Network Working Group S. Blake-Wilson

Network Working Group S. Blake-Wilson
Request for Comments: 4366 BCI
Obsoletes: 3546 M. Nystrom
Updates: 4346 RSA Security
Category: Standards Track D. Hopwood
Independent Consultant
J. Mikkelsen
Transactionware
T. Wright
Vodafone
April 2006

Transport Layer Security (TLS) Extensions

Status of This Memo

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

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

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

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

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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 7.4.1.2 of [TLS].

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

This document is a revision of the RFC3546 [RFC3546]. The only major
change concerns the definition of new extensions. New extensions can
now be defined via the IETF Consensus Process (rather than requiring
a standards track RFC). In addition, a few minor clarifications and
editorial improvements were made.

The remainder of this document is organized as follows. Section 2
describes general extension mechanisms for the client hello and
server hello handshake messages. Section 3 describes specific
extensions to TLS. Section 4 describes new error alerts for use with

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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 7.4.1.2, allows additional information to be
added to the client hello message. Thus, the use of the extended
client hello defined above should not "break" existing TLS servers.

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

2.2. Extended Server Hello

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

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

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

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

2.3. Hello Extensions

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

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

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Here:

- "extension_type" identifies the particular extension type.

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

The extension types defined in this document are:

enum {
server_name(0), max_fragment_length(1),
client_certificate_url(2), trusted_ca_keys(3),
truncated_hmac(4), status_request(5), (65535)
} ExtensionType;

The list of defined extension types is maintained by the IANA. The
current list can be found at:
http://www.iana.org/assignments/tls-extensiontype-values. See
Sections 5 and 8 for more information on how new values are added.

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

Nonetheless, "server-oriented" extensions may be provided in the
future within this framework. Such an extension (say, of type x)
would require the client to first send an extension of type x in the
(extended) client hello with empty extension_data to indicate that it
supports the extension type. In this case, the client is offering
the capability to understand the extension type, and the server is
taking the client up on its offer.

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

Finally, note that an extended client hello may be sent both when
starting a new session and when requesting session resumption.
Indeed, a client that requests resumption of a session does not in
general know whether the server will accept this request, and
therefore it SHOULD send an extended client hello if it would
normally do so for a new session. In general the specification of
each extension type must include a discussion of the effect of the
extension both during new sessions and during resumed sessions.

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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
empty.

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

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

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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
hashes.

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
ordering.

The hash corresponding to each URL at the client's discretion either
is not present or is the SHA-1 hash of the certificate or certificate
chain (in the case of X.509 certificates, the DER-encoded certificate
or the DER-encoded PkiPath).

Note that when a list of URLs for X.509 certificates is used, the
ordering of URLs is the same as that used in the TLS Certificate
message (see [TLS], Section 7.4.2), but opposite to the order in
which certificates are encoded in PkiPath. In either case, the
self-signed root certificate MAY be omitted from the chain, under the
assumption that the server must already possess it in order to
validate it.

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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
problems.

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

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

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

If a SHA-1 hash is present for an URL, then the server MUST check
that the SHA-1 hash of the contents of the object retrieved from that
URL (after decoding any MIME Content-Transfer-Encoding) matches the
given hash. If any retrieved object does not have the correct SHA-1
hash, the server MUST abort the handshake with a
"bad_certificate_hash_value" alert.

Note that clients may choose to send either "Certificate" or
"CertificateURL" after successfully negotiating the option to send
certificate URLs. The option to send a certificate is included to
provide flexibility to clients possessing multiple certificates.

If a server encounters an unreasonable delay in obtaining
certificates in a given CertificateURL, it SHOULD time out and signal
a "certificate_unobtainable" error alert.

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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
derivation.

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

3.6. Certificate Status Request

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

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

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

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

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

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

Blake-Wilson, et al. Standards Track [Page 17]

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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
request.

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

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

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

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

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

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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 {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
/* 41 is not defined, for historical reasons */
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110), /* new */
certificate_unobtainable(111), /* new */
unrecognized_name(112), /* new */
bad_certificate_status_response(113), /* new */
bad_certificate_hash_value(114), /* new */
(255)
} AlertDescription;

5. Procedure for Defining New Extensions

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

(This means that new assignments can be made only via RFCs approved
by the IESG.)

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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].

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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.

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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.

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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.

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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: [email protected]
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.

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This is identical to the definition published in [X509-4th-TC1];
note that it is different from that in [X509-4th].

All Certificates MUST conform to [PKIX]. (This should be
interpreted as a requirement to encode only PKIX-conformant
certificates using this type. It does not necessarily require
that all certificates that are not strictly PKIX-conformant must
be rejected by relying parties, although the security consequences
of accepting any such certificates should be considered
carefully.)

DER (as opposed to BER) encoding MUST be used. If this type is
sent over a 7-bit transport, base64 encoding SHOULD be used.

Security considerations:
The security considerations of [X509-4th] and [PKIX] (or any
updates to them) apply, as well as those of any protocol that uses
this type (e.g., TLS).

Note that this type only specifies a certificate chain that can be
assessed for validity according to the relying party's existing
configuration of trusted CAs; it is not intended to be used to
specify any change to that configuration.

Interoperability considerations:
No specific interoperability problems are known with this type,
but for recommendations relating to X.509 certificates in general,
see [PKIX].

Published specification: RFC 4366 (this memo), and [PKIX].

Applications which use this media type: TLS. It may also be used by
other protocols, or for general interchange of PKIX certificate
chains.

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

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

Intended usage: COMMON

Blake-Wilson, et al. Standards Track [Page 26]

RFC 4366 TLS Extensions April 2006

Change controller:
IESG <[email protected]>

9. Acknowledgements

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

10. Normative References

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

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

[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.

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

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

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

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

[PKIX] Housley, R., Polk, W., Ford, W., and D. Solo,
"Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC
3280, April 2002.

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

Blake-Wilson, et al. Standards Track [Page 27]

RFC 4366 TLS Extensions April 2006

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

[URI] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.

[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[X509-4th] ITU-T Recommendation X.509 (2000) | ISO/IEC
9594-8:2001, "Information Systems - Open Systems
Interconnection - The Directory: Public key and
attribute certificate frameworks."

[X509-4th-TC1] ITU-T Recommendation X.509(2000) Corrigendum 1(2001) |
ISO/IEC 9594-8:2001/Cor.1:2002, Technical Corrigendum
1 to ISO/IEC 9594:8:2001.

11. Informative References

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

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

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

[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
J., and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.

Blake-Wilson, et al. Standards Track [Page 28]

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Authors' Addresses

Simon Blake-Wilson
BCI

EMail: [email protected]

Magnus Nystrom
RSA Security

EMail: [email protected]

David Hopwood
Independent Consultant

EMail: [email protected]

Jan Mikkelsen
Transactionware

EMail: [email protected]

Tim Wright
Vodafone

EMail: [email protected]

Blake-Wilson, et al. Standards Track [Page 29]

RFC 4366 TLS Extensions April 2006

Full Copyright Statement

Copyright (C) The Internet Society (2006).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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The IETF takes no position regarding the validity or scope of any
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on the procedures with respect to rights in RFC documents can be
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Copies of IPR disclosures made to the IETF Secretariat and any
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The IETF invites any interested party to bring to its attention any
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this standard. Please address the information to the IETF at
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Acknowledgement

Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).

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