TLS Working Group Simon Blake-Wilson, Certicom
INTERNET-DRAFT Magnus Nystrom, RSA Security
June 20, 2001 David Hopwood, Independent Consultant
Expires December 20, 2001 Jan Mikkelsen, Transactionware
Intended Category: Standards track Tim Wright, Vodafone
TLS Extensions
<draft-ietf-tls-extensions-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or made obsolete by other documents at
any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as work in progress.
The list of current Internet-Drafts may be found at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories may be found at
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Abstract
This document describes extensions that may be used to add
functionality to 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.
This document is based on discussions within the TLS working group
and within the WAP security group.
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 RFC 2119 [KEYWORDS].
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Please send comments on this document to the TLS mailing list.
Table of Contents
1. Introduction .............................................. 2
2. General Extension Mechanisms .............................. 4
2.1. Extended Client Hello ................................... 4
2.2. Extended Server Hello ................................... 5
2.3. Hello Extensions ........................................ 5
2.4. Extensions to the handshake protocol .................... 6
3. Specific Extensions ....................................... 7
3.1. DNS Name Indication ..................................... 7
3.2. Maximum Record Size Negotiation ......................... 8
3.3. Client Certificate URLs ................................. 9
3.4. Trusted CA Indication .................................. 11
3.5. Truncated HMAC ......................................... 12
3.6. OCSP Status Request..................................... 13
4. Error alerts ............................................. 14
5. Procedure for Defining New Extensions..................... 16
6. Security Considerations .................................. 17
6.1. Security of dns_name ................................... 17
6.2. Security of max_record_size ............................ 17
6.3. Security of client_certificate_url ..................... 18
6.4. Security of trusted_ca_keys ............................ 18
6.5. Security of truncated_hmac ............................. 19
6.6. Security of status_request ............................. 19
7. Internationalisation Considerations .......................19
8. Intellectual Property Rights ............................. 20
9. Acknowledgments .......................................... 20
10. References ............................................... 20
11. Authors' Addresses ....................................... 21
1. Introduction
This document describes extensions that may be used to add
functionality to 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.
Wireless environments often suffer from a number of constraints not
commonly present in wired environments - these constraints may
include bandwidth limitations, computational power limitations,
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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 DNS name of the
server they are contacting. This functionality is desirable to
facilitate secure connections to servers which host multiple
'virtual' servers at a single underlying network address.
- Allow TLS clients and servers to negotiate the maximum record size
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 which 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 an OCSP [OCSP] response during a TLS handshake. This
functionality is desirable in order to avoid sending a 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 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.
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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, 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.
2.1. Extended Client Hello
The extended client hello message format MAY be sent in place of the
client hello message format when clients wish to request extended
functionality from servers. The extended client hello message format
is:
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
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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 clients request additional functionality using the
extended client hello, and this functionality is not supplied by the
server, clients 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.
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 1.0
clients.
2.3. Hello Extensions
The extension format for extended client hellos and extended server
hellos is:
struct {
ExtensionType extensionType;
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opaque extension_data<0..2^16-1>;
} Extension;
Here:
- "extensionType" identifies the particular extension type.
- "extension_data" contains information specific to the particular
extension type.
The extension types defined in this document are:
enum {
dns_name(0), max_record_size(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 should appear in the extended server
hello only if 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.
Also note that when multiple extensions are present in the extended
client hello or the extended server hello, the extensions must appear
in the order identified in "ExtensionType". Thus clients and servers
MUST abort the handshake if they receive an extended hello message in
which the extensions are not in the correct order.
Finally note that all the extensions defined in this document are
relevant only when a session is initiated. Extensions appearing in
client and server hellos sent during session resumption MUST be
ignored, and the extension functionality negotiated during session
initiation applied to the resumed session.
2.4. Extensions to the handshake protocol
This document suggests the use of two new handshake messages,
"CertificateURL" and "CertificateAndOCSPResponse". 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),
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certificate_verify(15), client_key_exchange(16),
finished(20), certificate_url(21), ocsp_response(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 ocsp_response: CertificateAndOCSPResponse;
} 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 clients to
indicate which server they are contacting. Section 3.2 describes the
extension to provide maximum record size negotiation. Section 3.3
describes the extension to allow client certificate URLs. Section 3.4
describes the extension to allow clients to indicate which CA root
keys they possess. Section 3.5 describes the extension to allow the
use of truncated HMAC. Section 3.6 describes the extension to
support integration of OCSP into TLS handshakes.
3.1. DNS Name Indication
TLS does not provide a mechanism for clients to tell servers the DNS
name of the server they are contacting. It may be desirable for
clients to provide this information to facilitate secure connections
to servers which host multiple 'virtual' servers at a single
underlying network address.
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In order to provide the server DNS name, clients MAY include an
extension of type "dns_name" in the (extended) client hello. The
"extension_data" field of this extension shall contain:
opaque DNSName<1..2^16-1>;
"DNSName" contains the fully qualified domain name of the server, as
understood by the client. The domain name is represented as a byte
string using UTF-8 encoding [UTF8], without a trailing dot. (Note
that the use of UTF-8 here for encoding internationalized domain
names is independent of the choice of encoding for these names in the
DNS protocol. The latter has yet to be decided by the IETF
International Domain Name Working Group.)
Literal IPv4 and IPv6 addresses are not permitted.
It is RECOMMENDED that clients include an extension of type "DNSName"
in the client hello whenever they locate a server by its domain name.
Servers that receive a client hello containing the "dns_name"
extension, MAY use the information contained in the extension to
guide their selection of an appropriate certificate to return to the
client. In this event, the server shall include an extension of type
"dns_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 DNS name as belonging to a domain it is responsible
for, it should send an unrecognised_domain alert (which may or may
not be fatal).
3.2. Maximum Record Size Negotiation
TLS specifies a fixed maximum record size of 2^14 bytes. It may be
desirable for constrained clients to negotiate a smaller maximum
record size due to memory limitations or bandwidth limitations.
In order to negotiate smaller maximum record sizes, clients MAY
include an extension of type "max_record_size" 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)
} MaxRecordSize;
whose value is the desired maximum record size. The allowed values
for this field are: 2^9, 2^10, 2^11, and 2^12.
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Servers that receive an extended client hello containing a
"max_record_size" extension, MAY accept the requested maximum record
size by including an extension of type "max_record_size" in the
(extended) server hello. The "extension_data" field of this extension
shall contain "MaxRecordSize" whose value is the same as the
requested maximum record size.
Servers receiving maximum record size negotiation requests for values
other than the allowed values MUST abort the handshake with an
"illegal_parameter" alert. Similarly, clients receiving maximum
record size negotiation responses that differ from the size they
requested MUST also abort the handshake with an "illegal_parameter"
alert.
Once a maximum record size other than 2^14 has been successfully
negotiated, the client and server MUST immediately begin fragmenting
messages (including handshake messages), to ensure that no message
larger than the negotiated size is sent. Note that TLS already
requires clients and servers to support fragmentation of handshake
messages.
The negotiated size applies for the duration of the session including
session resumptions.
The negotiated size limits the input that the record layer may
process without fragmentation. Note that the output of the record
layer may be larger. For example, if the negotiated size is 2^9=512,
then for currently defined cipher suites (those defined in [TLS],
[KERB], and planned AES ciphersuites), 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 output an error
without decrypting the message. The exact error message sent will
depend on the size of the received message - either "record_overflow"
if the message is longer than 2^14+2048 bytes, or "decryption_failed"
otherwise.
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.
In order to negotiate to send certificate URLs to a server, clients
MAY include an extension of type "client_certificate_url" in the
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(extended) client hello. The "extension_data" field of this extension
shall be empty.
(Note that it is necessary to negotiate use of client certificate
URLs in order to avoid "breaking" existing TLS 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:
struct {
URLAndHash url_and_hash_list<1..2^16-1>;
} CertificateURL;
struct {
opaque URL<1..2^16-1>;
CertHash certificate_hash;
} URLAndHash;
opaque CertHash[20];
Here "url_and_hash_list" contains a sequence of URLs and SHA-1
hashes, each URL referring to a single, DER-encoded X.509v3
certificate, and the SHA-1 hash of that certificate. The URLs should
occur in the list in the same order that the corresponding
certificates appear in the certificate chain.
Servers receiving "CertificateURL" shall attempt to retrieve the
client's certificate chain from the URLs, and then process the
certificate chain as usual. HTTP SHOULD be used to retrieve the
certificate chain from the URLs, and MUST be supported by servers
supporting this extension.
Servers MUST check that the SHA-1 hash of any certificates retrieved
from a CertificateURL matches the given hash, and then process the
certificate chain as usual.
If any retrieved certificate does not have the correct SHA-1 hash,
the server MUST abort the handshake with a bad_certificate alert.
Note that clients may choose to send either "Certificate" or
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"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 which, 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 null: struct {};
case key_hash_sha: KeyHash;
case x509_name: DistinguishedName;
case cert_hash: CertHash;
} Identifier;
} TrustedAuthority;
enum { null(0), key_hash_sha(1), x509_name(2), cert_hash(3),
(255)}
IdentifierType;
opaque DistinguishedName<1..2^16-1>;
opaque KeyHash[20];
Here "TrustedAuthorities" provides a list of CA root key identifiers
that the client possesses. Each CA root key is identified via either:
- "null" - no CA root key identity supplied.
- "key_hash_sha" - contains the SHA-1 hash of the CA root key. For
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DSA and ECDSA keys, this is the hash of the "subjectPublicKey"
value. For RSA keys, this is the hash of the byte string
representation of the modulus (without any initial 0-valued
bytes). (This copies the key hash formats deployed in other
environments.)
- "cert_hash" - contains the SHA-1 hash of a certificate containing
the CA root key.
- "x509_name" - contains the X.509 distinguished name of the CA.
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 distinguish 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.
3.5. Truncated HMAC
Currently defined TLS ciphersuites 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" in the extended server hello.
Note that if new ciphersuites are added that do not use HMAC, and the
session negotiates one of these ciphersuites, this extension will
have no effect. It is strongly recommended that any new cipher suites
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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 ciphersuite 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]. 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. OCSP Status Request
Constrained clients may wish to use OCSP [OCSP] to check the validity
of server certificates, in order to avoid transmission of CRLs and
therefore save bandwidth on constrained networks.
In order to indicate their desire to use OCSP, clients MAY include an
extension of type "status_request" in the (extended) client hello.
The "extension_data" field of this extension shall contain
"StatusRequest" where:
struct {
ResponderID responder_id_list<0..2^16-1>;
Extensions request_extensions;
} StatusRequest;
opaque ResponderID<1..2^16-1>;
opaque Extensions<0..2^16-1>;
Here "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
the OCSP request extensions.
Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as
defined in [OCSP].
Servers that receive a client hello containing the "status_request"
extension, MAY return an OCSP response to the client along with their
certificate. If so, they SHOULD use the information contained in the
extension when selecting an OCSP responder, and SHOULD include
request_extensions in the OCSP request.
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Servers return an OCSP response along with their certificate by
sending "CertificateAndOCSPResponse" in place of the "Certificate"
message. If a server returns an OCSP response, then the server MUST
have included an extension of type "status_request" with empty
"extension_data" in the extended server hello.
struct {
ASN.1Cert certificate_list<0..2^24-1>;
OCSPResponse ocsp_response;
} CertificateAndOCSPResponse;
opaque ASN.1Cert<1..2^24-1>;
opaque OCSPResponse<1..2^24-1>;
Here "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 "CertificateAndOCSPResponse" message is conveyed using the
handshake message type "ocsp_response".
Note that a server MAY also choose not to send the
"CertificateAndOCSPResponse" message, and instead send the
"Certificate" message, even if it receives a "status_request"
extension in the client hello message.
Note in addition that servers MUST NOT send the
"CertificateAndOCSPResponse" 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 "CertificateAndOCSPResponse" field:
- MUST process the certificate as if it was received in a
"Certificate" message, and;
- SHOULD 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
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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.
- "bad_extension_order" - this alert is sent by clients or servers
that receive an extended hello with the extensions in the wrong
order (see Section 2.3). This message is always fatal.
- "unrecognised_domain" - this alert is sent by servers that
receive a dns_name extension request, but do not recognize the
DNS name as belonging to a domain they are responsible for.
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 is always fatal.
- "bad_ocsp_response" - this alert is sent by clients that receive
an invalid OCSP response (see Section 3.6). 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),
certificate_unobtainable(41), /* new */
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),
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internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110), /* new */
bad_extension_order(111), /* new */
unrecognised_domain(112), /* new */
bad_ocsp_response(113), /* 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) should be forwarded to the IETF
TLS Working Group for discussion.
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.
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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
ciphersuite 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, and 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
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].
6.1. Security of dns_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, dns_name does not appear to
introduce significant security issues.
The length of the domain name should be checked for buffer overflow
(note that RFC 1035 restricts domain names to 255 bytes).
6.2. Security of max_record_size
The maximum record size 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 ciphersuite
has been activated, the effective maximum record size depends on the
ciphersuite, as well as on the negotiated max_record_size. This must
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be taken into account when sizing buffers, and checking for buffer
overflow.
6.3. Security of client_certificate_url
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 checking a
SHA-1 hash of the certificate chain contents, 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.
(The attack that this protects against is admittedly fairly
unrealistic: the attacker would have to get a valid certificate on
the client's key which is different from the client's certificate.
Nevertheless, including this hash guarantees that there can be no
unexpected problems due to the certificate chain not being directly
covered by the Finished hashes.)
A similar approach should also be taken if possible for any future
extensions that involve fetching information from external sources.
Note that 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.
Support for client_certificate_url involves the server acting as a
client in another protocol (usually HTTP, but other URL schemes are
not prohibited). It is therefore subject to many of the same security
considerations that apply to a publically accessible HTTP proxy
server. This includes the possibility that an attacker might use the
server to indirectly attack another host that is vulnerable to some
security flaw. It also includes potentially increased exposure to
denial of service attacks: an attacker can make many connections,
each of which results in the server making an HTTP request.
It is recommended that the client_certificate_url extension should
have to be specifically enabled by a server administrator, rather
than being enabled by default.
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).
6.4. Security of trusted_ca_keys
It is possible that which CA root keys a client possesses could be
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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 the handshake
messages 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 future, if either the client or the server for
a given session have been updated to take into account the problem,
they would be able to veto use of this extension.
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. A client that requires
OCSP validation of certificates SHOULD be prepared to contact the
OCSP server directly in this case.
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. Internationalisation Considerations
None of the extensions defined here directly use strings subject to
localisation. Domain names are encoded using UTF-8. If future
extensions use text strings, then internationalisation should be
considered in their design.
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8. Intellectual Property Rights
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this document. Please address the information to the IETF Executive
Director.
9. Acknowledgments
The authors wish to thank the TLS Working Group and the WAP Security
Group. This document is based on discussion within these groups.
10. References
[HMAC] Krawczyk, H., Bellare, M., and Canetti, R. - HMAC: Keyed-
hashing for message authentication. IETF RFC 2104, February 1997.
[HTTP] J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1," IETF RFC
2616, June 1999.
[KERB] A. Medvinsky, M. Hur, "Addition of Kerberos Cipher Suites to
Transport Layer Security (TLS)," IETF RFC 2712, October 1999.
[KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels," IETF RFC 2119, March 1997.
[MAILING LIST] Mikkelsen, J. Eberhard, R., and J. Kistler, "General
ClientHello extension mechanism and virtual hosting," Ietf-tls
mailing list posting, August 14, 2000.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "Internet X.509 Public Key Infrastructure: Online Certificate
Status Protocol - OCSP," IETF RFC 2560, June 1999.
[TLS] Dierks, T., and C. Allen, "The TLS Protocol - Version 1.0,"
IETF RFC 2246, January 1999.
[URI] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax," IETF RFC 2396, August 1998.
[UTF8] F. Yergeau, "UTF-8, a transformation format of ISO 10646,"
IETF RFC 2279, January 1998.
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11. Authors' Addresses
Simon Blake-Wilson
Certicom Corp.
sblake-wilson@certicom.com
Magnus Nystrom
RSA Security
magnus@rsasecurity.com
David Hopwood
Independent Consultant
david.hopwood@zetnet.co.uk
Jan Mikkelsen
Transactionware
jam@transactionware.com
Tim Wright
Vodafone
timothy.wright@vf.vodafone.co.uk
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