Internet DRAFT - draft-pettersen-tls-ext-multiple-ocsp
draft-pettersen-tls-ext-multiple-ocsp
Network Working Group Y. Pettersen
Internet-Draft Opera Software ASA
Intended status: Standards Track March 6, 2012
Expires: September 7, 2012
Adding Multiple TLS Certificate Status Extension requests
draft-pettersen-tls-ext-multiple-ocsp-03
Abstract
This document introduces a replacement of the TLS Certificate Status
Extension to allow clients to specify and support multiple
certificate status methods. Also being introduced is a new OCSP-
based method 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.
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].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 7, 2012.
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document authors. All rights reserved.
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1. Introduction
The Transport Layer Security Extension [RFC6066] framework defines,
among other extensions, the Certificate Status Extension 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 using other retrieval methods 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
as it is currently defined. First, it does not provide functionality
to request the status information about intermediate CA certificates,
which means the client has to request this through other retrieval
methods, such as CRLs, which can cause significant delays when the
revocation list has to be refreshed. Second, the current format of
the extension and requirements in the TLS protocol prevents a client
from offering the server multiple status methods.
Many Certificate Authorities are now issuing intermediate CA
certificates that not only specify a CRL Distribution Point[RFC5280],
but also a URL for OCSP [RFC2560] Certificate Status requests. Given
that CRLs, particularly those cached by the client, are frequently
less up to date than is possible for an OCSP responder, using OCSP to
access up-to-date status information about intermediate CA
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certificates will be of great benefit to clients. 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 subscriber
sites. The author is aware that the cost of providing this bandwidth
just for site certificates has been a concern to some CAs,
particularly the smaller ones, and it follows naturally that doubling
or tripling this cost could be problematic for the CAs. The author
is also aware of at least one case when OCSP requests for a single
high-traffic site caused significant network problems for the issuing
CA.
For these reasons, it will be beneficial to use the TLS server to
provide the certificate status information not just for the server
certificate, but also for the intermediate CA certificates. This
will reduce the roundtrips needed to complete the handshake by the
client to just those needed for negotiating the TLS connection.
Also, for the Certificate 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, it must be
possible for clients 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 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, 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 by using a list of CertificateStatusRequestItem 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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2. Multiple Certificate Status Extension
2.1. New extension
The extension added by this document is indicated by the
"status_request_v2" in the ExtensionType enum, which is updated with
this 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 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;
} CertificateStatusRequestItem;
enum { ocsp(1), ocsp_multi(YY), (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>;
struct {
CertificateStatusRequestItem certificate_status_req_list<1..2^16-1>
} CertificateStatusRequestList
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[[ EDITOR: The value used for ocsp_multi has been left as YY. This
value will be assigned when this draft progresses to RFC.]]
In the OCSPStatusRequestItem, 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 of OCSP request extensions.
Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as
defined in [RFC2560]. "Extensions" is imported from [RFC5280]. A
zero-length "request_extensions" value means that there are no
extensions (as opposed to a zero-length ASN.1 SEQUENCE, which is not
valid for the "Extensions" type).
In the case of the "id-pkix-ocsp-nonce" OCSP extension, [RFC2560] is
unclear about its encoding; for clarification, the nonce MUST be a
DER-encoded OCTET STRING, which is encapsulated as another OCTET
STRING (note that implementations based on an existing OCSP client
will need to be checked for conformance to this requirement).
The list of CertificateStatusRequestItem entries MUST be in order of
preference.
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 status 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 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]):
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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 contains a complete, DER-encoded OCSP
response (using the ASN.1 type OCSPResponse defined in [RFC2560]) .
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
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.
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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 is otherwise not satisfactory with a
bad_certificate_status_response(113) alert. This alert is always
fatal.
[[Open issue: At least one reviewer has suggested that the client
should treat an unsatisfactory (non-revoked) response as an empty
response for that particular response and fall back to the
alternative method described above]]
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
TLS category after IETF Concensus has decided to add the value.
Section 2.2 describes a TLS CertificateStatusType Registry to be
maintained by the IANA. 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.
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.
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5. Alternative methods for similar functionality
There may be alternative methods to accomplish the same objective as
this proposal, but without defining a new TLS Extension.
One possible method is that each OCSP response includes, recursively
or separately, the OCSP response for its own signer as a
responseExtensions. The problem with this method is that either the
responder has to send this to all requestors, leading to increased
bandwidth usage, or a special request extension or URL have to be
sent by the TLS server requesting the OCSP response, and it would
also lead to unnecessary bandwidth usage for the TLS server, if the
connecting client cannot use the extra OCSP responses. It is the
author's opinion that this approach would require the same level of
complexity as the proposed extension in the webservers, in order to
reduce bandwidth consumption, at least while phasing in the
technology, and the OCSP responders would require similar level of
complexity to produce the variant responses.
6. Acknowledgements
This document is based on [RFC6066] authored by Donald Eastlake 3rd.
7. Normative References
[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.
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Author's Address
Yngve N. Pettersen
Opera Software ASA
Email: yngve@opera.com
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