X.509v3 TLS Feature Extension
draft-hallambaker-tlsfeature-07

Internet Engineering Task Force (IETF)              Phillip Hallam-Baker
Internet-Draft                                         Comodo Group Inc.
Intended Status: id                                        March 9, 2015
Expires: September 10, 2015

                     X.509v3 TLS Feature Extension
                    draft-hallambaker-tlsfeature-07

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 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

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   Copyright (c) 2015 IETF Trust and the persons identified as the 
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Table of Contents

   1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
      1.1.  Requirements Language . . . . . . . . . . . . . . . . . .  3
      1.2.  TLS Feature . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
      3.1.  TLS Feature . . . . . . . . . . . . . . . . . . . . . . .  4
      3.2.  Use . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
         3.2.1.  Certificate Signing Request  . . . . . . . . . . . .  5
         3.2.2.  Certificate Signing Certificate  . . . . . . . . . .  5
         3.2.3.  End Entity Certificate . . . . . . . . . . . . . . .  5
      3.3.  Processing  . . . . . . . . . . . . . . . . . . . . . . .  6
         3.3.1.  Certification Authority  . . . . . . . . . . . . . .  6
         3.3.2.  Server . . . . . . . . . . . . . . . . . . . . . . .  6
         3.3.3.  Client . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
   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.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
      7.1.  Normative References  . . . . . . . . . . . . . . . . . .  7
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  8

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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].

1.2. TLS Feature

   In order to avoid the confusion that would occur in attempting to 
   describe 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 'feature' is used to refer to a TLS 
   extension.

2. Purpose

   The purpose of the TLS Feature 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 including downgrade attacks against cipher 
   suite choices offered and client credentials, the TLS Feature is only
   relevant to the validation of TLS protocol credentials.  In 
   particular to the revocation status of the server credentials 
   presented.

   At the time of writing, 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 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 inband.  When 
   this information is provided inband, the privacy, performance and 
   reliability concerns arising from the need to make a third party 
   connection during the TLS handshake are eliminated.  A client cannot 
   however draw any conclusion from the absence of inband 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 
   witholding 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 

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   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 RFC 6844 [RFC6844] that are 
   designed to prevent or mitigate mis-issue.  Nevertheless a 
   Certification Authority MAY consider the presence or absence of a 
   required TLS feature as one factor in determining the level of 
   additional scrutiny a request should be subject to.

   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.  Otherwise clients
   MAY refuse connection.  It is important therefore that a 
   Certification Authority only issue certificates that specify features
   that match the configuration of the server and that the server is 
   capable of verifying that its configuration is compatible with the 
   feature declaration of the certificates it offers.  Ideally, the TLS 
   feature declaration would be specified by the certificate request 
   generator as part of the certificate issue process.

   This document describes the use of the TLS feature in PKIX end entity
   and certificate signing certificate and a mechanism that MAY be used 
   to describe support for the specified features in-band for the most 
   commonly used certificate registration protocol.

3. Syntax

   The TLS Feature extension has the following format:

   tls-feature OBJECT IDENTIFIER ::=  { id-pe  24 }
   
   Features ::= SEQUENCE OF INTEGER

   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.

3.1. TLS Feature

   The object member Features is a sequence of TLS extension identifiers
   (features, in this specification's terminology).  If these features 
   are requested by the client in its ClientHello message, then they 

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   MUST be present in the server's ServerHello.

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

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

3.2. Use

3.2.1. 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.  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.

3.2.2. Certificate Signing Certificate

   When present in a Certificate Signing Certificate (i.e., CA 
   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 which MUST offer the same set or a 
   superset of the features advertised in the signing certificate.

   While relying parites (i.e., clients) MAY reject certificates that do
   not comply with this requirement, the use of TLS Feature extension 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.

3.2.3. End Entity Certificate

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

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

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   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.

3.3. Processing

3.3.1. Certification Authority

   A CA SHOULD NOT issue certs 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.

3.3.2. 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.  A 
   server MAY override local configuration options if necessary to 
   ensure consistency but 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.

3.3.3. Client

   A client MUST treat a certificate with a TLS Feature extension 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.

4. 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

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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.

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 risks of such refusal would appear to be negligible 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

   On approval, IANA shall add in the SMI Security for PKIX Certificate 
   Extension (1.3.6.1.5.5.7.1) registry the following entry:

   Decimal  Description                     References
   -------  ------------------------------  ---------------------
   24       id-pe-tlsfeature                {this RFC}

7. References

7.1. Normative References

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

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions: 
              Extension Definitions", RFC 6066, January 2011.

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   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS) 
              Multiple Certificate Status Request Extension", RFC 6961, 
              June 2013.

   [RFC6844]  Hallam-Baker, P.,Stradling, R., "DNS Certification 
              Authority Authorization (CAA) Resource Record", RFC 6844, 
              January 2013.

   [RFC5280]  Cooper, D.,Santesson, S.,Farrell, S.,Boeyen, S.,Housley, 
              R.,Polk, W., "Internet X.509 Public Key Infrastructure 
              Certificate and Certificate Revocation List (CRL) 
              Profile", RFC 5280, May 2008.

   [RFC2986]  Nystrom, M.,Kaliski, B., "PKCS #10: Certification Request 
              Syntax Specification Version 1.7", RFC 2986, November 
              2000.

Author's Address

   Phillip Hallam-Baker
   Comodo Group Inc.

   philliph@comodo.com

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