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12 RFC 6066
TLS Working Group Donald Eastlake 3rd
INTERNET-DRAFT Stellar Switches
Obsoletes: 4366
Intended status: Proposed Standard
Expires: January 29, 2011 July 30, 2010
Transport Layer Security (TLS) Extensions: Extension Definitions
<draft-ietf-tls-rfc4366-bis-10.txt>
Abstract
This document provides specifications for existing TLS extensions. It
is a companion document for the TLS 1.2 specification [RFC5246]. 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
Distribution of this document is unlimited. Comments should be sent
to the TLS working group mailing list <tls@ietf.org>.
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 working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
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Donald Eastlake 3rd [Page 1]
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Acknowledgements
This draft is based on material from RFC 4366 for which the authors
were S. Blake-Wilson, M. Nystron, D. Hopwood, J. Mikkelsen, and T.
Wright.
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....................6
3. Server Name Indication..................................7
4. Maximum Fragment Length Negotiation.....................9
5. Client Certificate URLs................................11
6. Trusted CA Indication..................................14
7. Truncated HMAC.........................................16
8. Certificate Status Request.............................17
9. Error Alerts...........................................19
10. IANA Considerations...................................20
10.1 pkipath MIME Type Registration.......................20
11. Security Considerations...............................22
11.1 Security Considerations for server_name..............22
11.2 Security Considerations for max_fragment_length......22
11.3 Security Considerations for client_certificate_url...22
11.4 Security Considerations for trusted_ca_keys..........24
11.5 Security Considerations for truncated_hmac...........24
11.6 Security Considerations for status_request...........24
12. Normative References..................................25
13. Informative References................................25
Annex A: Changes from RFC 4366............................27
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1. Introduction
The TLS (Transport Layer Security) 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 [RFC4366], which covered TLS extensions for TLS 1.0
[RFC2246] and TLS 1.1 [RFC4346].
1.1 Specific Extensions Covered
The extensions described here focus on extending the functionality
provided by the TLS protocol message formats. Other issues, such as
the addition of new cipher suites, are deferred.
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.
- 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
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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) [RFC2560] response) during a
TLS handshake. This functionality is desirable in order to avoid
sending a Certificate Revocation List (CRL) over a constrained
access network and therefore save bandwidth.
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.
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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 RFC
2119.
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2. Extensions to the Handshake Protocol
This document specifies the use of two new handshake messages,
"CertificateURL" and "CertificateStatus". These messages are
described in Section 5 and Section 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;
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3. Server Name Indication
TLS does not provide a mechanism for a client to tell a server the
name of the server it is contacting. It may be desirable for clients
to provide this information to facilitate secure connections to
servers that host multiple 'virtual' servers at a single underlying
network address.
In order to provide 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..
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
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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). However, for backward compatibility, all future 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.
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 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.
If an application negotiates a server name using an application
protocol and then upgrades to TLS, and if a server_name extension is
sent, then the extension SHOULD contain the same name that was
negotiated in the application protocol. If the server_name is
established in the TLS session handshake, the client SHOULD NOT
attempt to request a different server name at the application layer.
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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.
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 for currently defined cipher
suites (those defined in [RFC5246] and [RFC2712]), and when null
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compression is used, 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 DTLS implementations SHOULD NOT generate record_overflow alerts
unless the packet passes message authentication.
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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 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):
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
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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.
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 is not required to follow HTTP redirects when
retrieving the certificates or certificate chain. The URLs used in
this extension SHOULD therefore be chosen not to depend on such
redirects.
If the protocol used to retrieve certificates or certificate chains
returns a MIME-formatted response (as HTTP does), then the following
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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 encounters an unreasonable delay in obtaining
certificates in a given CertificateURL, it SHOULD time out and signal
a certificate_unobtainable(111) error alert. This alert MAY be fatal;
for example, if client authentication is required by the server for
the handshake to continue.
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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 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
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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.
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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
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.
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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>;
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"
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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.
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.
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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),
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.
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10. IANA Considerations
IANA Considerations for TLS Extensions and the creation of a Registry
therefore 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 [RFC4366] should be updated to
reference this document on its publication as an RFC.
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:
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
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).
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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: [RFC4366], and [RFC5280].
Applications which use this media type: TLS. It may also be used by
other protocols, or for general interchange of PKIX certificate
chains.
Additional information:
Magic number(s): DER-encoded ASN.1 can be easily recognized.
Further parsing is required to distinguish it from other ASN.1
types.
File extension(s): .pkipath
Macintosh File Type Code(s): not specified
Person & email address to contact for further information:
Magnus Nystrom <magnus@rsasecurity.com>
Intended usage: COMMON
Change controller: IESG <iesg@ietf.org>
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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.
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
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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.
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.
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11.4 Security Considerations for trusted_ca_keys
It is possible that which CA root keys a client possesses could be
regarded as confidential information. As a result, the CA root key
indication extension should be used with care.
The use of the SHA-1 certificate hash alternative ensures that each
certificate is specified unambiguously. 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.
Since the MAC algorithm only takes effect after all handshake
messages that affect extension parameters have been authenticated by
the hashes in the Finished messages, it is not possible for an active
attacker to force negotiation of the truncated HMAC extension where
it would not otherwise be used (to the extent that the handshake
authentication is secure). Therefore, in the event that any security
problem were found with truncated HMAC in the future, if either the
client or the server for a given session were updated to take the
problem into account, it would be able to veto use of this extension.
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.
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12. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online Certificate
Status Protocol - OCSP", RFC 2560, June 1999.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC 2585, May
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.
[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
13. Informative References
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)", RFC 2712, October 1999.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366,
April 2006.
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[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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Annex 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 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.
Furthermores, 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.
The material was also re-organized in minor ways. For example,
information as to which errors are fatal is moved from the one "Error
Alerts" section to the individual extension specifications.
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Author's Address
Donald Eastlake 3rd
Stellar Switches, Inc.
155 Beaver Street
Milford, MA 01757 USA
Tel: +1-508-333-2270
Email: d3e3e3@gmail.com
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This document is subject to BCP 78 and the IETF Trust's Legal
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language to the contrary, or terms, conditions or rights that differ
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Donald Eastlake 3rd [Page 29]
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