Network Working Group Y. Pettersen
Internet-Draft March 29, 2013
Intended status: Standards Track
Expires: September 30, 2013
The TLS Multiple Certificate Status Request Extension
draft-ietf-tls-multiple-cert-status-extension-05
Abstract
This document defines the Transport Layer Security (TLS) Certificate
Status Version 2 Extension to allow clients to specify and support
multiple certificate status methods. Also defined is a new method
based on the Online Certificate Status Protocol (OCSP) that servers
can use to provide status information not just about the server's own
certificate, but also the status of intermediate certificates in the
chain.
Status of This Memo
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1. Introduction
The Transport Layer Security (TLS) Extension [RFC6066] framework
defines, among other extensions, the Certificate Status Extension
(also referred to as "OCSP stapling") that clients can use to request
the server's copy of the current status of its certificate. The
benefits of this extension include a reduced number of roundtrips and
network delays for the client to verify the status of the server's
certificate and a reduced load on the certificate issuer's status
response servers, thus solving a problem that can become significant
when the issued certificate is presented by a frequently visited
server.
There are two problems with the existing Certificate Status
extension. First, it does not provide functionality to request the
status information about intermediate Certification Authority (CA)
certificates, which means the client has to request status
information through other methods, such as Certificate Revocation
Lists (CRLs), introducing further delays. Second, the current format
of the extension and requirements in the TLS protocol prevents a
client from offering the server multiple status methods.
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Many CAs are now issuing intermediate CA certificates that not only
specify the publication point for their CRLs in a CRL Distribution
Point [RFC5280], but also specify a URL for their OCSP [RFC2560]
server in Authority Information Access [RFC5280]. Given that client-
cached CRLs are frequently out of date, clients would benefit from
using OCSP to access up-to-date status information about intermediate
CA certificates. The benefit to the issuing CA is less clear, as
providing the bandwidth for the OCSP responder can be costly,
especially for CAs with many high-traffic subscriber sites, and this
cost is a concern for many CAs. There are cases where OCSP requests
for a single high-traffic site caused significant network problems
for the issuing CA.
Clients will benefit from the TLS server providing certificate status
information regardless of type, not just for the server certificate,
but also for the intermediate CA certificates. Combining the status
checks into one extension will reduce the roundtrips needed to
complete the handshake by the client to just those needed for
negotiating the TLS connection. Also, for the Certification
Authorities, the load on their servers will depend on the number of
certificates they have issued, not on the number of visitors to those
sites.
For such a new system to be introduced seamlessly, clients need to be
able to indicate support for the existing OCSP Certificate Status
method, and a new multiple-OCSP mode.
Unfortunately, the definition of the Certificate Status extension
only allows a single Certificate Status extension to be defined in a
single extension record in the handshake, and the TLS Protocol
[RFC5246] only allows a single record in the extension list for any
given extension. This means that it is not possible for clients to
indicate support for new methods while still supporting older
methods, which would cause problems for interoperability between
newer clients and older servers. This will not just be an issue for
the multiple status request mode proposed above, but also for any
other future status methods that might be introduced. This will be
true not just for the current PKIX infrastructure [RFC5280], but also
for alternative PKI structures.
The solution to this problem is to define a new extension,
status_request_v2, with an extended format that allows the client to
indicate support for multiple status request methods. This is
implemented using a list of CertificateStatusRequestItemV2 records in
the extension record. As the server will select the single status
method based on the selected cipher suite and the certificate
presented, no significant changes are needed in the server's
extension format.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Presentation language
This document defines protocol structures using the same conventions
and presentation language as defined in Section 4 of [RFC5246].
2. Multiple Certificate Status Extension
2.1. New extension
The extension defined by this document is indicated by the
"status_request_v2" in the ExtensionType enum (originally defined by
[RFC6066]), which uses the following value:
enum {
status_request_v2(XX), (65535)
} ExtensionType;
[[ EDITOR: The value used for status_request_v2 has been left as
"XX". This value will be assigned when this draft progresses to
RFC.]]
2.2. Multiple Certificate Status Request record
Clients that support a certificate status protocol like OCSP may send
the status_request_v2 extension to the server in order to use the TLS
handshake to transfer such data instead of downloading it through
separate connections. When using this extension, the
"extension_data" field (defined by Section 7.4.1.4 in[RFC5246]) of
the extension SHALL contain a CertificateStatusRequestList where:
struct {
CertificateStatusType status_type;
uint16 request_length; /* Length of request field in bytes */
select (status_type) {
case ocsp: OCSPStatusRequest;
case ocsp_multi: OCSPStatusRequest;
} request;
} CertificateStatusRequestItemV2;
enum { ocsp(1), ocsp_multi(2), (255) } CertificateStatusType;
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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>;
struct {
CertificateStatusRequestItemV2 certificate_status_req_list<1..2^16-1>;
} CertificateStatusRequestList;
In the OCSPStatusRequest (originally defined by [RFC6066]), 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, or are identfied by the certificates used by
the server. "Extensions" is a DER encoding [CCITT.X690.2002] of the
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).
The items in the list of CertificateStatusRequestItemV2 entries are
in order of the client's preference (favorite choice first).
A server that receives a client hello containing the
"status_request_v2" extension MAY return a suitable certificate
status response message to the client along with the server's
certificate message. If OCSP is requested, it SHOULD use the
information contained in the extension when selecting an OCSP
responder and SHOULD include request_extensions in the OCSP request.
The server returns a certificate status response along with its
certificate by sending a "CertificateStatus" message (originally
defined by [RFC6066]) immediately after the [RFC5246] (Section 7.4.2)
"Certificate" message (and before any "ServerKeyExchange" or
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"CertificateRequest" messages). If a server returns a
"CertificateStatus" message in response to a status_request_v2
request, then the server MUST have included an extension of type
"status_request_v2" 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
[RFC6066]):
struct {
CertificateStatusType status_type;
select (status_type) {
case ocsp: OCSPResponse;
case ocsp_multi: OCSPResponseList;
} response;
} CertificateStatus;
opaque OCSPResponse<0..2^24-1>;
struct {
OCSPResponse ocsp_response_list<1..2^24-1>;
} OCSPResponseList;
An "OCSPResponse" element (originally defined by [RFC6066]) contains
a complete, DER-encoded OCSP response (using the ASN.1
[CCITT.X680.2002] type OCSPResponse defined in [RFC2560]). Only one
OCSP response, with a length of at least one byte, may be sent for
status_type "ocsp".
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An "ocsp_response_list" contains a list of "OCSPResponse" elements,
as specified above, each containing the OCSP response for the
matching corresponding certificate in the server's Certificate TLS
handshake message. That is, the first entry is the OCSP response for
the first certificate in the Certificate list, the second entry is
the response for the second certificate, and so on. The list MAY
contain fewer OCSP responses than there were certificates in the
Certificate handshake message, but there MUST NOT be more responses
than there were certificates in the list. Individual elements of the
list MAY have a length of 0 (zero) bytes, if the server does not have
the OCSP response for that particular certificate stored, in which
case, the client MUST act as if a response was not received for that
particular certificate. If the client receives a
"ocsp_response_list" that does not contain a response for one or more
of the certificates in the completed certificate chain, the client
SHOULD attempt to validate the certificate using an alternative
retrieval method, such as downloading the relevant CRL; OCSP SHOULD
in this situation only be used for the end entity certificate, not
intermediate CA certificates, for reasons stated above.
Note that a server MAY also choose not to send a "CertificateStatus"
message, even if it has received a "status_request_v2" extension in
the client hello message and has sent a "status_request_v2" extension
in the server hello message. Additionally, note that that a server
MUST NOT send the "CertificateStatus" message unless it received
either a "status_request" or "status_request_v2" extension in the
client hello message and sent a corresponding "status_request" or
"status_request_v2" extension in the server hello message.
Clients requesting an OCSP response and receiving one or more OCSP
responses in a "CertificateStatus" message MUST check the OCSP
response(s) and abort the handshake if the response is a revoked
status or other unacceptable responses (as determined by client
policy), with a bad_certificate_status_response(113) alert. This
alert is always fatal.
If the response is inconclusive, then the client MAY decide to allow
the connection if it believes it will have the opportunity to check
the validity of the certificate through another means, e.g., by
directly querying the issuer's CRL or OCSP responders. The client
MUST abort the connection if it needs to engage in activities that
require trust in the server, and the server certificate has not been
sufficiently validated. An example of where the client might wish to
continue is with EAP-TLS, where the client can use another mechanism
to check the status of a certificate once it obtains network access.
In this case, the client could continue with the handshake, but it
would be inappropriate for the client to disclose a username and
password until it has fully validated the server certificate.
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3. IANA Considerations
Section 2.1 defines the new TLS Extension status_request_v2 enum,
which should be added to the ExtensionType Values list in the IANA
Transport Layer Security (TLS) Extensions registry.
Section 2.2 describes a TLS CertificateStatusType Registry to be
maintained by the IANA. The new registry is called TLS Certificate
Status Types and should be defined under the Transport Layer Security
(TLS) Extensions registry. CertificateStatusType values are to be
assigned via IETF Review as defined in [RFC5226]. The initial
registry corresponds to the definition of "ExtensionType" in
Section 2.2.
Value Description Reference
1 ocsp [This RFC]
2 ocsp_multi [This RFC]
4. Security Considerations
General Security Considerations for TLS Extensions are covered in
[RFC5246]. Security Considerations for the particular extension
specified in this document are given below. In general, implementers
should continue to monitor the state of the art and address any
weaknesses identified.
4.1. Security Considerations for status_request_v2
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.
The security considerations of [RFC2560] apply to OCSP requests and
responses.
5. Acknowledgements
This document is based on [RFC6066] authored by Donald Eastlake 3rd.
6. Normative References
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[CCITT.X680.2002]
International International Telephone and Telegraph
Consultative Committee, "Abstract Syntax Notation One
(ASN.1): Specification of basic notation", CCITT
Recommendation X.680, July 2002.
[CCITT.X690.2002]
International International Telephone and Telegraph
Consultative Committee, "ASN.1 encoding rules:
Specification of basic encoding Rules (BER), Canonical
encoding rules (CER) and Distinguished encoding rules
(DER)", CCITT Recommendation X.690, July 2002.
[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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[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.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
Author's Address
Yngve N. Pettersen
Email: yngve@spec-work.net
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