, 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
   servers.

   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
   clients.





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 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;

   Here:

   - "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



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   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
   session.

 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),
           (255)
       } HandshakeType;

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

3. Specific Extensions

   This section describes the specific TLS extensions specified in this
   document.

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



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   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;
           }
       } 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.




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   "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.  (Note
   that the use of UTF-8 here for encoding internationalized hostnames
   is independent of the choice of encoding for these names in the DNS
   protocol. The latter has yet to be decided by the IETF
   Internationalized Domain Name Working Group [IDNWG].)

   If the server needs to match the HostName against names that contain
   non-ASCII characters (that is, if it has one or more
   internationalized virtual host names), it MUST take account of name
   equivalence rules that will be defined by the IDN Working Group.  If
   the server only needs to match the HostName against names containing
   exclusively ASCII characters, it MUST NOT treat a HostName that
   contains a byte value >= 128 as matching any ASCII name, and it MUST
   compare ASCII names case-insensitively.

   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.



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

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

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

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

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

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

   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 planned AES ciphersuites), 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.



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 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:

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

       struct {
           opaque URL<1..2^16-1>;
           CertHash certificate_hash;
       } URLAndHash;

       opaque CertHash<0..20>;

   Here "url_and_hash_list" contains a sequence of URLs and hashes.

   When X.509 certificates are used, each URL refers either to a single,
   DER-encoded X.509v3 certificate, or to a PKCS #7 certificate chain.
   For ciphersuites using other certificate formats, the ciphersuite
   specification must define the encoding format of certificates or
   certificate chains, and any constraint on their ordering, if this
   message is to be used.

   The hash corresponding to each URL at the client's discretion is



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   either empty or it contains 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 CertificateSet, see [CMS]).

   For X.509, URLs MUST occur in the list in the same order that the
   corresponding certificates appear in the certificate chain.

   Servers receiving "CertificateURL" SHALL attempt to retrieve the
   client's certificate chain from the URLs, and then process the
   certificate chain as usual. Servers that support this extension MUST
   support the http: URL scheme for certificate URLs, and MAY support
   other schemes.

   In general, the format of the certificates retrieved by the server
   will depend on the protocol used by the server to retrieve them, as
   recommended by the PKIX working group. In the case of HTTP, the
   response MUST be a MIME formatted response. When a single X.509v3
   certificate is returned, the Content-type is application/pkix-cert
   [PKIOP]. When a chain of X.509v3 certificates is returned, the
   Content-type is application/pkcs7-mime [SMIME].

   Servers MUST check that the SHA-1 hash of any certificates retrieved
   from a CertificateURL matches the given hash if it is present.  If
   any retrieved certificate 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
   failures.

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




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

       struct {
           IdentifierType identifier_type;
           select (identifier_type) {
               case pre_agreed: struct {};
               case key_hash_sha: KeyHash;
               case x509_name: DistinguishedName;
               case cert_hash: CertHash;
           } Identifier;
       } TrustedAuthority;

       enum { pre_agreed(0), key_hash_sha(1), x509_name(2), cert_hash(3),
           (255)}
           IdentifierType;

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

       opaque KeyHash[20];

   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_hash_sha" - 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, this is the hash of the byte string
      representation of the modulus (without any initial 0-valued
      bytes). (This copies the key hash formats deployed in other
      environments.)

   - "cert_hash" - contains the SHA-1 hash of a certificate containing
      the CA root key.

   - "x509_name" - contains the X.509 Distinguished Name of the CA.

   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.




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   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 ciphersuites 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" in the extended server hello.

   Note that if new ciphersuites are added that do not use HMAC, and the
   session negotiates one of these ciphersuites, 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 ciphersuite 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.





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 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;
           }
       } 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].

   Servers that receive a client hello containing the "status_request"
   extension, MAY return a suitable certificate status response to the
   client along with their certificate. If OCSP is requested, they
   SHOULD use the information contained in the extension when selecting
   an OCSP responder, and SHOULD include request_extensions in the OCSP
   request.

   Servers return a certificate response along with their certificate by



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   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 ocsp_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



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     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 {
           close_notify(0),
           unexpected_message(10),
           bad_record_mac(20),
           decryption_failed(21),
           record_overflow(22),
           decompression_failure(30),
           handshake_failure(40),
           /* 41 is not defined, for historical reasons */
           bad_certificate(42),
           unsupported_certificate(43),
           certificate_revoked(44),
           certificate_expired(45),
           certificate_unknown(46),
           illegal_parameter(47),
           unknown_ca(48),
           access_denied(49),
           decode_error(50),
           decrypt_error(51),
           export_restriction(60),
           protocol_version(70),
           insufficient_security(71),
           internal_error(80),
           user_canceled(90),
           no_renegotiation(100),



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           unsupported_extension(110),           /* new */
           certificate_unobtainable(111),        /* new */
           unrecognized_name(112),               /* new */
           bad_certificate_status_response(113), /* new */
           bad_certificate_hash_value(114),      /* new */
           (255)
       } 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 security.

   Therefore, requests to define new extensions (including assigning
   extension and error alert numbers) should be forwarded to the IETF
   TLS Working Group for discussion.

   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.



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    - It would be technically possible to use extensions to change
      major aspects of the design of TLS; for example the design of
      ciphersuite 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
   [TLS].

 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.

 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
   messages.

   Note that as described in section 3.2, once a non-null ciphersuite
   has been activated, the effective maximum fragment length depends on
   the ciphersuite, 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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 6.3. Security of client_certificate_url

   The major issue with this extension 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
   certificate 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.

   Note that 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.

   Support for client_certificate_url involves the server acting as a
   client in another protocol (usually HTTP, but other URL schemes are
   not prohibited). It is therefore subject to many of the same security
   considerations that apply to a publicly accessible HTTP proxy server.
   This includes the possibility that an attacker might use the server
   to indirectly attack another host that is vulnerable to some security
   flaw. It also includes potentially increased exposure to denial of
   service attacks: an attacker can make many connections, each of which
   results in the server making an HTTP request.

   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.

   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).

 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.



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   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
   hashes.

 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 be prepared to contact the
   OCSP server directly in this case.

   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. DNS hostnames are encoded using UTF-8. If future
   extensions use text strings, then internationalization should be
   considered in their design.








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8. Intellectual Property Rights

   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
   Director.

9. Acknowledgments

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

10. References

   [CMS] R. Housley, "Cryptographic Message Syntax," IETF RFC 2630, June
   1999.

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

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

   [IDNWG] IETF Internationalized Domain Name Working Group,
   http://www.i-d-n.net/

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

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

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

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

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

   [SMIME] B. Ramsdell, "S/MIME Version 3 Message Specification," IETF
   RFC 2633, June 1999.



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

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

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

11. Authors' Addresses

   Simon Blake-Wilson
   Certicom Corp.
   sblake-wilson@certicom.com

   Magnus Nystrom
   RSA Security
   magnus@rsasecurity.com

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

   Jan Mikkelsen
   Transactionware
   janm@transactionware.com

   Tim Wright
   Vodafone
   timothy.wright@vf.vodafone.co.uk





















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