Internet Engineering Task Force P. M. Hallam-Baker Internet-Draft Comodo Group Inc. Intended status: Standards Track April 01, 2013 Expires: October 03, 2013 X.509v3 TLS Feature Extension draft-hallambaker-tlsfeature-01 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 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 03, 2013. Copyright Notice Copyright (c) 2013 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 Hallam-Baker Expires October 03, 2013 [Page 1] Internet-Draft X.509v3 TLS Feature Extension April 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 1.2. TLS Feature . . . . . . . . . . . . . . . . . . . . . . . 2 2. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. TLS Feature . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . 6 5.1. Alternative Certificates and Certificate Issuers . . . . 6 5.2. Denial of Service . . . . . . . . . . . . . . . . . . . . 7 5.3. Cipher Suite Downgrade Attack . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Normative References . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 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 Hallam-Baker Expires October 03, 2013 [Page 2] Internet-Draft X.509v3 TLS Feature Extension April 2013 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, the TLS Feature 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 [RFC6066]. The OCSP stapling mechanism described in [RFC6066] permits a TLS server to provide evidence of valid certificate status inband and thus improve client response. A TLS Feature extension 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 feature declaration MUST respond with a valid OCSP token for the End Entity Certificate it presents unless another extension offered by the client indicates that this is unnecessary. For example, if the client indicates that it already has a valid OCSP token cached. Use of the TLS Feature 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 feature permits a client to fail 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 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 approrpriately addressed by mechanisms such as Certificate 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 scruitiny a request should be subject to. Hallam-Baker Expires October 03, 2013 [Page 3] Internet-Draft X.509v3 TLS Feature Extension April 2013 Any TLS feature declaration specified in an End Entity certificate MUST be offered by the server or 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 a mechanism that MAY be used to provide this communication in-band for the most commonly used certificate registration protocol. 3. Syntax The TLS Feature extension has the following format: cabf-tls-feature OBJECT IDENTIFIER ::= { cabf 1 } Features ::= SEQUENCE OF INTEGER The TLS Feature Extension MAY be marked critical. RFC 5280 [RFC5280] requires that implementations that do not understand the extension MUST reject the certificate. Marking the TLS Feature Extension critical breaks backward compatibility and is not recommended unless this is the desired behavior. Implementations that process the extension MUST ignore the criticality bit setting. 3.1. TLS Feature The TLS Feature extension lists a sequence of TLS extension identifiers that a server compliant with the feature declaration MUST support and accept on client request. This specification does not require a TLS client to offer or support any TLS extension regardless of whether it is specified in the TLS Feature 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 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. Hallam-Baker Expires October 03, 2013 [Page 4] Internet-Draft X.509v3 TLS Feature Extension April 2013 3.1.1. status_request If the TLS status_request extension is specified in the TLS Feature 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) [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, 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 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 an End Entity Certificate, 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. 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. Hallam-Baker Expires October 03, 2013 [Page 5] Internet-Draft X.509v3 TLS Feature Extension April 2013 3.3. Processing 3.3.1. Certification Authority A CA SHOULD NOT issue certs with a 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 The TLS Feature extension MAY be used with any TLS server regardless of whether the server offers support. Server support for the TLS Feature extension 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 inconsitency is discovered. A server SHOULD support generation of the Feature extension in CSRs if key generation is supported. 3.3.3. Client A compliant client MUST process the TLS Feature 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 Feature 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 Hallam-Baker Expires October 03, 2013 [Page 6] Internet-Draft X.509v3 TLS Feature Extension April 2013 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 Certificate 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 No action by IANA is required. 7. 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. [RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, November 2000. [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. Hallam-Baker Expires October 03, 2013 [Page 7] Internet-Draft X.509v3 TLS Feature Extension April 2013 [RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification Authority Authorization (CAA) Resource Record", RFC 6844, January 2013. [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 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 Hallam-Baker Expires October 03, 2013 [Page 8]