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|
TLS Working Group Donald Eastlake 3rd
INTERNET-DRAFT Motorola Laboratories
Obsoletes: RFC 4366
Updates: RFC 2246, RFC 4346
Intended status: Proposed Standard
Expires: Decmeber 2007 June 2007
Transport Layer Security (TLS) Extensions: Extension Definitions
--------- ----- -------- ----- ----------- --------- -----------
<draft-ietf-tls-rfc4366-bis-00.txt>
Status of This Document
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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
http://www.ietf.org/shadow.html
Abstract
This document provides documentation for existing specific TLS
extensions. It is a companion document for the TLS 1.2 specification,
draft-ietf-tls-rfc4346-bis-03.txt.
Donald Eastlake 3rd [Page 1]
INTERNET-DRAFT TLS Extension Definitions
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
Status of This Document....................................1
Abstract...................................................1
Acknowledgements...........................................2
Table of Contents..........................................2
1. Introduction............................................3
1.1 Specific Extensions Covered............................3
1.2 Conventions Used in This Document......................4
3. Server Name Indication..................................5
4. Maximum Fragment Length Negotiation.....................6
5. Client Certificate URLs.................................7
6. Trusted CA Indication..................................10
7. Truncated HMAC.........................................11
8. Certificate Status Request.............................12
9. IANA Considerations....................................15
10. Security Considerations...............................15
10.1 Security Considerations for server_name..............15
10.2 Security Considerations for max_fragment_length......15
10.3 Security Considerations for client_certificate_url...16
10.4 Security Considerations for trusted_ca_keys..........17
10.5 Security Considerations for truncated_hmac...........17
10.6 Security Considerations for status_request...........18
11. Internationalization Considerations...................18
12. Normative References..................................19
13. Informative References................................19
Copyright, Disclaimer, and Additional IPR Provisions......21
Author's Address..........................................22
Expiration and File Name..................................22
Donald Eastlake 3rd [Page 2]
INTERNET-DRAFT TLS Extension Definitions
1. Introduction
The TLS (Transport Layer Security) Protocol Version 1.2 is specified
in [RFCTLS]. That specification includes the framework for extensions
to TLS, considerations in designing such extensions (see Section
7.4.1.4 of [RFCTLS]), and IANA Considerations for the allocation of
new extension code points; however, it does not specify any
particular extensions other than CertHashTypes (see Section
7.4.1.4.1of [RFCTLS]).
This document provides the specifications for existing TLS
extensions. It is, for the most part, the mere adaptation and editing
of material from [RFC4366], which covered all aspects of 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.
Specifically, the extensions described in this document:
- Allow TLS clients to provide to the TLS server the name of the
server they are contacting. This functionality is desirable in
order to facilitate secure connections to servers that host
multiple 'virtual' servers at a single underlying network address.
- Allow TLS clients and servers to negotiate the maximum fragment
length to be sent. This functionality is desirable as a result of
memory constraints among some clients, and bandwidth constraints
among some access networks.
- Allow TLS clients and servers to negotiate the use of client
certificate URLs. This functionality is desirable in order to
conserve memory on constrained clients.
- Allow TLS clients to indicate to TLS servers which CA root keys
they possess. This functionality is desirable in order to prevent
multiple handshake failures involving TLS clients that are only
able to store a small number of CA root keys due to memory
limitations.
- Allow TLS clients and servers to negotiate the use of truncated
MACs. This functionality is desirable in order to conserve
bandwidth in constrained access networks.
- Allow TLS clients and servers to negotiate that the server sends
Donald Eastlake 3rd [Page 3]
INTERNET-DRAFT TLS Extension Definitions
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.
The extensions described in this document may be used by TLS clients
and servers. The extensions are designed to be backwards compatible,
meaning that TLS clients that support the extensions can talk to TLS
servers that do not support the extensions, and vice versa. The
document therefore updates TLS 1.0 [RFC2246] and TLS 1.1 [RFC4346].
1.2 Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Donald Eastlake 3rd [Page 4]
INTERNET-DRAFT TLS Extension Definitions
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 the server name, clients MAY include an extension
of type "server_name" in the (extended) client hello. The
"extension_data" field of this extension SHALL contain
"ServerNameList" where:
struct {
NameType name_type;
select (name_type) {
case host_name: HostName;
} name;
} ServerName;
enum {
host_name(0), (255)
} NameType;
opaque HostName<1..2^16-1>;
struct {
ServerName server_name_list<1..2^16-1>
} ServerNameList;
Currently, the only server names supported are DNS hostnames;
however, this does not imply any dependency of TLS on DNS, and other
name types may be added in the future (by an RFC that updates this
document). TLS MAY treat provided server names as opaque data and
pass the names and types to the application.
"HostName" contains the fully qualified DNS hostname of the server,
as understood by the client. The hostname is represented as a byte
string using UTF-8 encoding [RFC3629], without a trailing dot.
If the hostname labels contain only US-ASCII characters, then the
client MUST ensure that labels are separated only by the byte 0x2E,
representing the dot character U+002E (requirement 1 in Section 3.1
of [RFC3490] notwithstanding). If the server needs to match the
HostName against names that contain non-US-ASCII characters, it MUST
perform the conversion operation described in Section 4 of [RFC3490],
treating the HostName as a "query string" (i.e., the AllowUnassigned
flag MUST be set). Note that IDNA allows labels to be separated by
any of the Unicode characters U+002E, U+3002, U+FF0E, and U+FF61;
therefore, servers MUST accept any of these characters as a label
Donald Eastlake 3rd [Page 5]
INTERNET-DRAFT TLS Extension Definitions
separator. If the server only needs to match the HostName against
names containing exclusively ASCII characters, it MUST compare ASCII
names case-insensitively.
Literal IPv4 and IPv6 addresses are not permitted in "HostName".
It is RECOMMENDED that clients include an extension of type
"server_name" in the client hello whenever they locate a server by a
supported name type.
A server that receives a client hello containing the "server_name"
extension MAY use the information contained in the extension to guide
its selection of an appropriate certificate to return to the client,
and/or other aspects of security policy. In this event, the server
SHALL include an extension of type "server_name" in the (extended)
server hello. The "extension_data" field of this extension SHALL be
empty.
If the server understood the client hello extension but does not
recognize the server name, it SHOULD send an "unrecognized_name"
alert (which MAY be fatal).
If an application negotiates a server name using an application
protocol and then upgrades to TLS, and if a server_name extension is
sent, then the extension SHOULD contain the same name that was
negotiated in the application protocol. If the server_name is
established in the TLS session handshake, the client SHOULD NOT
attempt to request a different server name at the application layer.
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.
Donald Eastlake 3rd [Page 6]
INTERNET-DRAFT TLS Extension Definitions
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 [RFCTLS], 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 [RFCTLS], [RFC2712], and [RFC3268]), and
when null compression is used, the record layer output can be at most
793 bytes: 5 bytes of headers, 512 bytes of application data, 256
bytes of padding, and 20 bytes of MAC. This means that in this event
a TLS record layer peer receiving a TLS record layer message larger
than 793 bytes may discard the message and send a "record_overflow"
alert, without decrypting the message.
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
Donald Eastlake 3rd [Page 7]
INTERNET-DRAFT TLS Extension Definitions
(extended) client hello. The "extension_data" field of this extension
SHALL be empty.
(Note that it is necessary to negotiate use of client certificate
URLs in order to avoid "breaking" existing TLS servers.)
Servers that receive an extended client hello containing a
"client_certificate_url" extension MAY indicate that they are willing
to accept certificate URLs by including an extension of type
"client_certificate_url" in the (extended) server hello. The
"extension_data" field of this extension SHALL be empty.
After negotiation of the use of client certificate URLs has been
successfully completed (by exchanging hellos including
"client_certificate_url" extensions), clients MAY send a
"CertificateURL" message in place of a "Certificate" message:
enum {
individual_certs(0), pkipath(1), (255)
} CertChainType;
enum {
false(0), true(1)
} Boolean;
struct {
CertChainType type;
URLAndOptionalHash url_and_hash_list<1..2^16-1>;
} CertificateURL;
struct {
opaque url<1..2^16-1>;
Boolean hash_present;
select (hash_present) {
case false: struct {};
case true: SHA1Hash;
} hash;
} URLAndOptionalHash;
opaque SHA1Hash[20];
Here "url_and_hash_list" contains a sequence of URLs and optional
hashes.
When X.509 certificates are used, there are two possibilities:
- If CertificateURL.type is "individual_certs", each URL refers to a
single DER-encoded X.509v3 certificate, with the URL for the client's
certificate first.
Donald Eastlake 3rd [Page 8]
INTERNET-DRAFT TLS Extension Definitions
- If CertificateURL.type is "pkipath", the list contains a single
URL referring to a DER-encoded certificate chain, using the type
PkiPath described in Section 8 of [RFCTLS].
When any other certificate format is used, the specification that
describes use of that format in TLS should define the encoding format
of certificates or certificate chains, and any constraint on their
ordering.
The hash corresponding to each URL at the client's discretion either
is not present or is the SHA-1 hash of the certificate or certificate
chain (in the case of X.509 certificates, the DER-encoded certificate
or the DER-encoded PkiPath).
Note that when a list of URLs for X.509 certificates is used, the
ordering of URLs is the same as that used in the TLS Certificate
message (see [RFCTLS], Section 7.4.2), but opposite to the order in
which certificates are encoded in PkiPath. In either case, the self-
signed root certificate MAY be omitted from the chain, under the
assumption that the server must already possess it in order to
validate it.
Servers receiving "CertificateURL" SHALL attempt to retrieve the
client's certificate chain from the URLs and then process the
certificate chain as usual. A cached copy of the content of any URL
in the chain MAY be used, provided that a SHA-1 hash is present for
that URL and it matches the hash of the cached copy.
Servers that support this extension MUST support the http: URL scheme
for certificate URLs, and MAY support other schemes. 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
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" (see Section 8 of [RFCTLS]).
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If a SHA-1 hash is present for an URL, then the server MUST check
that the SHA-1 hash of the contents of the object retrieved from that
URL (after decoding any MIME Content-Transfer-Encoding) matches the
given hash. If any retrieved object does not have the correct SHA-1
hash, the server MUST abort the handshake with a
"bad_certificate_hash_value" alert.
Note that clients may choose to send either "Certificate" or
"CertificateURL" after successfully negotiating the option to send
certificate URLs. The option to send a certificate is included to
provide flexibility to clients possessing multiple certificates.
If a server encounters an unreasonable delay in obtaining
certificates in a given CertificateURL, it SHOULD time out and signal
a "certificate_unobtainable" error alert.
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>;
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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
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
with either MD5 or SHA-1 [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
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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, CipherSpec.hash_size is 10 bytes,
and only the first 10 bytes of the HMAC output are transmitted and
checked. Note that this extension does not affect the calculation of
the pseudo-random function (PRF) as part of handshaking or key
derivation.
The negotiated HMAC truncation size applies for the duration of the
session including session resumptions.
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;
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} 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 [RFC3280]. 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.
struct {
CertificateStatusType status_type;
select (status_type) {
case ocsp: OCSPResponse;
} response;
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} 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]). Note that
only one OCSP response may be sent.
The "CertificateStatus" message is conveyed using the handshake
message type "certificate_status".
Note that a server MAY also choose not to send a "CertificateStatus"
message, even if it receives a "status_request" extension in the
client hello message.
Note in addition that servers MUST NOT send the "CertificateStatus"
message unless it received a "status_request" extension in the client
hello message.
Clients requesting an OCSP response and receiving an OCSP response in
a "CertificateStatus" message MUST check the OCSP response and abort
the handshake if the response is not satisfactory.
certificate_unobtainable(111), /* new */
unrecognized_name(112), /* new */
bad_certificate_status_response(113), /* new */
bad_certificate_hash_value(114), /* new */
(255)
} AlertDescription;
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9. IANA Considerations
IANA Considerations for TLS Extensions and the creation of a Registry
therefore are all covered in Section 12 of [RFCTLS]..
10. Security Considerations
General Security Considerations for TLS Extensions are covered in
[RFCTLS]. 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.
Additional security considerations are described in the TLS 1.0 RFC
[RFC2246] and the TLS 1.1 RFC [RFC4346].
10.1 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.
Implementations MUST ensure that a buffer overflow does not occur,
whatever the values of the length fields in server_name.
Although this document specifies an encoding for internationalized
hostnames in the server_name extension, it does not address any
security issues associated with the use of internationalized
hostnames in TLS (in particular, the consequences of "spoofed" names
that are indistinguishable from another name when displayed or
printed). It is recommended that server certificates not be issued
for internationalized hostnames unless procedures are in place to
mitigate the risk of spoofed hostnames.
10.2 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
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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.
10.3 Security Considerations for client_certificate_url
There are two major issues with this extension.
The first major issue is whether or not clients should include
certificate hashes when they send certificate URLs.
When client authentication is used *without* the
client_certificate_url extension, the client certificate chain is
covered by the Finished message hashes. The purpose of including
hashes and checking them against the retrieved certificate chain is
to ensure that the same property holds when this extension is used,
i.e., that all of the information in the certificate chain retrieved
by the server is as the client intended.
On the other hand, omitting certificate hashes enables functionality
that is desirable in some circumstances; for example, clients can be
issued daily certificates that are stored at a fixed URL and need not
be provided to the client. Clients that choose to omit certificate
hashes should be aware of the possibility of an attack in which the
attacker obtains a valid certificate on the client's key that is
different from the certificate the client intended to provide.
Although TLS uses both MD5 and SHA-1 hashes in several other places,
this was not believed to be necessary here. The property required of
SHA-1 is second pre-image resistance.
The second major issue is that support for client_certificate_url
involves the server's acting as a client in another URL protocol.
The server therefore becomes subject to many of the same security
concerns that clients of the URL scheme are subject to, with the
added concern that the client can attempt to prompt the server to
connect to some (possibly weird-looking) URL.
In general, this issue means that an attacker might use the server to
indirectly attack another host that is vulnerable to some security
flaw. It also introduces the possibility of denial of service attacks
in which an attacker makes many connections to the server, each of
which results in the server's attempting a connection to the target
of the attack.
Note that the server may be behind a firewall or otherwise able to
access hosts that would not be directly accessible from the public
Internet. This could exacerbate the potential security and denial of
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service problems described above, as well as allow the existence of
internal hosts to be confirmed when they would otherwise be hidden.
The detailed security concerns involved will depend on the URL
schemes supported by the server. In the case of HTTP, the concerns
are similar to those that apply to a publicly accessible HTTP proxy
server. In the case of HTTPS, loops and deadlocks may be created, and
this should be addressed. In the case of FTP, attacks arise that are
similar to FTP bounce attacks.
As a result of this issue, it is RECOMMENDED that the
client_certificate_url extension should have to be specifically
enabled by a server administrator, rather than be enabled by default.
It is also RECOMMENDED that URI protocols be enabled by the
administrator individually, and only a minimal set of protocols be
enabled. Unusual protocols that offer limited security or whose
security is not well-understood SHOULD be avoided.
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).
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.
10.4 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. As for the previous
extension, it was not believed necessary to use both MD5 and SHA-1
hashes.
10.5 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
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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.
10.6 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.
11. Internationalization Considerations
None of the extensions defined here directly use strings subject to
localization. Domain Name System (DNS) hostnames are encoded using
UTF-8. If future extensions use text strings, then
internationalization should be considered in their design.
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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.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[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.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)", RFC 3490,
March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January
2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFCTLS] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.2",
draft-ietf-tls-rfc4346-bis-03.txt, March 2007.
13. Informative References
[RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)", RFC 2712, October 1999.
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[RFC3268] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites
for Transport Layer Security (TLS)", RFC 3268, June 2002.
[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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Copyright, Disclaimer, and Additional IPR Provisions
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
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Author's Address
Donald Eastlake 3rd
Motorola Laboratories
111 Locke Drive
Marlborough, MA 01752
Tel: +1-508-786-7554
Email: Donald.Eastlake@motorola.com
Expiration and File Name
This draft expires in December 2007.
Its file name is draft-ietf-tls-rfc4366-bis-00.txt.
Donald Eastlake 3rd [Page 22]
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