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Internet Engineering Task Force (IETF) Y. Pettersen
Request for Comments: 6961 June 2013
Category: Standards Track
ISSN: 2070-1721
The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension
Abstract
This document defines the Transport Layer Security (TLS) Certificate
Status Version 2 Extension to allow clients to specify and support
several certificate status methods. (The use of the Certificate
Status extension is commonly referred to as "OCSP stapling".) Also
defined is a new method based on the Online Certificate Status
Protocol (OCSP) that servers can use to provide status information
about not only the server's own certificate but also the status of
intermediate certificates in the chain.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6961.
Pettersen Standards Track [Page 1]
RFC 6961 Multiple Certificate Status Extension June 2013
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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it for publication as an RFC or to translate it into languages other
than English.
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 prevent a
client from offering the server multiple status methods.
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RFC 6961 Multiple Certificate Status Extension June 2013
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 [RFC6960]
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. Additionally, using this extension significantly reduces
privacy concerns around the clients informing the certificate issuer
about which sites they are visiting.
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
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RFC 6961 Multiple Certificate Status Extension June 2013
method based on the selected cipher suite and the certificate
presented, no significant changes are needed in the server's
extension format.
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
"status_request_v2" in the ExtensionType enum (originally defined by
[RFC6066]), which uses the following value:
enum {
status_request_v2(17), (65535)
} ExtensionType;
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 in Section 7.4.1.4 of [RFC5246]) of
the extension SHALL contain a CertificateStatusRequestListV2 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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RFC 6961 Multiple Certificate Status Extension June 2013
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>;
} CertificateStatusRequestListV2;
In the OCSPStatusRequest (originally defined by [RFC6066]), the
"ResponderID" 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 identified by the certificates used by
the server. "Extensions" is a DER encoding [X.690] of the OCSP
request extensions, and if the server supports the forwarding of OCSP
request extensions, this value MUST be forwarded without
modification.
Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as
defined in [RFC6960]. "Extensions" is imported from [RFC5280]. A
zero-length "request_extensions" value means that there are no
extensions (as opposed to a DER-encoded zero-length ASN.1 SEQUENCE,
which is not valid for the "Extensions" type).
Servers that support a client's selection of responders using the
ResponderID field could implement this selection by matching the
responder ID values from the client's list with the ResponderIDs of
known OCSP responders, either by using a binary compare of the values
or a hash calculation and compare method.
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). This
has been clarified in [RFC6960].
The items in the list of CertificateStatusRequestItemV2 entries are
ordered according to 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
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RFC 6961 Multiple Certificate Status Extension June 2013
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 "Certificate" message
(Section 7.4.2 of [RFC5246]) (and before any "ServerKeyExchange" or
"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" (defined in [RFC6066]) as follows
(updated from the definition in [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 [X.680] type
OCSPResponse defined in [RFC6960]). Only one OCSP response, with a
length of at least one byte, may be sent for status_type "ocsp".
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
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RFC 6961 Multiple Certificate Status Extension June 2013
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 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 OCSP response received from the server does not result in a
definite "good" or "revoked" status, it is inconclusive. A TLS
client in such a case MAY check the validity of the server
certificate through other means, e.g., by directly querying the
certificate issuer. If such processing still results in an
inconclusive response, then the application using the TLS connection
will have to decide whether to close the connection or not. Note
that this problem cannot be decided by the generic TLS client code
without information from the application. If the application doesn't
provide any such information, then the client MUST abort the
connection, since the server certificate has not been sufficiently
validated.
An example of where the application might wish to continue is with
EAP-TLS (Extensible Authentication Protocol - TLS), where the
application can use another mechanism to check the status of a
certificate once it obtains network access. In this case, the
application could have the client continue with the handshake, but it
MUST NOT disclose a username and password until it has fully
validated the server certificate.
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RFC 6961 Multiple Certificate Status Extension June 2013
3. IANA Considerations
Section 2.1 defines the new TLS extension status_request_v2 (17)
enum, which has been added to the "ExtensionType Values" list in the
IANA "Transport Layer Security (TLS) Extensions" registry.
Section 2.2 describes a TLS CertificateStatusType registry that is
now maintained by IANA. The "TLS Certificate Status Types" registry
has been created 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 "CertificateStatusType" in
Section 2.2.
Value Description Reference
-----------------------------------------
0 Reserved [RFC6961]
1 ocsp [RFC6066] [RFC6961]
2 ocsp_multi [RFC6961]
3-255 Unassigned
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 [RFC6960] for further details.
This extension allows the client to send arbitrary data to the
server. The server implementers need to handle such data carefully
to avoid introducing security vulnerabilities.
The security considerations of [RFC6960] apply to OCSP requests and
responses.
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RFC 6961 Multiple Certificate Status Extension June 2013
5. Acknowledgements
This document is based on [RFC6066], authored by Donald Eastlake 3rd.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, June 2013.
[X.680] ITU-T Recommendation X.680 (2008) | ISO/IEC 8824-1:2008,
"Abstract Syntax Notation One (ASN.1): Specification of
basic notation", November 2008.
[X.690] ITU-T Recommendation X.690 (2008) | ISO/IEC 8825-1:2008,
"ASN.1 encoding rules: Specification of Basic Encoding
Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", November 2008.
6.2. Informative References
[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.
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RFC 6961 Multiple Certificate Status Extension June 2013
Author's Address
Yngve N. Pettersen
EMail: yngve@spec-work.net
Pettersen Standards Track [Page 10]
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