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rfc:rfc4366

Network Working Group S. Blake-Wilson Request for Comments: 4366 BCI Obsoletes: 3546 M. Nystrom Updates: 4346 RSA Security Category: Standards Track D. Hopwood

                                                Independent Consultant
                                                          J. Mikkelsen
                                                       Transactionware
                                                             T. Wright
                                                              Vodafone
                                                            April 2006
             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 (2006).

Abstract

 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 clients that support the extensions and TLS servers that
 do not support the extensions, and vice versa.

Blake-Wilson, et al. Standards Track [Page 1] RFC 4366 TLS Extensions April 2006

Table of Contents

 1. Introduction ....................................................3
    1.1. Conventions Used in This Document ..........................5
 2. General Extension Mechanisms ....................................5
    2.1. Extended Client Hello ......................................5
    2.2. Extended Server Hello ......................................6
    2.3. Hello Extensions ...........................................6
    2.4. Extensions to the Handshake Protocol .......................8
 3. Specific Extensions .............................................8
    3.1.  Server Name Indication ....................................9
    3.2.  Maximum Fragment Length Negotiation ......................11
    3.3.  Client Certificate URLs ..................................12
    3.4.  Trusted CA Indication ....................................15
    3.5. Truncated HMAC ............................................16
    3.6. Certificate Status Request ................................17
 4. Error Alerts ...................................................19
 5. Procedure for Defining New Extensions ..........................20
 6. Security Considerations ........................................21
    6.1. Security of server_name ...................................22
    6.2. Security of max_fragment_length ...........................22
    6.3. Security of client_certificate_url ........................22
    6.4. Security of trusted_ca_keys ...............................24
    6.5. Security of truncated_hmac ................................24
    6.6. Security of status_request ................................25
 7. Internationalization Considerations ............................25
 8. IANA Considerations ............................................25
 9. Acknowledgements ...............................................27
 10. Normative References ..........................................27
 11. Informative References ........................................28

Blake-Wilson, et al. Standards Track [Page 2] RFC 4366 TLS Extensions April 2006

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 such as 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:
  1. Allow TLS clients to provide to the TLS server the name of the

server they are contacting. This functionality is desirable in

    order to facilitate secure connections to servers that host
    multiple 'virtual' servers at a single underlying network address.
  1. 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.
  1. 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.
  1. 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.
  1. 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.

Blake-Wilson, et al. Standards Track [Page 3] RFC 4366 TLS Extensions April 2006

  1. 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 clients
 and servers.  The extensions are designed to be backwards compatible,
 meaning that TLS clients that support the extensions can talk to TLS
 servers that do not support the extensions, and vice versa.  The
 document therefore updates TLS 1.0 [TLS] and TLS 1.1 [TLSbis].
 Backwards compatibility is primarily achieved via two considerations:
  1. Clients typically request the use of extensions via the extended

client hello message described in Section 2.1. TLS requires

    servers to accept extended client hello messages, even if the
    server does not "understand" the extension.
  1. For the specific extensions described here, no mandatory server

response is required when clients request extended functionality.

 Essentially, backwards compatibility is achieved based on the TLS
 requirement that servers that are not "extensions-aware" ignore data
 added to client hellos that they do not recognize; for example, see
 Section 7.4.1.2 of [TLS].
 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.
 This document is a revision of the RFC3546 [RFC3546].  The only major
 change concerns the definition of new extensions.  New extensions can
 now be defined via the IETF Consensus Process (rather than requiring
 a standards track RFC).  In addition, a few minor clarifications and
 editorial improvements were made.
 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.  Section 4 describes new error alerts for use with

Blake-Wilson, et al. Standards Track [Page 4] RFC 4366 TLS Extensions April 2006

 the TLS extensions.  The final sections of the document address IPR,
 security considerations, registration of the application/pkix-pkipath
 MIME type, acknowledgements, and references.

1.1. Conventions Used in This Document

 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 BCP 14, RFC 2119
 [KEYWORDS].

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 [MAILINGLIST].
 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.

Blake-Wilson, et al. Standards Track [Page 5] RFC 4366 TLS Extensions April 2006

 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 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 it
 does 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
 clients.

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;

Blake-Wilson, et al. Standards Track [Page 6] RFC 4366 TLS Extensions April 2006

 Here:
  1. "extension_type" identifies the particular extension type.
  1. "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;
 The list of defined extension types is maintained by the IANA.  The
 current list can be found at:
 http://www.iana.org/assignments/tls-extensiontype-values.  See
 Sections 5 and 8 for more information on how new values are added.
 Note that for all extension types (including those defined in the
 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-oriented" extensions may be provided in the
 future within this framework.  Such an extension (say, of type x)
 would require the client to first send an extension of type x in the
 (extended) client hello with empty extension_data to indicate that it
 supports the extension type.  In this case, the client is offering
 the capability to understand the extension type, and the server is
 taking the client up on its offer.
 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 an extended client hello may be sent both when
 starting a new session and when requesting session resumption.
 Indeed, 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.  In general the specification of
 each extension type must include a discussion of the effect of the
 extension both during new sessions and during resumed sessions.

Blake-Wilson, et al. Standards Track [Page 7] RFC 4366 TLS Extensions April 2006

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
 involved in "Finished" messages.
 Note also that all the extensions defined in this section are
 relevant only when a session is initiated.  When a client includes
 one or more of the defined extension types in an extended client
 hello while requesting session resumption:

Blake-Wilson, et al. Standards Track [Page 8] RFC 4366 TLS Extensions April 2006

  1. If the resumption request is denied, the use of the extensions is

negotiated as normal.

  1. If, on the other hand, the older session is resumed, then the

server MUST ignore the extensions and send a server hello

    containing none of the extension types.  In this case, the
    functionality of these extensions negotiated during the original
    session initiation is applied to the resumed session.
 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 that provides maximum fragment length negotiation.  Section
 3.3 describes the extension that allows client certificate URLs.
 Section 3.4 describes the extension that allows a client to indicate
 which CA root keys it possesses.  Section 3.5 describes the extension
 that allows the use of truncated HMAC.  Section 3.6 describes the
 extension that supports 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;

Blake-Wilson, et al. Standards Track [Page 9] RFC 4366 TLS Extensions April 2006

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

Blake-Wilson, et al. Standards Track [Page 10] RFC 4366 TLS Extensions April 2006

3.2. Maximum Fragment Length Negotiation

 Without this extension, TLS specifies a fixed maximum plaintext
 fragment length of 2^14 bytes.  It may be desirable for constrained
 clients to negotiate a smaller maximum fragment length due to memory
 limitations or bandwidth limitations.
 In order to negotiate smaller maximum fragment lengths, clients MAY
 include an extension of type "max_fragment_length" in the (extended)
 client hello.  The "extension_data" field of this extension SHALL
 contain:
 enum{
     2^9(1), 2^10(2), 2^11(3), 2^12(4), (255)
 } MaxFragmentLength;
 whose value is the desired maximum fragment length.  The allowed
 values for this field are: 2^9, 2^10, 2^11, and 2^12.
 Servers that receive an extended client hello containing a
 "max_fragment_length" extension MAY accept the requested maximum
 fragment length by including an extension of type
 "max_fragment_length" in the (extended) server hello.  The
 "extension_data" field of this extension SHALL contain a
 "MaxFragmentLength" whose value is the same as the requested maximum
 fragment length.
 If a server receives a maximum fragment length negotiation request
 for a value other than the allowed values, it MUST abort the
 handshake with an "illegal_parameter" alert.  Similarly, if a client
 receives a maximum fragment length negotiation response that differs
 from the length it requested, it MUST also abort the handshake with
 an "illegal_parameter" alert.
 Once a maximum fragment length other than 2^14 has been successfully
 negotiated, the client and server MUST immediately begin fragmenting
 messages (including handshake messages), to ensure that no fragment
 larger than the negotiated length is sent.  Note that TLS already
 requires clients and servers to support fragmentation of handshake
 messages.
 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

Blake-Wilson, et al. Standards Track [Page 11] RFC 4366 TLS Extensions April 2006

 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.  This 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

 Without this extension, TLS specifies that when client authentication
 is performed, client certificates are sent by clients to servers
 during the TLS handshake.  It may be desirable for constrained
 clients to send certificate URLs in place of certificates, so that
 they do not need to store their certificates and can therefore save
 memory.
 In order to negotiate sending certificate URLs to a server, clients
 MAY include an extension of type "client_certificate_url" in the
 (extended) client hello.  The "extension_data" field of this
 extension SHALL be empty.
 (Note that it is necessary to negotiate use of client certificate
 URLs in order to avoid "breaking" existing TLS servers.)
 Servers that receive an extended client hello containing a
 "client_certificate_url" extension MAY indicate that they are willing
 to accept certificate URLs by including an extension of type
 "client_certificate_url" in the (extended) server hello.  The
 "extension_data" field of this extension SHALL be empty.
 After negotiation of the use of client certificate URLs has been
 successfully completed (by exchanging hellos including
 "client_certificate_url" extensions), clients MAY send a
 "CertificateURL" message in place of a "Certificate" message:
    enum {
        individual_certs(0), pkipath(1), (255)
    } CertChainType;
    enum {
        false(0), true(1)
    } Boolean;

Blake-Wilson, et al. Standards Track [Page 12] RFC 4366 TLS Extensions April 2006

    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
 hashes.
 When X.509 certificates are used, there are two possibilities:
  1. 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.
  1. 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
 ordering.
 The hash corresponding to each URL at the client's discretion either
 is 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.

Blake-Wilson, et al. Standards Track [Page 13] RFC 4366 TLS Extensions April 2006

 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.  Use of other
 schemes than "http", "https", or "ftp" may create unexpected
 problems.
 If the protocol used is HTTP, then the HTTP server can be configured
 to use the Cache-Control and Expires directives described in [HTTP]
 to specify whether and for how long certificates or certificate
 chains should be cached.
 The TLS server is not required to follow HTTP redirects when
 retrieving the certificates or certificate chain.  The URLs used in
 this extension SHOULD therefore be chosen not to depend on such
 redirects.
 If the protocol used to retrieve certificates or certificate chains
 returns a MIME-formatted response (as HTTP does), then the following
 MIME Content-Types SHALL be used: when a single X.509v3 certificate
 is returned, the Content-Type is "application/pkix-cert" [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.

Blake-Wilson, et al. Standards Track [Page 14] RFC 4366 TLS Extensions April 2006

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:
    struct {
        TrustedAuthority trusted_authorities_list<0..2^16-1>;
    } TrustedAuthorities;
    struct {
        IdentifierType identifier_type;
        select (identifier_type) {
            case pre_agreed: struct {};
            case key_sha1_hash: SHA1Hash;
            case x509_name: DistinguishedName;
            case cert_sha1_hash: SHA1Hash;
        } identifier;
    } TrustedAuthority;
    enum {
        pre_agreed(0), key_sha1_hash(1), x509_name(2),
        cert_sha1_hash(3), (255)
    } IdentifierType;
    opaque DistinguishedName<1..2^16-1>;
 Here "TrustedAuthorities" provides a list of CA root key identifiers
 that the client possesses.  Each CA root key is identified via
 either:
  1. "pre_agreed": no CA root key identity supplied.
  1. "key_sha1_hash": contains the SHA-1 hash of the CA root key. For

Digital Signature Algorithm (DSA) and Elliptic Curve Digital

    Signature Algorithm (ECDSA) keys, this is the hash of the
    "subjectPublicKey" value.  For RSA keys, the hash is of the big-
    endian byte string representation of the modulus without any
    initial 0-valued bytes.  (This copies the key hash formats
    deployed in other environments.)

Blake-Wilson, et al. Standards Track [Page 15] RFC 4366 TLS Extensions April 2006

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

the CA.

  1. "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

Blake-Wilson, et al. Standards Track [Page 16] RFC 4366 TLS Extensions April 2006

 suites using other MACs consider the MAC size an integral part of the
 cipher suite definition, taking into account both security and
 bandwidth considerations.
 If HMAC truncation has been successfully negotiated during a TLS
 handshake, and the negotiated cipher suite uses HMAC, both the client
 and the server pass this fact to the TLS record layer along with the
 other negotiated security parameters.  Subsequently during the
 session, clients and servers MUST use truncated HMACs, calculated as
 specified in [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 pseudo-random function (PRF) as part of handshaking or key
 derivation.
 The negotiated HMAC truncation size applies for the duration of the
 session including session resumptions.

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>;

Blake-Wilson, et al. Standards Track [Page 17] RFC 4366 TLS Extensions April 2006

 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
 request.
 Servers return a certificate response along with their certificate by
 sending a "CertificateStatus" message immediately after the
 "Certificate" message (and before any "ServerKeyExchange" or
 "CertificateRequest" messages).  If a server returns a
 "CertificateStatus" message, then the server MUST have included an
 extension of type "status_request" with empty "extension_data" in the
 extended server hello.
    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.

Blake-Wilson, et al. Standards Track [Page 18] RFC 4366 TLS Extensions April 2006

 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.
  1. "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.
  1. "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.
  1. "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.
  1. "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.
  1. "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.

Blake-Wilson, et al. Standards Track [Page 19] RFC 4366 TLS Extensions April 2006

 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),
        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

 The list of extension types, as defined in Section 2.3, is maintained
 by the Internet Assigned Numbers Authority (IANA).  Thus, an
 application needs to be made to the IANA in order to obtain a new
 extension type value.  Since there are subtle (and not-so-subtle)
 interactions that may occur in this protocol between new features and
 existing features that may result in a significant reduction in
 overall security, new values SHALL be defined only through the IETF
 Consensus process specified in [IANA].
 (This means that new assignments can be made only via RFCs approved
 by the IESG.)

Blake-Wilson, et al. Standards Track [Page 20] RFC 4366 TLS Extensions April 2006

 The following considerations should be taken into account when
 designing new extensions:
  1. 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.
  1. 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.
  1. 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.
  1. 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.  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
 for 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] and the TLS 1.1 RFC [TLSbis].

Blake-Wilson, et al. Standards Track [Page 21] RFC 4366 TLS Extensions April 2006

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 messages.
 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.

Blake-Wilson, et al. Standards Track [Page 22] RFC 4366 TLS Extensions April 2006

 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's 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's attempting a connection to the
 target of the attack.
 Note that the server may be behind a firewall or otherwise able to
 access hosts that would not be directly accessible from the public
 Internet.  This could exacerbate the potential security and denial of
 service problems described above, as well as allow the existence of
 internal hosts to be confirmed when they would otherwise be hidden.
 The detailed security concerns involved will depend on the 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, loops and deadlocks may be created,
 and this should be addressed.  In the case of FTP, attacks arise that
 are similar to FTP bounce attacks.
 As a result of this issue, it is RECOMMENDED that the
 client_certificate_url extension should have to be specifically
 enabled by a server administrator, rather than be enabled by default.
 It is also RECOMMENDED that URI protocols be enabled by the
 administrator individually, and only a minimal set of protocols be
 enabled.  Unusual protocols that offer limited security or whose
 security is not well-understood SHOULD be avoided.

Blake-Wilson, et al. Standards Track [Page 23] RFC 4366 TLS Extensions April 2006

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

Blake-Wilson, et al. Standards Track [Page 24] RFC 4366 TLS Extensions April 2006

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.  In this case, a client
 that requires OCSP validation of certificates SHOULD either contact
 the OCSP server directly or abort the handshake.
 Use of the OCSP nonce request extension (id-pkix-ocsp-nonce) may
 improve security against attacks that attempt to replay OCSP
 responses; see Section 4.4.1 of [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

 Sections 2.3 and 5 describe a registry of ExtensionType values to be
 maintained by the IANA.  ExtensionType values are to be assigned via
 IETF Consensus as defined in RFC 2434 [IANA].  The initial registry
 corresponds to the definition of "ExtensionType" in Section 2.3.
 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 application/pkix-pkipath
 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,
      etc.

Blake-Wilson, et al. Standards Track [Page 25] RFC 4366 TLS Extensions April 2006

    This is identical to the definition published in [X509-4th-TC1];
    note that it is different from that in [X509-4th].
    All Certificates MUST conform to [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
    carefully.)
    DER (as opposed to BER) encoding MUST be used.  If this type is
    sent over a 7-bit transport, base64 encoding SHOULD be used.
 Security considerations:
    The security considerations of [X509-4th] and [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: RFC 4366 (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
    chains.
 Additional information:
    Magic number(s): DER-encoded ASN.1 can be easily recognized.
      Further parsing is required to distinguish it from other ASN.1
      types.
    File extension(s): .pkipath
    Macintosh File Type Code(s): not specified
 Person & email address to contact for further information:
    Magnus Nystrom <magnus@rsasecurity.com>
 Intended usage: COMMON

Blake-Wilson, et al. Standards Track [Page 26] RFC 4366 TLS Extensions April 2006

 Change controller:
    IESG <iesg@ietf.org>

9. Acknowledgements

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

10. 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.
 [IANA]         Narten, T. and H. Alvestrand, "Guidelines for Writing
                an IANA Considerations Section in RFCs", BCP 26, RFC
                2434, October 1998.
 [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, "X.509 Internet Public Key Infrastructure
                Online Certificate Status Protocol - OCSP", RFC 2560,
                June 1999.
 [PKIOP]        Housley, R. and P. Hoffman, "Internet X.509 Public Key
                Infrastructure Operational Protocols: FTP and HTTP",
                RFC 2585, May 1999.
 [PKIX]         Housley, R., Polk, W., Ford, W., and D. Solo,
                "Internet X.509 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.

Blake-Wilson, et al. Standards Track [Page 27] RFC 4366 TLS Extensions April 2006

 [TLSbis]       Dierks, T. and E. Rescorla, "The Transport Layer
                Security (TLS) Protocol Version 1.1", RFC 4346, April
                2006.
 [URI]          Berners-Lee, T., Fielding, R., and L. Masinter,
                "Uniform Resource Identifier (URI): Generic Syntax",
                STD 66, RFC 3986, January 2005.
 [UTF8]         Yergeau, F., "UTF-8, a transformation format of ISO
                10646", STD 63, RFC 3629, November 2003.
 [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.

11. Informative References

 [AESSUITES]    Chown, P., "Advanced Encryption Standard (AES)
                Ciphersuites for Transport Layer Security (TLS)", RFC
                3268, June 2002.
 [KERB]         Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
                Suites to Transport Layer Security (TLS)", RFC 2712,
                October 1999.
 [MAILINGLIST]  J. Mikkelsen, R. Eberhard, and J. Kistler, "General
                ClientHello extension mechanism and virtual hosting,"
                ietf-tls mailing list posting, August 14, 2000.
 [RFC3546]      Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
                J., and T. Wright, "Transport Layer Security (TLS)
                Extensions", RFC 3546, June 2003.

Blake-Wilson, et al. Standards Track [Page 28] RFC 4366 TLS Extensions April 2006

Authors' Addresses

 Simon Blake-Wilson
 BCI
 EMail: sblakewilson@bcisse.com
 Magnus Nystrom
 RSA Security
 EMail: magnus@rsasecurity.com
 David Hopwood
 Independent Consultant
 EMail: david.hopwood@blueyonder.co.uk
 Jan Mikkelsen
 Transactionware
 EMail: janm@transactionware.com
 Tim Wright
 Vodafone
 EMail: timothy.wright@vodafone.com

Blake-Wilson, et al. Standards Track [Page 29] RFC 4366 TLS Extensions April 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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 Administrative Support Activity (IASA).

Blake-Wilson, et al. Standards Track [Page 30]

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