rfc3546.Transport Layer Security (TLS) Extensions
Network Working Group S. Blake-Wilson Request for Comments: 3546 BCI Updates: 2246 M. Nystrom Category: Standards Track RSA Security D. Hopwood Independent Consultant J. Mikkelsen Transactionware T. Wright Vodafone June 2003 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 (2003). All Rights Reserved.
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 1.0 clients that support the extensions and TLS 1.0
servers that do not support the extensions, and vice versa.
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].
Blake-Wilson, et. al. Standards Track [Page 1]
Table of Contents
1. Introduction (2)
2. General Extension Mechanisms (4)
2.1. Extended Client Hello (5)
2.2. Extended Server Hello (5)
2.3. Hello Extensions (6)
2.4. Extensions to the handshake protocol (7)
3. Specific Extensions (8)
3.1. Server Name Indication (8)
3.2. Maximum Fragment Length Negotiation (10)
3.3. Client Certificate URLs (11)
3.4. Trusted CA Indication (14)
3.5. Truncated HMAC (15)
3.6. Certificate Status Request (16)
4. Error alerts (18)
5. Procedure for Defining New Extensions (20)
6. Security Considerations (21)
6.1. Security of server_name (21)
6.2. Security of max_fragment_length (21)
6.3. Security of client_certificate_url (22)
6.4. Security of trusted_ca_keys (23)
6.5. Security of truncated_hmac (23)
6.6. Security of status_request (24)
7. Internationalization Considerations (24)
8. IANA Considerations (24)
9. Intellectual Property Rights (26)
10. Acknowledgments (26)
11. Normative References (27)
12. Informative References (28)
13. Authors’ Addresses (28)
14. Full Copyright Statement (29)
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 like wireless networks.
Blake-Wilson, et. al. Standards Track [Page 2]
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 are designed to:
- Allow TLS clients to provide to the TLS server the name of the
server they are contacting. This functionality is desirable to
facilitate secure connections to servers that host multiple
’virtual’ servers at a single underlying network address.
- Allow TLS clients and servers to negotiate the maximum fragment
length to be sent. This functionality is desirable as a result of memory constraints among some clients, and bandwidth constraints
among some access networks.
- Allow TLS clients and servers to negotiate the use of client
certificate URLs. This functionality is desirable in order to
conserve memory on constrained clients.
- Allow TLS clients to indicate to TLS servers which CA root keys
they possess. This functionality is desirable in order to prevent multiple handshake failures involving TLS clients that are only
able to store a small number of CA root keys due to memory
limitations.
- Allow TLS clients and servers to negotiate the use of truncated
MACs. This functionality is desirable in order to conserve
bandwidth in constrained access networks.
- Allow TLS clients and servers to negotiate that the server sends
the client certificate status information (e.g., an 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.
Blake-Wilson, et. al. Standards Track [Page 3]
The extensions described in this document may be used by TLS 1.0
clients and TLS 1.0 servers. The extensions are designed to be
backwards compatible - meaning that TLS 1.0 clients that support the extensions can talk to TLS 1.0 servers that do not support the
extensions, and vice versa.
Backwards compatibility is primarily achieved via two considerations: - Clients typically request the use of extensions via the extended
client hello message described in Section 2.1. TLS 1.0 [TLS]
requires servers to accept extended client hello messages, even if the server does not "understand" the extension.
- For the specific extensions described here, no mandatory server
response is required when clients request extended functionality. Note however, that although backwards compatibility is supported,
some constrained clients may be forced to reject communications with servers that do not support the extensions as a result of the limited capabilities of such clients.
The remainder of this document is organized as follows. Section 2
describes general extension mechanisms for the client hello and
server hello handshake messages. Section 3 describes specific
extensions to TLS 1.0. Section 4 describes new error alerts for use with the TLS extensions. The final sections of the document address IPR, security considerations, registration of the application/pkix-
pkipath MIME type, acknowledgements, and references.
2. General Extension Mechanisms
This section presents general extension mechanisms for the TLS
handshake client hello and server hello messages.
These general extension mechanisms are necessary in order to enable
clients and servers to negotiate whether to use specific extensions, and how to use specific extensions. The extension formats described are based on [MAILING LIST].
Section 2.1 specifies the extended client hello message format,
Section 2.2 specifies the extended server hello message format, and
Section 2.3 describes the actual extension format used with the
extended client and server hellos.
Blake-Wilson, et. al. Standards Track [Page 4]
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.
Note that [TLS], Section 7.4.1.2, allows additional information to be added to the client hello message. Thus the use of the extended
client hello defined above should not "break" existing TLS 1.0
servers.
A server that supports the extensions mechanism MUST accept only
client hello messages in either the original or extended ClientHello format, and (as for all other messages) MUST check that the amount of data in the message precisely matches one of these formats; if not
then it MUST send a fatal "decode_error" alert. This overrides the
"Forward compatibility note" in [TLS].
2.2. Extended Server Hello
The extended server hello message format MAY be sent in place of the server hello message when the client has requested extended
functionality via the extended client hello message specified in
Section 2.1. The extended server hello message format is:
Blake-Wilson, et. al. Standards Track [Page 5]
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
Extension server_hello_extension_list<0..2^16-1>;
} ServerHello;
Here the new "server_hello_extension_list" field contains a list of
extensions. The actual "Extension" format is defined in Section 2.3. Note that the extended server hello message is only sent in response to an extended client hello message. This prevents the possibility
that the extended server hello message could "break" existing TLS 1.0 clients.
2.3. Hello Extensions
The extension format for extended client hellos and extended server
hellos is:
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
Here:
- "extension_type" identifies the particular extension type.
- "extension_data" contains information specific to the particular
extension type.
The extension types defined in this document are:
enum {
server_name(0), max_fragment_length(1),
client_certificate_url(2), trusted_ca_keys(3),
truncated_hmac(4), status_request(5), (65535)
} ExtensionType;
Note that for all extension types (including those defined in
future), the extension type MUST NOT appear in the extended server
hello unless the same extension type appeared in the corresponding
client hello. Thus clients MUST abort the handshake if they receive an extension type in the extended server hello that they did not
request in the associated (extended) client hello.
Blake-Wilson, et. al. Standards Track [Page 6]
Nonetheless "server initiated" extensions may be provided in the
future within this framework by requiring the client to first send an empty extension to indicate that it supports a particular extension. Also note that when multiple extensions of different types are
present in the extended client hello or the extended server hello,
the extensions may appear in any order. There MUST NOT be more than one extension of the same type.
Finally note that all the extensions defined in this document are
relevant only when a session is initiated. However, a client that
requests resumption of a session does not in general know whether the server will accept this request, and therefore it SHOULD send an
extended client hello if it would normally do so for a new session.
If the resumption request is denied, then a new set of extensions
will be negotiated as normal. If, on the other hand, the older
session is resumed, then the server MUST ignore extensions appearing in the client hello, and send a server hello containing no
extensions; in this case the extension functionality negotiated
during the original session initiation is applied to the resumed
session.
2.4. Extensions to the handshake protocol
This document suggests the use of two new handshake messages,
"CertificateURL" and "CertificateStatus". These messages are
described in Section 3.3 and Section 3.6, respectively. The new
handshake message structure therefore becomes:
enum {
hello_request(0), client_hello(1), server_hello(2),
certificate(11), server_key_exchange (12),
certificate_request(13), server_hello_done(14),
certificate_verify(15), client_key_exchange(16),
finished(20), certificate_url(21), certificate_status(22),
(255)
} HandshakeType;
Blake-Wilson, et. al. Standards Track [Page 7]
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.
Section 3.1 describes the extension of TLS to allow a client to
indicate which server it is contacting. Section 3.2 describes the
extension to provide maximum fragment length negotiation. Section
3.3 describes the extension to allow client certificate URLs.
Section 3.4 describes the extension to allow a client to indicate
which CA root keys it possesses. Section 3.5 describes the extension to allow the use of truncated HMAC. Section 3.6 describes the
extension to support integration of certificate status information
messages into TLS handshakes.
3.1. Server Name Indication
[TLS] does not provide a mechanism for a client to tell a server the name of the server it is contacting. It may be desirable for clients to provide this information to facilitate secure connections to
servers that host multiple ’virtual’ servers at a single underlying
network address.
Blake-Wilson, et. al. Standards Track [Page 8]
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 [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".
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It is RECOMMENDED that clients include an extension of type
"server_name" in the client hello whenever they locate a server by a supported name type.
A server that receives a client hello containing the "server_name"
extension, MAY use the information contained in the extension to
guide its selection of an appropriate certificate to return to the
client, and/or other aspects of security policy. In this event, the server SHALL include an extension of type "server_name" in the
(extended) server hello. The "extension_data" field of this
extension SHALL be empty.
If the server understood the client hello extension but does not
recognize the server name, it SHOULD send an "unrecognized_name"
alert (which MAY be fatal).
If an application negotiates a server name using an application
protocol, then upgrades to TLS, and a server_name extension is sent, then the extension SHOULD contain the same name that was negotiated
in the application protocol. If the server_name is established in
the TLS session handshake, the client SHOULD NOT attempt to request a different server name at the application layer.
3.2. Maximum Fragment Length Negotiation
[TLS] specifies a fixed maximum plaintext fragment length of 2^14
bytes. It may be desirable for constrained clients to negotiate a
smaller maximum fragment length due to memory limitations or
bandwidth limitations.
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.
Blake-Wilson, et. al. Standards Track [Page 10]
Servers that receive an extended client hello containing a
"max_fragment_length" extension, MAY accept the requested maximum
fragment length by including an extension of type
"max_fragment_length" in the (extended) server hello. The
"extension_data" field of this extension SHALL contain
"MaxFragmentLength" whose value is the same as the requested maximum fragment length.
If a server receives a maximum fragment length negotiation request
for a value other than the allowed values, it MUST abort the
handshake with an "illegal_parameter" alert. Similarly, if a client receives a maximum fragment length negotiation response that differs from the length it requested, it MUST also abort the handshake with
an "illegal_parameter" alert.
Once a maximum fragment length other than 2^14 has been successfully negotiated, the client and server MUST immediately begin fragmenting messages (including handshake messages), to ensure that no fragment
larger than the negotiated length is sent. Note that TLS already
requires clients and servers to support fragmentation of handshake
messages.
The negotiated length applies for the duration of the session
including session resumptions.
The negotiated length limits the input that the record layer may
process without fragmentation (that is, the maximum value of
TLSPlaintext.length; see [TLS] section 6.2.1). Note that the output of the record layer may be larger. For example, if the negotiated
length is 2^9=512, then for currently defined cipher suites (those
defined in [TLS], [KERB], and [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. That means that in this event a TLS record layer peer receiving a TLS record layer message larger than 793 bytes may
discard the message and send a "record_overflow" alert, without
decrypting the message.
3.3. Client Certificate URLs
[TLS] specifies that when client authentication is performed, client certificates are sent by clients to servers during the TLS handshake. It may be desirable for constrained clients to send certificate URLs in place of certificates, so that they do not need to store their
certificates and can therefore save memory.
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In order to negotiate to send certificate URLs to a server, clients
MAY include an extension of type "client_certificate_url" in the
(extended) client hello. The "extension_data" field of this
extension SHALL be empty.
(Note that it is necessary to negotiate use of client certificate
URLs in order to avoid "breaking" existing TLS 1.0 servers.)
Servers that receive an extended client hello containing a
"client_certificate_url" extension, MAY indicate that they are
willing to accept certificate URLs by including an extension of type "client_certificate_url" in the (extended) server hello. The
"extension_data" field of this extension SHALL be empty.
After negotiation of the use of client certificate URLs has been
successfully completed (by exchanging hellos including
"client_certificate_url" extensions), clients MAY send a
"CertificateURL" message in place of a "Certificate" message:
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
hashes.
Blake-Wilson, et. al. Standards Track [Page 12]
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, or
- 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 is
either 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.
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.
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).
Blake-Wilson, et. al. Standards Track [Page 13]
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.
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>;
Blake-Wilson, et. al. Standards Track [Page 14]
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 DSA and 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.
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.
Blake-Wilson, et. al. Standards Track [Page 15]
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 suites using other MACs consider the MAC size as an integral part of the cipher suite definition, taking into account both security and
bandwidth considerations.
If HMAC truncation has been successfully negotiated during a TLS
handshake, and the negotiated 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 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:
Blake-Wilson, et. al. Standards Track [Page 16]
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>;
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
Blake-Wilson, et. al. Standards Track [Page 17]
"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.
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.
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- "unrecognized_name" - this alert is sent by servers that receive a server_name extension request, but do not recognize the server
name. This message MAY be fatal.
- "certificate_unobtainable" - this alert is sent by servers who are unable to retrieve a certificate chain from the URL supplied by
the client (see Section 3.3). This message MAY be fatal - for
example if client authentication is required by the server for the handshake to continue and the server is unable to retrieve the
certificate chain, it may send a fatal alert.
- "bad_certificate_status_response" - this alert is sent by clients that receive an invalid certificate status response (see Section
3.6). This message is always fatal.
- "bad_certificate_hash_value" - this alert is sent by servers when a certificate hash does not match a client provided
certificate_hash. This message is always fatal.
These error alerts are conveyed using the following syntax:
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
/* 41 is not defined, for historical reasons */
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110), /* new */
certificate_unobtainable(111), /* new */
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unrecognized_name(112), /* new */
bad_certificate_status_response(113), /* new */
bad_certificate_hash_value(114), /* new */
(255)
} AlertDescription;
5. Procedure for Defining New Extensions
Traditionally for Internet protocols, the Internet Assigned Numbers
Authority (IANA) handles the allocation of new values for future
expansion, and RFCs usually define the procedure to be used by the
IANA. However, there are subtle (and not so subtle) interactions
that may occur in this protocol between new features and existing
features which may result in a significant reduction in overall
security.
Therefore, requests to define new extensions (including assigning
extension and error alert numbers) must be approved by IETF Standards Action.
The following considerations should be taken into account when
designing new extensions:
- All of the extensions defined in this document follow the
convention that for each extension that a client requests and that the server understands, the server replies with an extension of
the same type.
- Some cases where a server does not agree to an extension are error conditions, and some simply a refusal to support a particular
feature. In general error alerts should be used for the former,
and a field in the server extension response for the latter.
- Extensions should as far as possible be designed to prevent any
attack that forces use (or non-use) of a particular feature by
manipulation of handshake messages. This principle should be
followed regardless of whether the feature is believed to cause a security problem.
Often the fact that the extension fields are included in the
inputs to the Finished message hashes will be sufficient, but
extreme care is needed when the extension changes the meaning of
messages sent in the handshake phase. Designers and implementors
should be aware of the fact that until the handshake has been
authenticated, active attackers can modify messages and insert,
remove, or replace extensions.
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