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+TLS Working Group Donald Eastlake 3rd
+INTERNET-DRAFT Motorola Laboratories
+Obsoletes: RFC 4366
+Intended status: Proposed Standard
+Expires: August 2008 February 20, 2008
+
+
+ Transport Layer Security (TLS) Extensions: Extension Definitions
+
+ <draft-ietf-tls-rfc4366-bis-02.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-07.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
+
+ 2. Extensions to the Handshake Protocol....................5
+
+ 3. Server Name Indication..................................6
+ 4. Maximum Fragment Length Negotiation.....................7
+ 5. Client Certificate URLs.................................8
+ 6. Trusted CA Indication..................................11
+ 7. Truncated HMAC.........................................12
+ 8. Certificate Status Request.............................13
+
+ 9. Error Alerts...........................................16
+
+ 10. IANA Considerations...................................17
+ 11. Security Considerations...............................17
+ 11.1 Security Considerations for server_name..............17
+ 11.2 Security Considerations for max_fragment_length......17
+ 11.3 Security Considerations for client_certificate_url...18
+ 11.4 Security Considerations for trusted_ca_keys..........19
+ 11.5 Security Considerations for truncated_hmac...........19
+ 11.6 Security Considerations for status_request...........20
+
+ 12. Normative References..................................21
+ 13. Informative References................................21
+
+ Copyright, Disclaimer, and Additional IPR Provisions......22
+
+ Author's Address..........................................23
+ Expiration and File Name..................................23
+
+
+
+
+
+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 Signature Algorithms (see Section
+ 7.4.1.4.1 of [RFCTLS]).
+
+ 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.
+
+ 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.
+
+
+
+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].
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+Donald Eastlake 3rd [Page 4]
+
+INTERNET-DRAFT TLS Extension Definitions
+
+
+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;
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Donald Eastlake 3rd [Page 5]
+
+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;
+
+ If the server understood the client hello extension but does not
+ recognize any of the server names, it SHOULD send an
+ unrecognized_name(112) alert (which MAY be fatal).
+
+ 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 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.
+
+
+
+Donald Eastlake 3rd [Page 6]
+
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+
+ 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.
+
+ 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
+
+
+Donald Eastlake 3rd [Page 7]
+
+INTERNET-DRAFT TLS Extension Definitions
+
+
+ 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
+ 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 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
+ (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
+
+
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+
+ "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;
+
+ 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.
+
+ - 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.
+
+
+
+Donald Eastlake 3rd [Page 9]
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+ 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]).
+
+ 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(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.
+
+
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+ 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.
+
+
+
+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
+
+
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+
+ 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
+ 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
+
+
+Donald Eastlake 3rd [Page 12]
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+ 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.
+
+
+
+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>;
+
+
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+
+ 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. 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>;
+
+
+
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+
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+
+
+ 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 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 with
+ bad_certificate_status_response(113) alert. This alert is always
+ fatal.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Donald Eastlake 3rd [Page 15]
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+INTERNET-DRAFT TLS Extension Definitions
+
+
+9. Error Alerts
+
+ This section defines new error alerts for use with the TLS extensions
+ defined in this document.
+
+ Four new error alerts are defined. To avoid "breaking" existing
+ clients and servers, these alerts MUST NOT be sent unless the sending
+ party has received an extended hello message from the party they are
+ communicating with. These error alerts are conveyed using the
+ following syntax:
+
+ enum {
+ close_notify(0),
+ unexpected_message(10),
+ bad_record_mac(20),
+ decryption_failed(21),
+ record_overflow(22),
+ decompression_failure(30),
+ handshake_failure(40),
+ /* 41 is not defined, for historical reasons */
+ bad_certificate(42),
+ unsupported_certificate(43),
+ certificate_revoked(44),
+ certificate_expired(45),
+ certificate_unknown(46),
+ illegal_parameter(47),
+ unknown_ca(48),
+ access_denied(49),
+ decode_error(50),
+ decrypt_error(51),
+ export_restriction(60),
+ protocol_version(70),
+ insufficient_security(71),
+ internal_error(80),
+ user_canceled(90),
+ no_renegotiation(100),
+ unsupported_extension(110),
+ 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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+
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+
+
+10. IANA Considerations
+
+ IANA Considerations for TLS Extensions and the creation of a Registry
+ therefore are all covered in Section 12 of [RFCTLS]..
+
+
+
+11. 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].
+
+
+
+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.
+
+ 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.
+
+
+
+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
+
+
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+
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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.
+
+
+
+11.3 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
+
+
+Donald Eastlake 3rd [Page 18]
+
+INTERNET-DRAFT TLS Extension Definitions
+
+
+ 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.
+
+
+
+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. As for the previous
+ extension, it was not believed necessary to use both MD5 and SHA-1
+ hashes.
+
+
+
+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
+
+
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+
+INTERNET-DRAFT TLS Extension Definitions
+
+
+ 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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+
+INTERNET-DRAFT TLS Extension Definitions
+
+
+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.
+
+ [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.
+
+ [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-*.txt, March 2007.
+
+
+
+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.
+
+ [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.
+
+
+Donald Eastlake 3rd [Page 21]
+
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+
+
+Copyright, Disclaimer, and Additional IPR Provisions
+
+ Copyright (C) The IETF Trust (2008).
+
+ 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
+ assurances of licenses to be made available, or the result of an
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Donald Eastlake 3rd [Page 22]
+
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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 August 2008.
+
+ Its file name is draft-ietf-tls-rfc4366-bis-02.txt.
+
+
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