TLS Working Group                                    Donald Eastlake 3rd
INTERNET-DRAFT                                     Motorola Laboratories
Obsoletes: RFC 4366
Updates: RFC 2246, RFC 4346
Intended status: Proposed Standard
Expires: July 2008                                      January 14, 2009

    Transport Layer Security (TLS) Extensions: Extension Definitions


Status of This Document

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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

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

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This document provides documentation for existing specific TLS
   extensions. It is a companion document for the TLS 1.2 specification,

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

Table of Contents

      Status of This Document....................................1

      Table of Contents..........................................2

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

      3. Server Name Indication..................................5
      4. Maximum Fragment Length Negotiation.....................6
      5. Client Certificate URLs.................................7
      6. Trusted CA Indication..................................10
      7. Truncated HMAC.........................................11
      8. Certificate Status Request.............................12

      9. IANA Considerations....................................15
      10. Security Considerations...............................15
      10.1 Security Considerations for server_name..............15
      10.2 Security Considerations for max_fragment_length......15
      10.3 Security Considerations for client_certificate_url...16
      10.4 Security Considerations for trusted_ca_keys..........17
      10.5 Security Considerations for truncated_hmac...........17
      10.6 Security Considerations for status_request...........18
      11. Internationalization Considerations...................18

      12. Normative References..................................19
      13. Informative References................................19

      Copyright, Disclaimer, and Additional IPR Provisions......21

      Author's Address..........................................22
      Expiration and File Name..................................22

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1. Introduction

   The TLS (Transport Layer Security) Protocol Version 1.2 is specified
   in [RFCTLS]. That specification includes the framework for extensions
   to TLS, considerations in designing such extensions (see Section of [RFCTLS]), and IANA Considerations for the allocation of
   new extension code points; however, it does not specify any
   particular extensions other than CertHashTypes (see Section [RFCTLS]).

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

1.1 Specific Extensions Covered

   The extensions described here focus on extending the functionality
   provided by the TLS protocol message formats. Other issues, such as
   the addition of new cipher suites, are deferred.

   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

   -  Allow TLS clients and servers to negotiate the use of truncated
      MACs. This functionality is desirable in order to conserve
      bandwidth in constrained access networks.

   -  Allow TLS clients and servers to negotiate that the server sends

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      the client certificate status information (e.g., an Online
      Certificate Status Protocol (OCSP) [RFC2560] response) during a
      TLS handshake. This functionality is desirable in order to avoid
      sending a Certificate Revocation List (CRL) over a constrained
      access network and therefore save bandwidth.

   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 [RFC2246] and TLS 1.1 [RFC4346].

1.2 Conventions Used in This Document

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

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3. Server Name Indication

   TLS does not provide a mechanism for a client to tell a server the
   name of the server it is contacting. It may be desirable for clients
   to provide this information to facilitate secure connections to
   servers that host multiple 'virtual' servers at a single underlying
   network address.

   In order to provide the server name, clients MAY include an extension
   of type "server_name" in the (extended) client hello. The
   "extension_data" field of this extension SHALL contain
   "ServerNameList" where:

      struct {
          NameType name_type;
          select (name_type) {
              case host_name: HostName;
          } name;
      } ServerName;

      enum {
          host_name(0), (255)
      } NameType;

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

      struct {
          ServerName server_name_list<1..2^16-1>
      } ServerNameList;

   Currently, the only server names supported are DNS hostnames;
   however, this does not imply any dependency of TLS on DNS, and other
   name types may be added in the future (by an RFC that updates this
   document). TLS MAY treat provided server names as opaque data and
   pass the names and types to the application.

   "HostName" contains the fully qualified DNS hostname of the server,
   as understood by the client. The hostname is represented as a byte
   string using UTF-8 encoding [RFC3629], 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 [RFC3490] 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 [RFC3490],
   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

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   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.

4. Maximum Fragment Length Negotiation

   Without this extension, TLS specifies a fixed maximum plaintext
   fragment length of 2^14 bytes. It may be desirable for constrained
   clients to negotiate a smaller maximum fragment length due to memory
   limitations or bandwidth limitations.

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

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

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

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

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

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

   The negotiated length applies for the duration of the session
   including session resumptions.

   The negotiated length limits the input that the record layer may
   process without fragmentation (that is, the maximum value of
   TLSPlaintext.length; see [RFCTLS], Section 6.2.1). Note that the
   output of the record layer may be larger. For example, if the
   negotiated length is 2^9=512, then for currently defined cipher
   suites (those defined in [RFCTLS], [RFC2712], and [RFC3268]), 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.

5. Client Certificate URLs

   Without this extension, TLS specifies that when client authentication
   is performed, client certificates are sent by clients to servers
   during the TLS handshake. It may be desirable for constrained clients
   to send certificate URLs in place of certificates, so that they do
   not need to store their certificates and can therefore save memory.

   In order to negotiate sending certificate URLs to a server, clients
   MAY include an extension of type "client_certificate_url" in the

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   (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;

      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.

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   -  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 of [RFCTLS].

   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 [RFCTLS], Section 7.4.2), but opposite to the order in
   which certificates are encoded in PkiPath. In either case, the self-
   signed root certificate MAY be omitted from the chain, under the
   assumption that the server must already possess it in order to
   validate it.

   Servers receiving "CertificateURL" SHALL attempt to retrieve the
   client's certificate chain from the URLs and then process the
   certificate chain as usual. A cached copy of the content of any URL
   in the chain MAY be used, provided that a SHA-1 hash is present for
   that URL and it matches the hash of the cached copy.

   Servers that support this extension MUST support the http: URL scheme
   for certificate URLs, and MAY support other schemes. Use of other
   schemes than "http", "https", or "ftp" may create unexpected

   If the protocol used is HTTP, then the HTTP server can be configured
   to use the Cache-Control and Expires directives described in
   [RFC2616] to specify whether and for how long certificates or
   certificate chains should be cached.

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

   If the protocol used to retrieve certificates or certificate chains
   returns a MIME-formatted response (as HTTP does), then the following
   MIME Content-Types SHALL be used: when a single X.509v3 certificate
   is returned, the Content-Type is "application/pkix-cert" [RFC2585],
   and when a chain of X.509v3 certificates is returned, the Content-
   Type is "application/pkix-pkipath" (see Section 8 of [RFCTLS]).

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   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.

6. Trusted CA Indication

   Constrained clients that, due to memory limitations, possess only a
   small number of CA root keys may wish to indicate to servers which
   root keys they possess, in order to avoid repeated handshake

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

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

      struct {
          IdentifierType identifier_type;
          select (identifier_type) {
              case pre_agreed: struct {};
              case key_sha1_hash: SHA1Hash;
              case x509_name: DistinguishedName;
              case cert_sha1_hash: SHA1Hash;
          } identifier;
      } TrustedAuthority;

      enum {
          pre_agreed(0), key_sha1_hash(1), x509_name(2),
          cert_sha1_hash(3), (255)
      } IdentifierType;

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

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   Here "TrustedAuthorities" provides a list of CA root key identifiers
   that the client possesses. Each CA root key is identified via either:

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

   -  "key_sha1_hash": contains the SHA-1 hash of the CA root key. For
      Digital Signature Algorithm (DSA) and Elliptic Curve Digital
      Signature Algorithm (ECDSA) keys, this is the hash of the
      "subjectPublicKey" value. For RSA keys, the hash is of the big-
      endian byte string representation of the modulus without any
      initial 0-valued bytes. (This copies the key hash formats deployed
      in other environments.)

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

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

7. Truncated HMAC

   Currently defined TLS cipher suites use the MAC construction HMAC
   with either MD5 or SHA-1 [RFC2104] to authenticate record layer
   communications. In TLS, the entire output of the hash function is
   used as the MAC tag. However, it may be desirable in constrained
   environments to save bandwidth by truncating the output of the hash
   function to 80 bits when forming MAC tags.

   In order to negotiate the use of 80-bit truncated HMAC, clients MAY

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   include an extension of type "truncated_hmac" in the extended client
   hello. The "extension_data" field of this extension SHALL be empty.

   Servers that receive an extended hello containing a "truncated_hmac"
   extension MAY agree to use a truncated HMAC by including an extension
   of type "truncated_hmac", with empty "extension_data", in the
   extended server hello.

   Note that if new cipher suites are added that do not use HMAC, and
   the session negotiates one of these cipher suites, this extension
   will have no effect. It is strongly recommended that any new cipher
   suites using other MACs consider the MAC size an integral part of the
   cipher suite definition, taking into account both security and
   bandwidth considerations.

   If HMAC truncation has been successfully negotiated during a TLS
   handshake, and the negotiated cipher suite uses HMAC, both the client
   and the server pass this fact to the TLS record layer along with the
   other negotiated security parameters. Subsequently during the
   session, clients and servers MUST use truncated HMACs, calculated as
   specified in [RFC2104]. That is, 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.

8. Certificate Status Request

   Constrained clients may wish to use a certificate-status protocol
   such as OCSP [RFC2560] to check the validity of server certificates,
   in order to avoid transmission of CRLs and therefore save bandwidth
   on constrained networks. This extension allows for such information
   to be sent in the TLS handshake, saving roundtrips and resources.

   In order to indicate their desire to receive certificate status
   information, clients MAY include an extension of type
   "status_request" in the (extended) client hello. The "extension_data"
   field of this extension SHALL contain "CertificateStatusRequest"

      struct {
          CertificateStatusType status_type;
          select (status_type) {
              case ocsp: OCSPStatusRequest;
          } request;

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      } CertificateStatusRequest;

      enum { ocsp(1), (255) } CertificateStatusType;

      struct {
          ResponderID responder_id_list<0..2^16-1>;
          Extensions  request_extensions;
      } OCSPStatusRequest;

      opaque ResponderID<1..2^16-1>;
      opaque Extensions<0..2^16-1>;

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

   Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as
   defined in [RFC2560]. "Extensions" is imported from [RFC3280].  A
   zero-length "request_extensions" value means that there are no
   extensions (as opposed to a zero-length ASN.1 SEQUENCE, which is not
   valid for the "Extensions" type).

   In the case of the "id-pkix-ocsp-nonce" OCSP extension, [RFC2560] is
   unclear about its encoding; for clarification, the nonce MUST be a
   DER-encoded OCTET STRING, which is encapsulated as another OCTET
   STRING (note that implementations based on an existing OCSP client
   will need to be checked for conformance to this requirement).

   Servers that receive a client hello containing the "status_request"
   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;

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      } CertificateStatus;

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

   An "ocsp_response" contains a complete, DER-encoded OCSP response
   (using the ASN.1 type OCSPResponse defined in [RFC2560]). Note that
   only one OCSP response may be sent.

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

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

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

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

          certificate_unobtainable(111),        /* new */
          unrecognized_name(112),               /* new */
          bad_certificate_status_response(113), /* new */
          bad_certificate_hash_value(114),      /* new */
      } AlertDescription;

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9. IANA Considerations

   IANA Considerations for TLS Extensions and the creation of a Registry
   therefore are all covered in Section 12 of [RFCTLS]..

10. Security Considerations

   General Security Considerations for TLS Extensions are covered in
   [RFCTLS]. Security Considerations for particular extensions specified
   in this document are given below.

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

   Additional security considerations are described in the TLS 1.0 RFC
   [RFC2246] and the TLS 1.1 RFC [RFC4346].

10.1 Security Considerations for server_name

   If a single server hosts several domains, then clearly it is
   necessary for the owners of each domain to ensure that this satisfies
   their security needs. Apart from this, server_name does not appear to
   introduce significant security issues.

   Implementations MUST ensure that a buffer overflow does not occur,
   whatever the values of the length fields in server_name.

   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.

10.2 Security Considerations for max_fragment_length

   The maximum fragment length takes effect immediately, including for
   handshake messages. However, that does not introduce any security
   complications that are not already present in TLS, since TLS requires
   implementations to be able to handle fragmented handshake messages.

   Note that as described in Section 4, once a non-null cipher suite has

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   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.

10.3 Security Considerations for client_certificate_url

   There are two major issues with this extension.

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

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

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

   The second major issue is that support for client_certificate_url
   involves the server'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

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   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.

   As discussed in [RFC3986], URLs that specify ports other than the
   default may cause problems, as may very long URLs (which are more
   likely to be useful in exploiting buffer overflow bugs).

   Also note that HTTP caching proxies are common on the Internet, and
   some proxies do not check for the latest version of an object
   correctly. If a request using HTTP (or another caching protocol) goes
   through a misconfigured or otherwise broken proxy, the proxy may
   return an out-of-date response.

10.4 Security Considerations for trusted_ca_keys

   It is possible that which CA root keys a client possesses could be
   regarded as confidential information. As a result, the CA root key
   indication extension should be used with care.

   The use of the SHA-1 certificate hash alternative ensures that each
   certificate is specified unambiguously. As for the previous
   extension, it was not believed necessary to use both MD5 and SHA-1

10.5 Security Considerations for truncated_hmac

   It is possible that truncated MACs are weaker than "un-truncated"
   MACs. However, no significant weaknesses are currently known or
   expected to exist for HMAC with MD5 or SHA-1, truncated to 80 bits.

   Note that the output length of a MAC need not be as long as the

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   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.

10.6 Security Considerations for status_request

   If a client requests an OCSP response, it must take into account that
   an attacker's server using a compromised key could (and probably
   would) pretend not to support the extension. In this case, a client
   that requires OCSP validation of certificates SHOULD either contact
   the OCSP server directly or abort the handshake.

   Use of the OCSP nonce request extension (id-pkix-ocsp-nonce) may
   improve security against attacks that attempt to replay OCSP
   responses; see Section 4.4.1 of [RFC2560] for further details.

11. 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.

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12. Normative References

   [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
   Hashing for Message Authentication", RFC 2104, February 1997.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
   Adams, "X.509 Internet Public Key Infrastructure Online Certificate
   Status Protocol - OCSP", RFC 2560, June 1999.

   [RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
   Infrastructure Operational Protocols: FTP and HTTP", RFC 2585, May

   [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
   L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
   HTTP/1.1", RFC 2616, June 1999.

   [RFC3280] 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.

   [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
   "Internationalizing Domain Names in Applications (IDNA)", RFC 3490,
   March 2003.

   [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
   STD 63, RFC 3629, November 2003.

   [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
   Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January

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

   [RFCTLS] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.2",
   draft-ietf-tls-rfc4346-bis-*.txt, March 2007.

13. Informative References

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

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

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

   [RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
   and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366,
   April 2006.

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Copyright, Disclaimer, and Additional IPR Provisions

   Copyright (C) The IETF Trust (2008).

   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

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-

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Author's Address

   Donald Eastlake 3rd
   Motorola Laboratories
   111 Locke Drive
   Marlborough, MA 01752

   Tel:   +1-508-786-7554

Expiration and File Name

   This draft expires in July 2008.

   Its file name is draft-ietf-tls-rfc4366-bis-01.txt.

Donald Eastlake 3rd                                            [Page 22]