





Internet Engineering Task Force (IETF)                   P. Hallam-Baker
Request for Comments: 7633                             Comodo Group Inc.
Category: Standards Track                                   October 2015
ISSN: 2070-1721


        X.509v3 Transport Layer Security (TLS) Feature Extension

Abstract

   The purpose of the TLS feature extension is to prevent downgrade
   attacks that are not otherwise prevented by the TLS protocol.  In
   particular, the TLS feature extension may be used to mandate support
   for revocation checking features in the TLS protocol such as Online
   Certificate Status Protocol (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 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/rfc7633.

Copyright Notice

   Copyright (c) 2015 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     2.2.  TLS Feature, X.509 Extension  . . . . . . . . . . . . . .   3
   3.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  TLS Feature . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Use . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Certificate Signing Request . . . . . . . . . . . . .   5
       4.2.2.  Certificate Signing Certificate . . . . . . . . . . .   5
       4.2.3.  End-Entity Certificate  . . . . . . . . . . . . . . .   5
     4.3.  Processing  . . . . . . . . . . . . . . . . . . . . . . .   6
       4.3.1.  Certification Authority . . . . . . . . . . . . . . .   6
       4.3.2.  Server  . . . . . . . . . . . . . . . . . . . . . . .   7
       4.3.3.  Client  . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     5.1.  Alternative Certificates and Certificate Issuers  . . . .   7
     5.2.  Denial of Service . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Cipher Suite Downgrade Attack . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Transport Layer Security (TLS) feature extension provides a means
   of preventing 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 including downgrade attacks against cipher
   suite choices offered and client credentials, the TLS feature
   extension is only relevant to the validation of TLS protocol
   credentials.

2.  Definitions

2.1.  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].





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2.2.  TLS Feature, X.509 Extension

   In order to avoid the confusion that would occur in attempting to
   specify an X.509 extension describing the use of TLS extensions, in
   this document the term "extension" is reserved to refer to X.509v3
   extensions and the term "TLS feature extension" is used to refer to
   what the TLS specification [RFC5246] refers to as an "extension".

3.  Purpose

   Currently, the only TLS feature extensions that are relevant to the
   revocation status of credentials are the Certificate Status Request
   extension (status_request) and the Multiple Certificate Status
   Extension (status_request_v2).  These extensions are used to support
   in-band exchange of Online Certificate Status Protocol (OCSP) tokens,
   otherwise known as OCSP stapling.  These extensions are described in
   [RFC6066] and [RFC6961].

   The OCSP stapling mechanism described in [RFC6066] permits a TLS
   server to provide evidence of valid certificate status in-band.  When
   this information is provided in-band, the privacy, performance, and
   reliability concerns arising from the need to make a third-party
   connection during the TLS handshake are eliminated.  However, a
   client cannot draw any conclusion from the absence of in-band status
   information unless it knows that the legitimate server would have
   provided it.  The status information might have been omitted because
   the server does not support the extension or because the server is
   withholding the information intentionally, knowing the certificate to
   be invalid.

   The inclusion of a TLS feature extension advertising the
   status_request feature in the server end-entity certificate permits a
   client to fail immediately if the certificate status information is
   not provided by the server.  The need to query the OCSP responder is
   eliminated entirely.  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 feature 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 appropriately addressed by mechanisms such as Certification
   Authority Authorization DNS records [RFC6844] that are designed to
   prevent or mitigate mis-issue.






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   A server offering an end-entity certificate with a TLS feature
   extension MUST satisfy a client request for the specified feature
   unless this would be redundant as described below.  Clients MAY
   refuse to accept the connection if the server does not accept a
   request for a specified feature.

   A Certification Authority SHOULD NOT issue certificates that specify
   a TLS feature extension advertising features that the server does not
   support.

   A server MAY advise a Certification Authority that it is capable of
   supporting a feature by including the corresponding TLS feature
   extension in a Certificate Signing Request [RFC2986].  A server
   SHOULD verify that its configuration supports the features advertised
   in the credentials presented to a client requesting connection.

   This document describes the use of the TLS feature in PKIX end-entity
   certificates and Certificate Signing Certificates.  A mechanism that
   MAY be used to describe support for the specified features in-band
   for the most commonly used certificate registration protocol is also
   provided.

4.  Syntax

   See Appendix A for an ASN.1 module

   The TLS feature extension has the following format:

   id-pe-tlsfeature OBJECT IDENTIFIER ::=  { id-pe 24 }

   Features ::= SEQUENCE OF INTEGER

   The extnValue of the id-pe-tlsfeature extension is the ASN.1 DER
   encoding of the Features structure.

   The TLS feature extension SHOULD NOT be marked critical.  RFC 5280
   [RFC5280] requires that implementations that do not understand
   critical extensions MUST reject the certificate.  Marking the TLS
   feature extension critical breaks backward compatibility and is not
   recommended unless this is the desired behavior.

4.1.  TLS Feature

   The object member "Features" is a sequence of TLS extension
   identifiers (features, in this specification's terminology) as
   specified in the IANA Transport Layer Security (TLS) Extensions





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   registry.  If these features are requested by the client in its
   ClientHello message, then the server MUST return a ServerHello
   message that satisfies this request.

   This specification does not require a TLS client to offer or support
   any TLS feature regardless of whether or not it is specified in the
   server certificate's TLS feature extension.  In particular, a client
   MAY request and a server MAY support any TLS extension regardless of
   whether or not it is specified in a TLS feature extension.

   A server that offers a certificate that contains a TLS feature
   extension MUST support the features specified and comply with the
   corresponding requirements.

4.2.  Use

4.2.1.  Certificate Signing Request

   If the certificate issue mechanism makes use of the PKCS #10
   Certificate Signing Request (CSR) [RFC2986], the CSR MAY specify a
   TLS feature extension as a CSR Attribute as defined in Section 4.1 of
   [RFC2986].  A server or server administration tool should only
   generate key signing requests that it knows can be supported by the
   server for which the certificate is intended.

4.2.2.  Certificate Signing Certificate

   When present in a Certificate Signing Certificate (i.e.,
   Certification Authority certificate with the key usage extension
   value set to keyCertSign), the TLS feature extension specifies a
   constraint on valid certificate chains.  Specifically, a certificate
   that is signed by a Certificate Signing Certificate that contains a
   TLS feature extension MUST contain a TLS feature extension that
   offers the same set or a superset of the features advertised in the
   signing certificate.

   This behavior provides a means of requiring support for a particular
   set of features for certificates issued under a particular
   Certificate Signing Certificate without requiring TLS clients to
   verify compliance with TLS feature extensions in multiple
   certificates.

4.2.3.  End-Entity Certificate

   When specified in a server end-entity certificate (i.e., a
   certificate that specifies the id-kp-serverAuth Extended Key Usage
   (EKU)), the TLS feature extension specifies criteria that a server
   MUST meet to be compliant with the feature declaration.



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

4.2.3.1.  TLS status_request

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

   A client MAY accept a TLS configuration despite it being inconsistent
   with the TLS feature declaration if the validity of the certificate
   chain presented can be established through other means (for example,
   by successfully obtaining the OCSP data from another source).

   There are certain situations in which the alternative to establishing
   a connection with imperfect TLS security is to transmit the same
   information with no security controls whatsoever.  Accordingly, a
   client MAY accept a TLS configuration despite it being inconsistent
   with the TLS feature declaration but MUST NOT distinguish that
   connection as secure.

4.3.  Processing

   Advertising a TLS feature extension may change the expectations of
   relying parties.  If these expectations are not met, a valid
   certificate may be rejected as invalid.  Particular attention is
   required at the start of a certificate lifecycle.  A server will be
   unable to comply with a TLS feature extension if the certificate is
   issued and released to the subject before the corresponding status
   token is published.

4.3.1.  Certification Authority

   A Certification Authority SHOULD NOT issue certificates with a TLS
   feature extension unless there is an affirmative statement to the
   effect that the end entity intends to support the specified features
   (for example, the use of a feature extension in the CSR or through an
   out-of-band communication).

   A Certification Authority SHOULD ensure that the certificate
   provisioning process for certificates containing a TLS feature
   extension permits the certificate subject to meet the requirements
   (for example, ensuring that OCSP tokens are published before the
   corresponding certificate is released to the subscriber).






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4.3.2.  Server

   A TLS server certificate containing a TLS feature extension MAY be
   used with any TLS server that supports the specified features.  It is
   not necessary for the server to provide support for the TLS feature
   extension itself.  Such support is nevertheless desirable as it can
   reduce the risk of administrative error.

   A server SHOULD verify that its configuration is compatible with the
   TLS feature extension expressed in a certificate it presents.  When
   an existing certificate is to be replaced by a new one, the server
   SHOULD NOT begin using the new certificate until the necessary OCSP
   status token or tokens are available.

   A server MAY override local configuration options if necessary to
   ensure consistency, but it SHOULD inform the administrator whenever
   such an inconsistency is discovered.

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

4.3.3.  Client

   A client MUST treat a certificate with a TLS feature extension as an
   invalid certificate if the features offered by the server do not
   contain all features present in both the client's ClientHello message
   and the TLS feature extension.

   In the case that use of TLS with a valid certificate is mandated by
   explicit security policy, application protocol specification, or
   other means, the client MUST refuse the connection.  If the use of
   TLS with a valid certificate is optional, a client MAY accept the
   connection but MUST NOT treat the certificate as valid.

5.  Security Considerations

5.1.  Alternative Certificates and Certificate Issuers

   Use of the TLS feature extension 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 feature is
   compromised after issue but not in the case that the attacker obtains
   an unmarked certificate from an issuer through fraud.

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




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5.2.  Denial of Service

   A certificate issuer could issue a certificate that intentionally
   specified a feature statement that they knew the server could not
   support.

   The consequences of such refusal would appear to be limited since a
   Certification Authority could equally refuse to issue the
   certificate.

5.3.  Cipher Suite Downgrade Attack

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

6.  IANA Considerations

   IANA has added the following entry in the "SMI Security for PKIX
   Certificate Extension" (1.3.6.1.5.5.7.1) registry:

   Decimal  Description                     References
   -------  ------------------------------  ---------------------

     24     id-pe-tlsfeature                this document (RFC 7633)

   IANA has added the following entry in the "SMI Security for PKIX
   Module Identifier" (1.3.6.1.5.5.7.0) registry:

   Decimal  Description                     References
   -------  ------------------------------  ---------------------

     86     id-mod-tls-feature-2015         this document (RFC 7633)


















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7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <http://www.rfc-editor.org/info/rfc2986>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [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, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,
              DOI 10.17487/RFC6844, January 2013,
              <http://www.rfc-editor.org/info/rfc6844>.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,
              <http://www.rfc-editor.org/info/rfc6961>.














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Appendix A.  ASN.1 Module

   TLS-Feature-Module-2015 {
       iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-tls-feature-2015(86)}

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

       IMPORTS -- From RFC 5912

       id-pe
       FROM PKIX1Explicit-2009 {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-pkix1-explicit-02(51)}

       EXTENSION
       FROM PKIX-CommonTypes-2009 {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-pkixCommon-02(57)}
       ;

       CertExtensions EXTENSION ::= {
           ext-TLSFeatures, ... }

           -- TLS Features Extension

       ext-TLSFeatures EXTENSION ::= { SYNTAX
           Features IDENTIFIED BY id-pe-tlsfeature }

       id-pe-tlsfeature OBJECT IDENTIFIER ::= { id-pe 24 }

       Features ::= SEQUENCE OF INTEGER

   END













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Acknowledgements

   This proposal incorporates text and other contributions from
   participants in the IETF and CA-Browser forum -- in particular, Robin
   Alden, Richard Barnes, Viktor Dukhovni, Stephen Farrell, Gervase
   Markham, Yoav Nir, Tom Ritter, Jeremy Rowley, Stefan Santesson, Ryan
   Sleevi, Brian Smith, Rob Stradling, and Sean Turner.

Author's Address

   Phillip Hallam-Baker
   Comodo Group Inc.

   Email: philliph@comodo.com





































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