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RFC 4366

Transport Layer Security (TLS) Extensions

Pages: 30
Obsoletes:  3546
Obsoleted by:  52466066
Updates:  4346
Updated by:  5746

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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
                                                               T. Wright
                                                              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).


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.
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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
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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: - Allow TLS clients to provide to the TLS server the name of the server they are contacting. This functionality is desirable in order to facilitate secure connections to servers that host multiple 'virtual' servers at a single underlying network address. - Allow TLS clients and servers to negotiate the maximum fragment length to be sent. This functionality is desirable as a result of memory constraints among some clients, and bandwidth constraints among some access networks. - Allow TLS clients and servers to negotiate the use of client certificate URLs. This functionality is desirable in order to conserve memory on constrained clients. - Allow TLS clients to indicate to TLS servers which CA root keys they possess. This functionality is desirable in order to prevent multiple handshake failures involving TLS clients that are only 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.
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   -  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:

   -  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.

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

   The list of defined extension types is maintained by the IANA.  The
   current list can be found at:  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.
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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:
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   -  If the resumption request is denied, the use of the extensions is
      negotiated as normal.

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

   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;
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   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

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

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

      opaque SHA1Hash[20];

   Here "url_and_hash_list" contains a sequence of URLs and optional

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

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

   -  If CertificateURL.type is "pkipath", the list contains a single
      URL referring to a DER-encoded certificate chain, using the type
      PkiPath described in Section 8.

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

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

   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

   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.
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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: - "pre_agreed": no CA root key identity supplied. - "key_sha1_hash": contains the SHA-1 hash of the CA root key. For Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA) keys, this is the hash of the "subjectPublicKey" value. For RSA keys, the hash is of the big- endian byte string representation of the modulus without any initial 0-valued bytes. (This copies the key hash formats deployed in other environments.)
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   -  "x509_name": contains the DER-encoded X.509 DistinguishedName of
      the CA.

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

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

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

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

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

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
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   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

   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>;
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   In the OCSPStatusRequest, the "ResponderIDs" provides a list of OCSP
   responders that the client trusts.  A zero-length "responder_id_list"
   sequence has the special meaning that the responders are implicitly
   known to the server, e.g., by prior arrangement.  "Extensions" is a
   DER encoding of OCSP request extensions.

   Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as
   defined in [OCSP].  "Extensions" is imported from [PKIX].  A zero-
   length "request_extensions" value means that there are no extensions
   (as opposed to a zero-length ASN.1 SEQUENCE, which is not valid for
   the "Extensions" type).

   In the case of the "id-pkix-ocsp-nonce" OCSP extension, [OCSP] is
   unclear about its encoding; for clarification, the nonce MUST be a
   DER-encoded OCTET STRING, which is encapsulated as another OCTET
   STRING (note that implementations based on an existing OCSP client
   will need to be checked for conformance to this requirement).

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

   Servers return a certificate response along with their certificate by
   sending a "CertificateStatus" message immediately after the
   "Certificate" message (and before any "ServerKeyExchange" or
   "CertificateRequest" messages).  If a server returns a

   "CertificateStatus" message, then the server MUST have included an
   extension of type "status_request" with empty "extension_data" in the
   extended server hello.

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

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

   An "ocsp_response" contains a complete, DER-encoded OCSP response
   (using the ASN.1 type OCSPResponse defined in [OCSP]).  Note that
   only one OCSP response may be sent.
ToP   noToC   RFC4366 - Page 19
   The "CertificateStatus" message is conveyed using the handshake
   message type "certificate_status".

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

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

   Clients requesting an OCSP response and receiving an OCSP response in
   a "CertificateStatus" message MUST check the OCSP response and abort
   the handshake if the response is not satisfactory.

4. Error Alerts

This section defines new error alerts for use with the TLS extensions defined in this document. The following new error alerts are defined. To avoid "breaking" existing clients and servers, these alerts MUST NOT be sent unless the sending party has received an extended hello message from the party they are communicating with. - "unsupported_extension": this alert is sent by clients that receive an extended server hello containing an extension that they did not put in the corresponding client hello (see Section 2.3). This message is always fatal. - "unrecognized_name": this alert is sent by servers that receive a server_name extension request, but do not recognize the server name. This message MAY be fatal. - "certificate_unobtainable": this alert is sent by servers who are unable to retrieve a certificate chain from the URL supplied by the client (see Section 3.3). This message MAY be fatal; for example, if client authentication is required by the server for the handshake to continue and the server is unable to retrieve the certificate chain, it may send a fatal alert. - "bad_certificate_status_response": this alert is sent by clients that receive an invalid certificate status response (see Section 3.6). This message is always fatal. - "bad_certificate_hash_value": this alert is sent by servers when a certificate hash does not match a client-provided certificate_hash. This message is always fatal.
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   These error alerts are conveyed using the following syntax:

      enum {
          /* 41 is not defined, for historical reasons */
          unsupported_extension(110),           /* new */
          certificate_unobtainable(111),        /* new */
          unrecognized_name(112),               /* new */
          bad_certificate_status_response(113), /* new */
          bad_certificate_hash_value(114),      /* new */
      } AlertDescription;

5. Procedure for Defining New Extensions

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

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

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

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

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

   -  It would be technically possible to use extensions to change major
      aspects of the design of TLS; for example, the design of cipher
      suite negotiation.  This is not recommended; it would be more
      appropriate to define a new version of TLS, particularly since the
      TLS handshake algorithms have specific protection against version
      rollback attacks based on the version number.  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].
ToP   noToC   RFC4366 - Page 22

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.
ToP   noToC   RFC4366 - Page 23
   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.
ToP   noToC   RFC4366 - Page 24
   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.
ToP   noToC   RFC4366 - Page 25

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: 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.
ToP   noToC   RFC4366 - Page 26
      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

      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

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

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

   Intended usage: COMMON
ToP   noToC   RFC4366 - Page 27
   Change controller:
      IESG <>

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.
ToP   noToC   RFC4366 - Page 28
   [TLSbis]       Dierks, T. and E. Rescorla, "The Transport Layer
                  Security (TLS) Protocol Version 1.1", RFC 4346, April

   [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.
ToP   noToC   RFC4366 - Page 29

Authors' Addresses

Simon Blake-Wilson BCI EMail: Magnus Nystrom RSA Security EMail: David Hopwood Independent Consultant EMail: Jan Mikkelsen Transactionware EMail: Tim Wright Vodafone EMail:
ToP   noToC   RFC4366 - Page 30
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