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Internet Engineering Task Force                          P. Hallam-Baker
Internet-Draft                                         Comodo Group Inc.
Intended status: Standards Track                       November 14, 2012
Expires: May 18, 2013


               X.509v3 Extension: OCSP Stapling Required
                 draft-hallambaker-tlssecuritypolicy-03

Abstract

   The purpose of the TLS Security Policy extension is to prevent
   downgrade attacks that are not otherwise prevented by the TLS
   protocol.  In particular, the TLS Security Policy extension may be
   used to mandate support for revocation checking features in the TLS
   protocol such as OCSP stapling.  Informing clients that an OCSP
   status response will always be stapled permits an immediate failure
   in the case that the response is not stapled.  This in turn prevents
   a denial of service attack that might otherwise be possible.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 18, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     3.1.  SecurityPolicy  . . . . . . . . . . . . . . . . . . . . . . 4
       3.1.1.  status_request  . . . . . . . . . . . . . . . . . . . . 5
     3.2.  Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
       3.2.1.  Certificate Signing Request . . . . . . . . . . . . . . 5
       3.2.2.  Certificate Signing Certificate . . . . . . . . . . . . 5
       3.2.3.  End Entity Certificate  . . . . . . . . . . . . . . . . 6
     3.3.  Processing  . . . . . . . . . . . . . . . . . . . . . . . . 6
       3.3.1.  Certification Authority . . . . . . . . . . . . . . . . 6
       3.3.2.  Server  . . . . . . . . . . . . . . . . . . . . . . . . 6
       3.3.3.  Client  . . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
     5.1.  Alternative Certificates and Certificate Issuers  . . . . . 7
     5.2.  Denial of Service . . . . . . . . . . . . . . . . . . . . . 7
     5.3.  Cipher Suite Downgrade Attack . . . . . . . . . . . . . . . 7
   6.  For discussion. . . . . . . . . . . . . . . . . . . . . . . . . 7
     6.1.  Mandated client behavior  . . . . . . . . . . . . . . . . . 8
     6.2.  OCSP Analog . . . . . . . . . . . . . . . . . . . . . . . . 8
     6.3.  Other protocols and applications  . . . . . . . . . . . . . 8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9


















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1.  Definitions

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].


2.  Purpose

   The purpose of the TLS Security Policy extension is to prevent
   downgrade attacks that are not otherwise prevented by the TLS
   protocol.

   Since the TLS protocol itself provides strong protection against most
   forms of downgrade attack, the TLS Security Policy is only relevant
   to the validation of TLS protocol credentials.  In particular to the
   revocation status of the credentials presented.

   At the time of writing, the only TLS feature that is relevant to the
   revocation status of credentials is the Certificate Status Request
   extension (status_request) used to support in-band exchange of OCSP
   tokens, otherwise known as OCSP stapling.  This extension is
   described in RFC 4366 [RFC4366].

   The TLS Security Policy mechanism described in this document is
   designed to support policy statements that prevent a downgrade attack
   against the current OCSP stapling mechanism and possible future
   certificate revocation mechanisms such as stapling of multiple OCSP
   tokens.

   The OCSP stapling mechanism described in RFC 4366 [RFC4366] permits a
   TLS server to provide evidence of valid certificate status inband and
   thus improve client response.  A TLS Security Policy that advertises
   the status_request extension informs a client that if the
   status_request is specified in a TLS Client Helo, that a server
   compliant with the policy MUST respond with a valid OCSP token for
   the End Entity Certificate it presents.

   Use of the TLS Security Policy extension in this fashion permits a
   client to avoid reliance on certificates that are revoked for the
   reasons that occur most frequently.  In particular it allows a client
   to avoid mis-reliance on certificates that are revoked for cause or
   at the request of the subject (e.g. because of a compromised private
   key).

   Advertising the status_request extension permits a client to fail



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   immediately in the case that the token is not provided by the server
   without the need to query the OCSP responder in addition.  This
   improves client efficiency and more importantly prevents a denial of
   service attack against the client by either blocking the OCSP
   response or mounting a denial of service attack against the OCSP
   responder.

   Since the TLS Security Policy extension is an option, it is not
   likely that an attacker attempting to obtain a certificate through
   fraud will choose to have a certificate issued with this extension.
   Such risks are more approrpriately addressed by mechanisms such as
   Certificate Authority Authorization records that are designed to
   prevent or mitigate mis-issue.  Nevertheless a Certification
   Authority MAY consider the presence or absence of a required security
   policy as one factor in determining the level of additional scruitiny
   a request should be subject to.

   Any security policy specified in an End Entity certificate MUST be
   followed by the server or clients MAY refuse connection.  It is
   important therefore that a Certification Authority only issue
   certificates that specify policies that match the configuration of
   the server and that the server is capable of verifying that its
   configuration is compatible with the security policy of the
   certificates it offers.  Ideally, the TLS security policy would be
   specified by the client as part of the certificate issue process.

   This document describes a mechanism that MAY be used to provide this
   communication in-band for the most commonly used certificate
   registration protocol.


3.  Syntax

   The TLS Security Policy extension has the following format:

   cabf-tls-security-policy OBJECT IDENTIFIER ::=  { cabf 1 }

   SecurityPolicy ::= SEQUENCE OF INTEGER

   The TLS Security Policy Extension MAY be marked critical.
   Implementations that process the extension MUST ignore the
   criticality bit setting.

3.1.  SecurityPolicy

   The SecurityPolicy extension lists a sequence of TLS extension
   identifiers that a server compliant with the policy MUST support and
   accept on client request.



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   This specification does not require a TLS client to offer or support
   any TLS extension regardless of whether it is specified in the TLS
   Security Policy or not.  In particular a client MAY request and a
   server MAY support any TLS extension regardless of whether it is
   specified in a TLS security extension or not.

   If a TLS Security Policy extension specifies a TLS extension, a
   server offering the certificate MUST support the extension specified
   and MUST comply with any specific requirements specified for that
   extension in this document or in the document that specifies the TLS
   extension.

3.1.1.  status_request

   If the TLS status_request extension is specified in the TLS Security
   Policy extension and a TLS client specifies the status_request
   extensionin the Client Hello, a server MUST return a valid OCSP token
   for the specified End Entity certificate in the response.

3.2.  Use

3.2.1.  Certificate Signing Request

   If the certificate issue mechanism makes use of the PKCS#10
   Certificate Signing Request (CSR) RFC 4366 [RFC4366], the CSR MAY
   specify a TLS Security Policy extension as a CSR attribute.  A server
   or server administration tool should only generate key signing
   requests that it knows can be supperted by the server for which the
   certificate is intended.

3.2.2.  Certificate Signing Certificate

   When present in a Certificate Signing Certificate, the TLS Security
   Policy extension specifies a constraint on valid certificate chains.
   Specifically, a certificate chain is only valid if each certificate
   in the chain specifies a TLS Security Policy that is at least as
   restrictive as that specified in the certificate for the key used to
   sign it.

   While relying clients MAY reject certificates that do not comply with
   this particular requirement, the use of TLs Security Policy in
   Certificate Signing Certificates is primarily intended for use by
   parties seeking to evaluate the performance of certificate issuers
   and MAY be ignored by clients.







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3.2.3.  End Entity Certificate

   When specified in an End Entity Certificate, the TLS Security Policy
   extension specifies criteria that a server MUST meet to be compliant
   with the policy.

   In the case that a client determines that the server configuration is
   inconsistent with the specified policy it MAY reject the TLS
   configuration.

   In the case that a client determines that the server configuration is
   inconsistent with a policy specifying support for the TLS
   status_request extension it SHOULD reject the TLS configuration.

3.3.  Processing

3.3.1.  Certification Authority

   A CA SHOULD NOT issue certs with the extension unless there is an
   affirmative statement to the effect that stapling is requested.

   For example the use of the extension in the CSR or through an out of
   band communication.

3.3.2.  Server

   A server SHOULD verify that its configuration is compatible with the
   TLS Security Policy extension expressed in a certificate it presents.
   A server MAY override local configuration options if necessary to
   ensure consistency but SHOULD inform the administrator whenever such
   an inconsitency is discovered.

   A server SHOULD NOT override local configuration options to offer use
   of cipher suites that have been otherwise deprecated as insecure but
   MAY override local configuration to offer stronger cipher suites that
   would otherwise have been the case.  For example, a server that would
   normally only offer DES might offer 3DES but not vice versa.

   A server SHOULD support generation of the extension in CSRs if key
   generation is supported.

3.3.3.  Client

   A compliant client MUST process the TLS Security Policy Extension and
   MUST ignore the setting of the X.509 criticality flag.

   A compliant client SHOULD reject a TLS connection with security
   properties that are inconsistent with the specified TLS Security



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   Policy extension.  A compliant client MAY accept such a TLS
   connection request however if it is determined that doing so is
   appropriate in particular circumstances.


4.  Acknowledgements

   [List of CABForum and PKIX contributors]


5.  Security Considerations

5.1.  Alternative Certificates and Certificate Issuers

   Use of the TLS Security Policy to mandate support for a particular
   form of revocation checking is optional.  This control can provide
   protection in the case that a certificate with a TLS Security Policy
   is compromised after issue but not in the case that the attacker
   obtains an unmarked certificate from an issuer through fraud.

   TLS Security Policy is a post-issue security control.  Such risks can
   only be addressed by security controls that take effect before issue.

5.2.  Denial of Service

   A certificate Issuer could issue a certificate that intentionally
   specified a security policy that they knew the server could not
   support.

   The risks of such refusal would appear to be negligible since a
   Certificate Authority could equally refuse to issue the certificate.

5.3.  Cipher Suite Downgrade Attack

   The TLS Security Policy extension does not provide protection against
   a cipher suite downgrade attack.  This is left to the existing
   controls in the TLS protocol itself.


6.  For discussion.

   [RFC EDITOR: DELETE PRIOR TO PUBLICATION]

   During the design of the extension, various proposals were made to
   add functionality.  This section explains the reasons that particular
   functionality was chosen to be supported.  It should be deleted by
   the RFC editor prior to publication.




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6.1.  Mandated client behavior

   Many certificate issuers (including this one) would like to mandate a
   particular set of client behavior when a certificate is processed.
   For example, requiring that a certificate 'hard fail' in cases where
   the server is unable to obtain OCSP status.

   Desirable though this functionality is to certificate issuers, it is
   hard to see how client providers are likely to provide support.  In
   particular the browser providers are already aware that CAs would
   prefer that they 'hard fail' on OCSP status.  Will expressing that
   request in a certificate make them any more likely to comply?

6.2.  OCSP Analog

   A related extension for OCSP may also be useful.  In particular an
   OCSP security policy extension might specify that the service
   supported particular OCSP extensions such as the digest locator.

6.3.  Other protocols and applications

   Should we describe a similar extension for code signing?

   While such an extension would clearly be useful, execution would
   require a specification for code signing to refer to.


7.  IANA Considerations

   No action by IANA is required.


8.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, April 2006.

   [X.509]    International Telecommunication Union, "ITU-T
              Recommendation X.509 (11/2008): Information technology -
              Open systems interconnection - The Directory: Public-key
              and attribute certificate frameworks", ITU-T



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              Recommendation X.509, November 2008.

   [X.680]    International Telecommunication Union, "ITU-T
              Recommendation X.680 (11/2008): Information technology -
              Abstract Syntax Notation One (ASN.1): Specification of
              basic notation", ITU-T Recommendation X.680,
              November 2008.


Author's Address

   Phillip Hallam-Baker
   Comodo Group Inc.

   Email: philliph@comodo.com




































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