Internet Engineering Task Force (IETF) D. Eastlake 3rd
Request for Comments: 6066 Huawei
Obsoletes: 4366 January 2011
Category: Standards Track
ISSN: 2070-1721
Transport Layer Security (TLS) Extensions: Extension Definitions
Abstract
This document provides specifications for existing TLS extensions.
It is a companion document for RFC 5246, "The Transport Layer
Security (TLS) Protocol Version 1.2". The extensions specified are
server_name, max_fragment_length, client_certificate_url,
trusted_ca_keys, truncated_hmac, and status_request.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6066.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Eastlake Standards Track [Page 1]RFC 6066 TLS Extension Definitions January 2011
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Table of Contents
1. Introduction ....................................................3
1.1. Specific Extensions Covered ................................3
1.2. Conventions Used in This Document ..........................5
2. Extensions to the Handshake Protocol ............................5
3. Server Name Indication ..........................................6
4. Maximum Fragment Length Negotiation .............................8
5. Client Certificate URLs .........................................9
6. Trusted CA Indication ..........................................12
7. Truncated HMAC .................................................13
8. Certificate Status Request .....................................14
9. Error Alerts ...................................................16
10. IANA Considerations ...........................................17
10.1. pkipath MIME Type Registration ...........................17
10.2. Reference for TLS Alerts, TLS HandshakeTypes, and
ExtensionTypes ...........................................19
11. Security Considerations .......................................19
11.1. Security Considerations for server_name ..................19
11.2. Security Considerations for max_fragment_length ..........20
11.3. Security Considerations for client_certificate_url .......20
11.4. Security Considerations for trusted_ca_keys ..............21
11.5. Security Considerations for truncated_hmac ...............21
11.6. Security Considerations for status_request ...............22
12. Normative References ..........................................22
13. Informative References ........................................23
Appendix A. Changes from RFC 4366 .................................24
Appendix B. Acknowledgements ......................................25
Eastlake Standards Track [Page 2]RFC 6066 TLS Extension Definitions January 2011
1. Introduction
The Transport Layer Security (TLS) Protocol Version 1.2 is specified
in [RFC5246]. That specification includes the framework for
extensions to TLS, considerations in designing such extensions (see
Section 7.4.1.4 of [RFC5246]), and IANA Considerations for the
allocation of new extension code points; however, it does not specify
any particular extensions other than Signature Algorithms (see
Section 7.4.1.4.1 of [RFC5246]).
This document provides the specifications for existing TLS
extensions. It is, for the most part, the adaptation and editing of
material from RFC 4366, which covered TLS extensions for TLS 1.0 (RFC
2246) and TLS 1.1 (RFC 4346).
1.1. Specific Extensions Covered
The extensions described here focus on extending the functionality
provided by the TLS protocol message formats. Other issues, such as
the addition of new cipher suites, are deferred.
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;
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.
Eastlake Standards Track [Page 3]RFC 6066 TLS Extension Definitions January 2011
- Allow TLS clients to indicate to TLS servers which certification
authority (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
Message Authentication Codes (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) [RFC2560] response) during a
TLS handshake. This functionality is desirable in order to avoid
sending a Certificate Revocation List (CRL) over a constrained
access network and therefore saving bandwidth.
TLS clients and servers may use the extensions described in this
document. 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.
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 document are
relevant only when a session is initiated. A client that requests
session resumption does not in general know whether the server will
accept this request, and therefore it SHOULD send the same extensions
as it would send if it were not attempting resumption. When a client
includes one or more of the defined extension types in an extended
client hello while requesting session resumption:
- The server name indication extension MAY be used by the server
when deciding whether or not to resume a session as described in
Section 3.
- 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.
Eastlake Standards Track [Page 4]RFC 6066 TLS Extension Definitions January 2011
1.2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
2. Extensions to the Handshake Protocol
This document specifies the use of two new handshake messages,
"CertificateURL" and "CertificateStatus". These messages are
described in Sections 5 and 8, 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;
Eastlake Standards Track [Page 5]RFC 6066 TLS Extension Definitions January 2011
3. Server Name Indication
TLS does not provide a mechanism for a client to tell a server the
name of the server it is contacting. It may be desirable for clients
to provide this information to facilitate secure connections to
servers that host multiple 'virtual' servers at a single underlying
network address.
In order to provide any of the server names, 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;
The ServerNameList MUST NOT contain more than one name of the same
name_type. If the server understood the ClientHello extension but
does not recognize the server name, the server SHOULD take one of two
actions: either abort the handshake by sending a fatal-level
unrecognized_name(112) alert or continue the handshake. It is NOT
RECOMMENDED to send a warning-level unrecognized_name(112) alert,
because the client's behavior in response to warning-level alerts is
unpredictable. If there is a mismatch between the server name used
by the client application and the server name of the credential
chosen by the server, this mismatch will become apparent when the
client application performs the server endpoint identification, at
which point the client application will have to decide whether to
proceed with the communication. TLS implementations are encouraged
to make information available to application callers about warning-
level alerts that were received or sent during a TLS handshake. Such
information can be useful for diagnostic purposes.
Eastlake Standards Track [Page 6]RFC 6066 TLS Extension Definitions January 2011
Note: Earlier versions of this specification permitted multiple
names of the same name_type. In practice, current client
implementations only send one name, and the client cannot
necessarily find out which name the server selected. Multiple
names of the same name_type are therefore now prohibited.
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). The data structure associated with the host_name NameType
is a variable-length vector that begins with a 16-bit length. For
backward compatibility, all future data structures associated with
new NameTypes MUST begin with a 16-bit length field. 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 ASCII encoding without a trailing dot. This allows the
support of internationalized domain names through the use of A-labels
defined in [RFC5890]. DNS hostnames are case-insensitive. The
algorithm to compare hostnames is described in [RFC5890], Section
2.3.2.4.
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.
When the server is deciding whether or not to accept a request to
resume a session, the contents of a server_name extension MAY be used
in the lookup of the session in the session cache. The client SHOULD
include the same server_name extension in the session resumption
request as it did in the full handshake that established the session.
A server that implements this extension MUST NOT accept the request
to resume the session if the server_name extension contains a
different name. Instead, it proceeds with a full handshake to
establish a new session. When resuming a session, the server MUST
NOT include a server_name extension in the server hello.
Eastlake Standards Track [Page 7]RFC 6066 TLS Extension Definitions January 2011
If an application negotiates a server name using an application
protocol and then upgrades to TLS, and if a server_name extension is
sent, then the extension SHOULD contain the same name that was
negotiated in the application protocol. If the server_name is
established in the TLS session handshake, the client SHOULD NOT
attempt to request a different server name at the application layer.
4. Maximum Fragment Length Negotiation
Without this extension, TLS specifies a fixed maximum plaintext
fragment length of 2^14 bytes. It may be desirable for constrained
clients to negotiate a smaller maximum fragment length due to memory
limitations or bandwidth limitations.
In order to negotiate smaller maximum fragment lengths, clients MAY
include an extension of type "max_fragment_length" in the (extended)
client hello. The "extension_data" field of this extension SHALL
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.
Eastlake Standards Track [Page 8]RFC 6066 TLS Extension Definitions January 2011
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 [RFC5246], 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, when using currently defined
cipher suites (those defined in [RFC5246] and [RFC2712]) and null
compression, the record-layer output can be at most 805 bytes: 5
bytes of headers, 512 bytes of application data, 256 bytes of
padding, and 32 bytes of MAC. This means that in this event a TLS
record-layer peer receiving a TLS record-layer message larger than
805 bytes MUST discard the message and send a "record_overflow"
alert, without decrypting the message. When this extension is used
with Datagram Transport Layer Security (DTLS), implementations SHOULD
NOT generate record_overflow alerts unless the packet passes message
authentication.
5. Client Certificate URLs
Without this extension, TLS specifies that when client authentication
is performed, client certificates are sent by clients to servers
during the TLS handshake. It may be desirable for constrained
clients to send certificate URLs in place of certificates, so that
they do not need to store their certificates and can therefore save
memory.
In order to negotiate sending certificate URLs to a server, clients
MAY include an extension of type "client_certificate_url" in the
(extended) client hello. The "extension_data" field of this
extension SHALL be empty.
(Note that it is necessary to negotiate the 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 as
follows (see also Section 2):
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RFC 6066 TLS Extension Definitions January 2011
enum {
individual_certs(0), pkipath(1), (255)
} CertChainType;
struct {
CertChainType type;
URLAndHash url_and_hash_list<1..2^16-1>;
} CertificateURL;
struct {
opaque url<1..2^16-1>;
unint8 padding;
opaque SHA1Hash[20];
} URLAndHash;
Here, "url_and_hash_list" contains a sequence of URLs and hashes.
Each "url" MUST be an absolute URI reference according to [RFC3986]
that can be immediately used to fetch the certificate(s).
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 10.1.
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 "padding" byte MUST be 0x01. It is present to make the structure
backwards compatible.
The hash corresponding to each URL 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 [RFC5246], 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.
Eastlake Standards Track [Page 10]RFC 6066 TLS Extension Definitions January 2011
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 the SHA-1 hash matches the
hash of the cached copy.
Servers that support this extension MUST support the 'http' URI
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
[RFC2616] to specify whether and for how long certificates or
certificate chains should be cached.
The TLS server MUST NOT follow HTTP redirects when retrieving the
certificates or certificate chain. The URLs used in this extension
MUST NOT be chosen 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" [RFC2585],
and when a chain of X.509v3 certificates is returned, the Content-
Type is "application/pkix-pkipath" (Section 10.1).
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(114) alert. This alert
is always fatal.
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 is unable to obtain certificates in a given
CertificateURL, it MUST send a fatal certificate_unobtainable(111)
alert if it requires the certificates to complete the handshake. If
the server does not require the certificates, then the server
continues the handshake. The server MAY send a warning-level alert
in this case. Clients receiving such an alert SHOULD log the alert
and continue with the handshake if possible.
Eastlake Standards Track [Page 11]
RFC 6066 TLS Extension Definitions January 2011
6. Trusted CA Indication
Constrained clients that, due to memory limitations, possess only a
small number of CA root keys may wish to indicate to servers which
root keys they possess, in order to avoid repeated handshake
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 zero-valued bytes. (This copies the key hash formats
deployed in other environments.)
Eastlake Standards Track [Page 12]RFC 6066 TLS Extension Definitions January 2011
- "x509_name": contains the DER-encoded X.509 DistinguishedName of
the CA.
- "cert_sha1_hash": contains the SHA-1 hash of a DER-encoded
Certificate containing the CA root key.
Note that clients may include none, some, or all of the CA root keys
they possess in this extension.
Note also that it is possible that a key hash or a Distinguished Name
alone may not uniquely identify a certificate issuer (for example, if
a particular CA has multiple key pairs). However, here we assume
this is the case following the use of Distinguished Names to identify
certificate issuers in TLS.
The option to include no CA root keys is included to allow the client
to indicate possession of some pre-defined set of CA root keys.
Servers that receive a client hello containing the "trusted_ca_keys"
extension MAY use the information contained in the extension to guide
their selection of an appropriate certificate chain to return to the
client. In this event, the server SHALL include an extension of type
"trusted_ca_keys" in the (extended) server hello. The
"extension_data" field of this extension SHALL be empty.
7. Truncated HMAC
Currently defined TLS cipher suites use the MAC construction HMAC
[RFC2104] to authenticate record-layer communications. In TLS, the
entire output of the hash function is used as the MAC tag. However,
it may be desirable in constrained environments to save bandwidth by
truncating the output of the hash function to 80 bits when forming
MAC tags.
In order to negotiate the use of 80-bit truncated HMAC, clients MAY
include an extension of type "truncated_hmac" in the extended client
hello. The "extension_data" field of this extension SHALL be empty.
Servers that receive an extended hello containing a "truncated_hmac"
extension MAY agree to use a truncated HMAC by including an extension
of type "truncated_hmac", with empty "extension_data", in the
extended server hello.
Note that if new cipher suites are added that do not use HMAC, and
the session negotiates one of these cipher suites, this extension
will have no effect. It is strongly recommended that any new cipher
suites using other MACs consider the MAC size an integral part of the
Eastlake Standards Track [Page 13]RFC 6066 TLS Extension Definitions January 2011
cipher suite definition, taking into account both security and
bandwidth considerations.
If HMAC truncation has been successfully negotiated during a TLS
handshake, and the negotiated cipher suite uses HMAC, both the client
and the server pass this fact to the TLS record layer along with the
other negotiated security parameters. Subsequently during the
session, clients and servers MUST use truncated HMACs, calculated as
specified in [RFC2104]. That is, SecurityParameters.mac_length 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.
8. Certificate Status Request
Constrained clients may wish to use a certificate-status protocol
such as OCSP [RFC2560] to check the validity of server certificates,
in order to avoid transmission of CRLs and therefore save bandwidth
on constrained networks. This extension allows for such information
to be sent in the TLS handshake, saving roundtrips and resources.
In order to indicate their desire to receive certificate status
information, clients MAY include an extension of type
"status_request" in the (extended) client hello. The
"extension_data" field of this extension SHALL contain
"CertificateStatusRequest" 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>;
Eastlake Standards Track [Page 14]RFC 6066 TLS Extension Definitions January 2011
In the OCSPStatusRequest, the "ResponderIDs" provides a list of OCSP
responders that the client trusts. A zero-length "responder_id_list"
sequence has the special meaning that the responders are implicitly
known to the server, e.g., by prior arrangement. "Extensions" is a
DER encoding of OCSP request extensions.
Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as
defined in [RFC2560]. "Extensions" is imported from [RFC5280]. A
zero-length "request_extensions" value means that there are no
extensions (as opposed to a zero-length ASN.1 SEQUENCE, which is not
valid for the "Extensions" type).
In the case of the "id-pkix-ocsp-nonce" OCSP extension, [RFC2560] is
unclear about its encoding; for clarification, the nonce MUST be a
DER-encoded OCTET STRING, which is encapsulated as another OCTET
STRING (note that implementations based on an existing OCSP client
will need to be checked for conformance to this requirement).
Servers that receive a client hello containing the "status_request"
extension MAY return a suitable certificate status response to the
client along with their certificate. If OCSP is requested, they
SHOULD use the information contained in the extension when selecting
an OCSP responder and SHOULD include request_extensions in the OCSP
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. The "CertificateStatus" message is conveyed
using the handshake message type "certificate_status" as follows (see
also Section 2):
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 [RFC2560]). Only one
OCSP response may be sent.
Eastlake Standards Track [Page 15]RFC 6066 TLS Extension Definitions January 2011
Note that a server MAY also choose not to send a "CertificateStatus"
message, even if has received a "status_request" extension in the
client hello message and has sent a "status_request" extension in the
server hello message.
Note in addition that a server MUST NOT send the "CertificateStatus"
message unless it received a "status_request" extension in the client
hello message and sent a "status_request" extension in the server
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 with
bad_certificate_status_response(113) alert. This alert is always
fatal.
9. Error Alerts
Four new error alerts are defined for use with the TLS extensions
defined in this document. 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. These error alerts are conveyed using the
following syntax. The new alerts are the last four, as indicated by
the comments on the same line as the error alert number.
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),
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internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
certificate_unobtainable(111), /* new */
unrecognized_name(112), /* new */
bad_certificate_status_response(113), /* new */
bad_certificate_hash_value(114), /* new */
(255)
} AlertDescription;
"certificate_unobtainable" is described in Section 5.
"unrecognized_name" is described in Section 3.
"bad_certificate_status_response" is described in Section 8.
"bad_certificate_hash_value" is described in Section 5.
10. IANA Considerations
IANA Considerations for TLS extensions and the creation of a registry
are covered in Section 12 of [RFC5246] except for the registration of
MIME type application/pkix-pkipath, which appears below.
The IANA TLS extensions and MIME type application/pkix-pkipath
registry entries that reference RFC 4366 have been updated to
reference this document.
10.1. pkipath MIME Type Registration
MIME media type name: application
MIME subtype name: pkix-pkipath
Required parameters: none
Optional parameters: version (default value is "1")
Encoding considerations:
Binary; 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.
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 [RFC5280]. (This should be
interpreted as a requirement to encode only PKIX-conformant
certificates using this type. It does not necessarily require
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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 [RFC5280] (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 [RFC5280].
Published specification: This document and [RFC5280].
Applications that 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 <mnystrom@microsoft.com>
Intended usage: COMMON
Change controller: IESG <iesg@ietf.org>
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10.2. Reference for TLS Alerts, TLS HandshakeTypes, and ExtensionTypes
The following values in the TLS Alert Registry have been updated to
reference this document:
111 certificate_unobtainable
112 unrecognized_name
113 bad_certificate_status_response
114 bad_certificate_hash_value
The following values in the TLS HandshakeType Registry have been
updated to reference this document:
21 certificate_url
22 certificate_status
The following ExtensionType values have been updated to reference
this document:
0 server_name
1 max_fragment_length
2 client_certificate_url
3 trusted_ca_keys
4 truncated_hmac
5 status_request
11. Security Considerations
General security considerations for TLS extensions are covered in
[RFC5246]. Security Considerations for particular extensions
specified in this document are given below.
In general, implementers should continue to monitor the state of the
art and address any weaknesses identified.
11.1. Security Considerations for server_name
If a single server hosts several domains, then clearly it is
necessary for the owners of each domain to ensure that this satisfies
their security needs. Apart from this, server_name does not appear
to introduce significant security issues.
Since it is possible for a client to present a different server_name
in the application protocol, application server implementations that
rely upon these names being the same MUST check to make sure the
client did not present a different name in the application protocol.
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Implementations MUST ensure that a buffer overflow does not occur,
whatever the values of the length fields in server_name.
11.2. Security Considerations for max_fragment_length
The maximum fragment length takes effect immediately, including for
handshake messages. However, that does not introduce any security
complications that are not already present in TLS, since TLS requires
implementations to be able to handle fragmented handshake messages.
Note that, as described in Section 4, once a non-null cipher suite
has 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.
11.3. Security Considerations for client_certificate_url
Support for client_certificate_url involves the server's acting as a
client in another URI-scheme-dependent protocol. The server
therefore becomes subject to many of the same security concerns that
clients of the URI 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 URI
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.
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RFC 6066 TLS Extension Definitions January 2011
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 schemes be enabled by the
administrator individually, and only a minimal set of schemes be
enabled. Unusual protocols that offer limited security or whose
security is not well understood SHOULD be avoided.
As discussed in [RFC3986], URLs that specify ports other than the
default may cause problems, as may very long URLs (which are more
likely to be useful in exploiting buffer overflow bugs).
This extension continues to use SHA-1 (as in RFC 4366) and does not
provide algorithm agility. The property required of SHA-1 in this
case is second pre-image resistance, not collision resistance.
Furthermore, even if second pre-image attacks against SHA-1 are found
in the future, an attack against client_certificate_url would require
a second pre-image that is accepted as a valid certificate by the
server and contains the same public key.
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.
11.4. Security Considerations for trusted_ca_keys
Potentially, the 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. This context does not
require a cryptographic hash function, so the use of SHA-1 is
considered acceptable, and no algorithm agility is provided.
11.5. Security Considerations for truncated_hmac
It is possible that truncated MACs are weaker than "un-truncated"
MACs. However, no significant weaknesses are currently known or
expected to exist for HMAC with MD5 or SHA-1, truncated to 80 bits.
Note that the output length of a MAC need not be as long as the
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.
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RFC 6066 TLS Extension Definitions January 2011
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
problems 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.
11.6. Security Considerations for status_request
If a client requests an OCSP response, it must take into account that
an attacker's server using a compromised key could (and probably
would) pretend not to support the extension. In this case, a client
that requires OCSP validation of certificates SHOULD either contact
the OCSP server directly or abort the handshake.
Use of the OCSP nonce request extension (id-pkix-ocsp-nonce) may
improve security against attacks that attempt to replay OCSP
responses; see Section 4.4.1 of [RFC2560] for further details.
12. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and
C. Adams, "X.509 Internet Public Key Infrastructure
Online Certificate Status Protocol - OCSP", RFC 2560,
June 1999.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP",
RFC 2585, May 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246, August
2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
13. Informative References
[RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)", RFC 2712,
October 1999.
[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.
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Appendix A. Changes from RFC 4366
The significant changes between RFC 4366 and this document are
described below.
RFC 4366 described both general extension mechanisms (for the TLS
handshake and client and server hellos) as well as specific
extensions. RFC 4366 was associated with RFC 4346, TLS 1.1. The
client and server hello extension mechanisms have been moved into RFC
5246, TLS 1.2, so this document, which is associated with RFC 5246,
includes only the handshake extension mechanisms and the specific
extensions from RFC 4366. RFC 5246 also specifies the unknown
extension error and new extension specification considerations, so
that material has been removed from this document.
The Server Name extension now specifies only ASCII representation,
eliminating UTF-8. It is provided that the ServerNameList can
contain more than only one name of any particular name_type. If a
server name is provided but not recognized, the server should either
continue the handshake without an error or send a fatal error.
Sending a warning-level message is not recommended because client
behavior will be unpredictable. Provision was added for the user
using the server_name extension in deciding whether or not to resume
a session. Furthermore, this extension should be the same in a
session resumption request as it was in the full handshake that
established the session. Such a resumption request must not be
accepted if the server_name extension is different, but instead a
full handshake must be done to possibly establish a new session.
The Client Certificate URLs extension has been changed to make the
presence of a hash mandatory.
For the case of DTLS, the requirement to report an overflow of the
negotiated maximum fragment length is made conditional on passing
authentication.
TLS servers are now prohibited from following HTTP redirects when
retrieving certificates.
The material was also re-organized in minor ways. For example,
information as to which errors are fatal is moved from the "Error
Alerts" section to the individual extension specifications.
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RFC 6066 TLS Extension Definitions January 2011
Appendix B. Acknowledgements
This document is based on material from RFC 4366 for which the
authors were S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen,
and T. Wright. Other contributors include Joseph Salowey, Alexey
Melnikov, Peter Saint-Andre, and Adrian Farrel.
Author's Address
Donald Eastlake 3rd
Huawei
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
EMail: d3e3e3@gmail.com
Eastlake Standards Track [Page 25]
